Wednesday, March 4, 2009

THE MOON
The moon is Earth's only natural satellite. The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles.

The same side of the moon always faces the Earth. The far side of the moon was first observed by humans in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin (on NASA's Apollo 11 mission, which also included Michael Collins) were the first people to walk on the moon, on July 20, 1969.

If you were standing on the moon, the sky would always appear dark, even during the daytime. Also, from any spot on the moon (except on the far side of the moon where you cannot see the Earth), the Earth would always be in the same place in the sky; the phase of the Earth changes and the Earth rotates, displaying various continents.

THE MOON'S ORBIT
The moon is about 238,900 miles (384,000 km) from Earth on average. At its closest approach (the lunar perigee) the moon is 221,460 miles (356,410 km) from the Earth. At its farthest approach (its apogee) the moon is 252,700 miles (406,700 km) from the Earth.

The moon revolves around the Earth in about one month (27 days 8 hours). It rotates around its own axis in the same amount of time. The same side of the moon always faces the Earth; it is in a synchronous rotation with the Earth.

The Moon's orbit is expanding over time as it slows down (the Earth is also slowing down as it loses energy). For example, a billion years ago, the Moon was much closer to the Earth (roughly 200,000 kilometers) and took only 20 days to orbit the Earth. Also, one Earth 'day' was about 18 hours long (instead of our 24 hour day). The tides on Earth were also much stronger since the moon was closer to the Earth.

SAROS
The saros is the roughly 18-year periodic cycle of the Earth-Moon-Sun system. Every 6,585 days, the Earth, Moon and Sun are in exactly the same position. When there is a lunar eclipse, there will also be one exactly 6,585 days later.

SIZE
The moon's diameter is 2,140 miles (3,476 km), 27% of the diameter of the Earth (a bit over a quarter of the Earth's diameter).

The gravitational tidal influence of the Moon on the Earth is about twice as strong as the Sun's gravitational tidal influence. The Earth:moon size ratio is quite small in comparison to ratios of most other planet:moon systems (for most planets in our Solar System, the moons are much smaller in comparison to the planet and have less of an effect on the planet).

MASS AND GRAVITY
The moon's mass is (7.35 x 10 22 kg), about 1/81 of the Earth's mass.

The moon's gravitational force is only 17% of the Earth's gravity. For example, a 100 pound (45 kg) person would weigh only 17 pounds (7.6 kg) on the Moon.

The moon's density is 3340 kg/m 3. This is about 3/5 the density of the Earth.

TEMPERATURE
The temperature on the Moon ranges from daytime highs of about 130°C = 265°F to nighttime lows of about -110°C = -170°F

ATMOSPHERE
The moon has no atmosphere. On the moon, the sky is always appears dark, even on the bright side (because there is no atmosphere). Also, since sound waves travel through air, the moon is silent; there can be no sound transmission on the moon.

MARE
Mare (plural maria) means "sea," but maria on the moon are plains on the moon. They are called maria because very early astronomers thought that these areas on the moon were great seas. The first moon landing was in the Mare Tranquillitatis (the Sea of Tranquility). Maria are concentrated on the side of the moon that faces the Earth; the far side has very few of these plains. Scientists don't know why this is so.

CRATERS AND RILLES

The lunar crater Aristarchus ( on the NW edge of the Oceanus Procellarum). This huge, circular crater is 25 miles (40 km) in diameter and 2.2 miles (3.6 km) deep (from rim to floor). There is a lot of ejecta (material thrown from the crater at impact) surrounding the crater.
The surface of the moon is scarred by millions of (mostly circular) impact craters, caused by asteroids, comets, and meteorites. There is no atmosphere on the moon to help protect it from bombardment from potential impactors (most objects from space burn up in our atmosphere). Also, there is no erosion (wind or precipitation) and little geologic activity to wear away these craters, so they remain unchanged until another new impact changes it.

These craters range in size up to many hundreds of kilometers, but the most enormous craters have been flooded by lava, and only parts of the outline are visible. The low elevation maria (seas) have fewer craters than other areas. This is because these areas formed more recently, and have had less time to be hit. The biggest intact lunar crater is Clavius which is 100 miles (160 km) in diameter.

A rille is a long, narrow valley on the surface of the moon. Hadley Rille is a long valley on the surface of the moon. This rille is 75 miles (125 km) long, 1300 feet (400 m) deep, and almost 1 mile (1500 m) wide at its widest point. It was formed by molten basaltic lava that carved out a steep channel along the base of the Apennine Front (which was explored by the Apollo 15 astronauts in 1971).

MOON OR DOUBLE PLANET?
The Earth and the Moon are relatively close in size (4:1 in diameter, 81:1 in mass), unlike most planet/moon systems. Many people consider the Earth and Moon to be a double planet system (rather than a planet/moon system). The moon does not actually revolve around the Earth; it revolves around the Sun in concert with the Earth (like a double planet system).

LIBRATION
Libration is a rocking movement of the Moon. Librations cause us to view the Moon from different angles at different times, enabling us to see about 59 percent of the Moon's surface from Earth, even though the same side always faces us. There are librations due to variations in the rate of the Moon's orbital motion (longitudinal libration) and to the inclination of the Moon's equator with respect to its orbital plane (latitudinal libration). There is also an apparent libration due to an observer on Earth viewing the Moon from different angles as the Earth rotates (diurnal libration, which occurs each day).
TWO LUNAR MONTHS
The sidereal and synodic lunar months have different lengths. The sidereal month is the amount of time it takes the Moon to return to the same position in the sky with respect to the stars; the sidereal month is 27.321 days long. The synodic month is the time between similar lunar phases (e.g., between two full moons); the synodic month is 29.530 days long.

LUNAR EXPLORATION

Astronaut Buzz Aldrin's footprint on the moon's Sea of Tranquility, from the Apollo 11 mission in 1969.
There have been many missions to the moon, including orbiters missions and moon landings. NASA's Apollo missions sent people to the moon for the first time. Apollo 11's LEM (Lunar Excursion Module) landed on the moon on July 20, 1969 with Neil Armstrong and Edwin "Buzz" Aldrin (Michael Collins was in the orbiter). Neil Armstrong was the first person to set foot on the moon. His first words upon stepping down the Lunar Module's ladder onto the lunar surface were, "That's one small step for man, one giant leap for mankind." Aldrin described the lunar scenery as "magnificent desolation." Apollo 12-17 continued lunar exploration.

MOON ROCKS
NASA astronauts have retrieved 842 pounds (382 kg) of moon rocks (in many missions), which have been closely studied. The composition of the moon rocks is very similar to that of Earth rocks. Using radioisotope dating, it has been found that moon rocks are about 4.3 billion years old.

THE ORIGIN OF THE MOON
Most scientists believe that the moon was formed from the ejected material after the Earth collided with a Mars-sized object. This ejected material coalesced into the moon that went into orbit around th Earth. This catastrophic collision occurred about 60 million years after Earth itself formed (about 4.3 billion years ago). This is determined by the radioisotope dating of moon rocks

BLUE MOON

When two full moons occur in a single month, the second full moon is called a "Blue Moon." Another definition of the blue moon is the third full moon that occurs in a season of the year which has four full moons (usually each season has only three full moons.)
Advertisement.

EnchantedLearning.com is a user-supported site.
As a bonus, site members have access to a banner-ad-free version of the site, with print-friendly pages.
Click here to learn more.

Join Enchanted Learning
Site subscriptions last 12 months.
Click here for more information on site membership.
As low as $20.00/year (directly by Credit Card)

Site members have access to the entire website with print-friendly pages and no ads.

(Already a member? Click here.)

Zoom Astronomy
The Moon
General
Description Inside the Moon Craters Phases of the Moon Why Do We See Only One Side of the Moon? Tides Activities,
Web Links
Map Lunar Eclipses



THE MOON
The moon is Earth's only natural satellite. The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles.

The same side of the moon always faces the Earth. The far side of the moon was first observed by humans in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin (on NASA's Apollo 11 mission, which also included Michael Collins) were the first people to walk on the moon, on July 20, 1969.

If you were standing on the moon, the sky would always appear dark, even during the daytime. Also, from any spot on the moon (except on the far side of the moon where you cannot see the Earth), the Earth would always be in the same place in the sky; the phase of the Earth changes and the Earth rotates, displaying various continents.

THE MOON'S ORBIT
The moon is about 238,900 miles (384,000 km) from Earth on average. At its closest approach (the lunar perigee) the moon is 221,460 miles (356,410 km) from the Earth. At its farthest approach (its apogee) the moon is 252,700 miles (406,700 km) from the Earth.

The moon revolves around the Earth in about one month (27 days 8 hours). It rotates around its own axis in the same amount of time. The same side of the moon always faces the Earth; it is in a synchronous rotation with the Earth.

The Moon's orbit is expanding over time as it slows down (the Earth is also slowing down as it loses energy). For example, a billion years ago, the Moon was much closer to the Earth (roughly 200,000 kilometers) and took only 20 days to orbit the Earth. Also, one Earth 'day' was about 18 hours long (instead of our 24 hour day). The tides on Earth were also much stronger since the moon was closer to the Earth.

SAROS
The saros is the roughly 18-year periodic cycle of the Earth-Moon-Sun system. Every 6,585 days, the Earth, Moon and Sun are in exactly the same position. When there is a lunar eclipse, there will also be one exactly 6,585 days later.

SIZE
The moon's diameter is 2,140 miles (3,476 km), 27% of the diameter of the Earth (a bit over a quarter of the Earth's diameter).

The gravitational tidal influence of the Moon on the Earth is about twice as strong as the Sun's gravitational tidal influence. The Earth:moon size ratio is quite small in comparison to ratios of most other planet:moon systems (for most planets in our Solar System, the moons are much smaller in comparison to the planet and have less of an effect on the planet).

MASS AND GRAVITY
The moon's mass is (7.35 x 10 22 kg), about 1/81 of the Earth's mass.

The moon's gravitational force is only 17% of the Earth's gravity. For example, a 100 pound (45 kg) person would weigh only 17 pounds (7.6 kg) on the Moon.

The moon's density is 3340 kg/m 3. This is about 3/5 the density of the Earth.

TEMPERATURE
The temperature on the Moon ranges from daytime highs of about 130°C = 265°F to nighttime lows of about -110°C = -170°F

ATMOSPHERE
The moon has no atmosphere. On the moon, the sky is always appears dark, even on the bright side (because there is no atmosphere). Also, since sound waves travel through air, the moon is silent; there can be no sound transmission on the moon.

MARE
Mare (plural maria) means "sea," but maria on the moon are plains on the moon. They are called maria because very early astronomers thought that these areas on the moon were great seas. The first moon landing was in the Mare Tranquillitatis (the Sea of Tranquility). Maria are concentrated on the side of the moon that faces the Earth; the far side has very few of these plains. Scientists don't know why this is so.

CRATERS AND RILLES

The lunar crater Aristarchus ( on the NW edge of the Oceanus Procellarum). This huge, circular crater is 25 miles (40 km) in diameter and 2.2 miles (3.6 km) deep (from rim to floor). There is a lot of ejecta (material thrown from the crater at impact) surrounding the crater.
The surface of the moon is scarred by millions of (mostly circular) impact craters, caused by asteroids, comets, and meteorites. There is no atmosphere on the moon to help protect it from bombardment from potential impactors (most objects from space burn up in our atmosphere). Also, there is no erosion (wind or precipitation) and little geologic activity to wear away these craters, so they remain unchanged until another new impact changes it.

These craters range in size up to many hundreds of kilometers, but the most enormous craters have been flooded by lava, and only parts of the outline are visible. The low elevation maria (seas) have fewer craters than other areas. This is because these areas formed more recently, and have had less time to be hit. The biggest intact lunar crater is Clavius which is 100 miles (160 km) in diameter.

A rille is a long, narrow valley on the surface of the moon. Hadley Rille is a long valley on the surface of the moon. This rille is 75 miles (125 km) long, 1300 feet (400 m) deep, and almost 1 mile (1500 m) wide at its widest point. It was formed by molten basaltic lava that carved out a steep channel along the base of the Apennine Front (which was explored by the Apollo 15 astronauts in 1971).

MOON OR DOUBLE PLANET?
The Earth and the Moon are relatively close in size (4:1 in diameter, 81:1 in mass), unlike most planet/moon systems. Many people consider the Earth and Moon to be a double planet system (rather than a planet/moon system). The moon does not actually revolve around the Earth; it revolves around the Sun in concert with the Earth (like a double planet system).

LIBRATION
Libration is a rocking movement of the Moon. Librations cause us to view the Moon from different angles at different times, enabling us to see about 59 percent of the Moon's surface from Earth, even though the same side always faces us. There are librations due to variations in the rate of the Moon's orbital motion (longitudinal libration) and to the inclination of the Moon's equator with respect to its orbital plane (latitudinal libration). There is also an apparent libration due to an observer on Earth viewing the Moon from different angles as the Earth rotates (diurnal libration, which occurs each day).
TWO LUNAR MONTHS
The sidereal and synodic lunar months have different lengths. The sidereal month is the amount of time it takes the Moon to return to the same position in the sky with respect to the stars; the sidereal month is 27.321 days long. The synodic month is the time between similar lunar phases (e.g., between two full moons); the synodic month is 29.530 days long.

LUNAR EXPLORATION

Astronaut Buzz Aldrin's footprint on the moon's Sea of Tranquility, from the Apollo 11 mission in 1969.
There have been many missions to the moon, including orbiters missions and moon landings. NASA's Apollo missions sent people to the moon for the first time. Apollo 11's LEM (Lunar Excursion Module) landed on the moon on July 20, 1969 with Neil Armstrong and Edwin "Buzz" Aldrin (Michael Collins was in the orbiter). Neil Armstrong was the first person to set foot on the moon. His first words upon stepping down the Lunar Module's ladder onto the lunar surface were, "That's one small step for man, one giant leap for mankind." Aldrin described the lunar scenery as "magnificent desolation." Apollo 12-17 continued lunar exploration.

MOON ROCKS
NASA astronauts have retrieved 842 pounds (382 kg) of moon rocks (in many missions), which have been closely studied. The composition of the moon rocks is very similar to that of Earth rocks. Using radioisotope dating, it has been found that moon rocks are about 4.3 billion years old.

THE ORIGIN OF THE MOON
Most scientists believe that the moon was formed from the ejected material after the Earth collided with a Mars-sized object. This ejected material coalesced into the moon that went into orbit around th Earth. This catastrophic collision occurred about 60 million years after Earth itself formed (about 4.3 billion years ago). This is determined by the radioisotope dating of moon rocks

BLUE MOON

When two full moons occur in a single month, the second full moon is called a "Blue Moon." Another definition of the blue moon is the third full moon that occurs in a season of the year which has four full moons (usually each season has only three full moons.)

Tuesday, February 3, 2009

rock formation

        Today we have discussed about the rock formation. there are 3 types of rocks the   igneous,sedimentary and metamorphic. We saw the different rocks came from australia. it is very nice because different colors,sizes ,textures and many others.

Monday, December 8, 2008

earthquake


[Collapse]
Wikipedia relies on your donations: please give today.
$3,491,160
Our Goal: $6 million
Donate Now »
[Expand]
Support Wikipedia: a non-profit project
Donate Now »
[Expand]
Support Wikipedia: a non-profit project — Donate Now
Earthquake
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the natural seismic phenomenon. For other uses, see Earthquake (disambiguation).

An earthquake (also known as a tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.

At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.

In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above this.
Global earthquake epicenters, 1963–1998
Global plate tectonic movement
Contents
[hide]

* 1 Naturally occurring earthquakes
o 1.1 Earthquake fault types
o 1.2 Earthquakes away from plate boundaries
o 1.3 Shallow-focus and deep-focus earthquakes
o 1.4 Earthquakes and volcanic activity
o 1.5 Earthquake clusters
+ 1.5.1 Aftershocks
+ 1.5.2 Earthquake swarms
+ 1.5.3 Earthquake storms
* 2 Size and frequency of occurrence
* 3 Effects/impacts of earthquakes
o 3.1 Shaking and ground rupture
o 3.2 Landslides and avalanches
o 3.3 Fires
o 3.4 Soil liquefaction
o 3.5 Tsunami
o 3.6 Floods
o 3.7 Human impacts
* 4 Preparation for earthquakes
* 5 Earthquakes in culture
o 5.1 Mythology and religion
o 5.2 Popular culture
* 6 See also
* 7 References
* 8 External links
o 8.1 Educational
o 8.2 Seismological data centers
+ 8.2.1 Europe
+ 8.2.2 Japan
+ 8.2.3 New Zealand
+ 8.2.4 United States
o 8.3 Seismic scales
o 8.4 Scientific information
o 8.5 Miscellaneous

[edit] Naturally occurring earthquakes
Fault types

Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.[1]

[edit] Earthquake fault types

Main article: Fault (geology)

There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.

[edit] Earthquakes away from plate boundaries

Where plate boundaries occur within continental lithosphere, deformation is spread out a over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms. [2]

All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.[3]

[edit] Shallow-focus and deep-focus earthquakes

The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).[4] These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.[5]

[edit] Earthquakes and volcanic activity

Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helens eruption of 1980.[6]

[edit] Earthquake clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time.[7]

[edit] Aftershocks

Main article: Aftershock

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.[7]

[edit] Earthquake swarms
February 2008 earthquake swarm near Mexicali

Main article: Earthquake swarm

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.[8]

[edit] Earthquake storms

Main article: Earthquake storm

Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.[9][10]

[edit] Size and frequency of occurrence

Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.[11] Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years. [12] This is an example of the Gutenberg-Richter law.

The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.[13] In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.[14]

Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate.[15][16] Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains. Humans can cause earthquakes for example by constructing large dams and buildings, drilling and injecting liquid into wells, and by coal mining and oil drilling.[17]

With the rapid growth of mega-cities such as Mexico City, Tokyo or Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.[18][19]

[edit] Effects/impacts of earthquakes
1755 copper engraving depicting Lisbon in ruins and in flames after the 1755 Lisbon earthquake. A tsunami overwhelms the ships in the harbor.

There are many effects of earthquakes including, but not limited to the following:

[edit] Shaking and ground rupture

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings or other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation.[20] The ground-shaking is measured by ground acceleration.

Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.

Ground rupture is a visible breaking and displacement of the earth's surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.[21]

[edit] Landslides and avalanches

Main article: Landslide

Landslides are a major geologic hazard because they can happen at any place in the world, much like earthquakes. Severe storms, earthquakes, volcanic activity, coastal wave attack, and wildfires can all produce slope instability. Landslide danger may be possible even though emergency personnel are attempting rescue.[22]

[edit] Fires
Fires of the 1906 San Francisco earthquake

Following an earthquake, fires can be generated by break of the electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, the deaths in the 1906 San Francisco earthquake were caused more by the fires than by the earthquake itself.[23]

[edit] Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, as buildings or bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, many buildings were sunk into the ground by soil liquefaction, eventually collapsing upon themselves.[24]

[edit] Tsunami
The tsunami of the 2004 Indian Ocean earthquake

Main article: Tsunami

Tsunamis are long-wavelength, long-period sea waves produced by an sudden or abrupt movement of large volumes of water. In the open ocean, the distance between wave crests can surpass 100 kilometers, and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour, depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.[25]

Ordinarily, subduction earthquakes under magnitude 7.5 on the richter scale do not cause tsunamis. However, there have been recorded instances, yet most destructive tsunamis are caused by magnitude 7.5 plus earthquakes.[25]

Tsunamis are distinct from tidal waves, because in a tsunami, water flows straight instead of in a circle like the typical wave. Earthquake-triggered landslides into the sea can also cause tsunamis.[26]

[edit] Floods

Main article: Flood

A flood is an overflow of any amount of water that reaches land.[27] Floods usually occur because of the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which then collapse and cause floods.[28]

The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.[29]

[edit] Human impacts

Earthquakes may result in disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse of buildings or destabilization of the base of buildings which may lead to collapse in future earthquakes. Earthquakes can also lead to volcanic eruptions, which cause further damages such as substantial crop damage, like in the "Year Without a Summer" (1816).[30]

Most of civilization agrees that human death is the most significant human impact of earthquakes.[31]

[edit] Preparation for earthquakes

Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: Earthquake engineering, Earthquake preparedness, Household seismic safety, Seismic retrofit (including special fasteners, materials, and techniques), Seismic hazard, Mitigation of seismic motion, and Earthquake prediction.

[edit] Earthquakes in culture

[edit] Mythology and religion

In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr, god of beauty and light, he was punished by being bound in a cave with a poisonous serpent placed above his head dripping venom. Loki's wife Sigyn stood by him with a bowl to catch the poison, but whenever she had to empty the bowl the poison would drip on Loki's face, forcing him to jerk his head away and thrash against his bonds, causing the earth to tremble.[32]

In Greek mythology, Poseidon was the god of and cause earthquakes. When he was in a bad mood, he would strike the ground with a trident, causing this and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.[33]

[edit] Popular culture

In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906.[34] Fictional earthquakes tend to strike suddenly and without warning.[34] For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1998).[34] The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977) among other works.[34]

[edit] See also
Look up earthquake in Wiktionary, the free dictionary.

* Earthquake insurance
* Earthquake loss
* List of earthquakes
* List of all deadly earthquakes since 1900
* List of earthquakes by death toll

[edit] References

1. ^ Spence, William; S. A. Sipkin, G. L. Choy (1989). "Measuring the Size of an Earthquake". United States Geological Survey. Retrieved on 2006-11-03.
2. ^ Talebian, M. Jackson, J. 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran. Geophysical Journal International, 156, pages 506-526
3. ^ Noson, Qamar, and Thorsen (1988). Washington State Earthquake Hazards: Washington State Department of Natural Resources, Washington Division of Geology and Earth Resources Information Circular 85.
4. ^ "M7.5 Northern Peru Earthquake of 26 September 2005" (pdf). Retrieved on 2008-08-01.
5. ^ Greene, H. W.; Burnley, P. C. (26 October 1989). "A new self-organizing mechanism for deep-focus earthquakes". Nature 341: 733–737. doi:10.1038/341733a0.
6. ^ Foxworthy and Hill (1982). Volcanic Eruptions of 1980 at Mount St. Helens, The First 100 Days: USGS Professional Paper 1249.
7. ^ a b "What are Aftershocks, Foreshocks, and Earthquake Clusters?".
8. ^ "Earthquake Swarms at Yellowstone". USGS. Retrieved on 2008-09-15.
9. ^ Amos Nur (2000). "Poseidon’s Horses: Plate Tectonics and Earthquake Storms in the Late Bronze Age Aegean and Eastern Mediterranean". Journal of Archaeological Science 27: 43–63. doi:10.1006/jasc.1999.0431. ISSN 0305-4403, http://water.stanford.edu/nur/EndBronzeage.pdf.
10. ^ "Earthquake Storms". Horizon (9pm 1 April 2003). Retrieved on 2007-05-02.
11. ^ "Earthquake Hazards Program". USGS. Retrieved on 2006-08-14.
12. ^ Seismicity and earthquake hazard in the UK
13. ^ "Common Myths about Earthquakes". USGS. Retrieved on 2006-08-14.
14. ^ "Earthquake Facts and Statistics: Are earthquakes increasing?". USGS. Retrieved on 2006-08-14.
15. ^ "Historic Earthquakes and Earthquake Statistics: Where do earthquakes occur?". USGS. Retrieved on 2006-08-14.
16. ^ "Visual Glossary - Ring of Fire". USGS. Retrieved on 2006-08-14.
17. ^ Madrigal, Alexis (4 June 2008). "Top 5 Ways to Cause a Man-Made Earthquake", Wired News, CondéNet. Retrieved on 5 June 2008.
18. ^ Global urban seismic risk
19. ^ Earthquake safety in Iran and other developing countries
20. ^ On Shaky Ground, Association of Bay Area Governments, San Francisco, reports 1995,1998 (updated 2003)
21. ^ Guidelines for evaluating the hazard of surface fault rupture, California Geological Survey
22. ^ "Natural Hazards - Landslides". USGS. Retrieved on 2008-09-15.
23. ^ "The Great 1906 San Francisco earthquake of 1906". USGS. Retrieved on 2008-09-15.
24. ^ "Historic Earthquakes -1946 Anchorage Earthquake". USGS. Retrieved on 2008-09-15.
25. ^ a b Noson, Qamar, and Thorsen (1988). Washington Division of Geology and Earth Resources Information Circular 85, Washington State Earthquake Hazards.
26. ^ Wicker, Crystal. "Earthquakes". Crystal Wicker/Weather Wiz Kids.
27. ^ MSN Encarta Dictionary. Flood. Retrieved on 2006-12-28.
28. ^ "Notes on Historical Earthquakes". British Geological Survey. Retrieved on 2008-09-15.
29. ^ "Fresh alert over Tajik flood threat", BBC News (2003-08-03). Retrieved on 15 September 2008.
30. ^ "Facts about The Year Without a Summer". National Geographic UK.
31. ^ "Earthquakes and Volcanoes". University of Michigan.
32. ^ Sturluson, Snorri (1220). Prose Edda.
33. ^ Sellers, Paige (1997-03-03). "Poseidon". Encyclopedia Mythica. Retrieved on 2008-09-02.
34. ^ a b c d Van Riper, A. Bowdoin (2002). Science in popular culture: a reference guide. Westport: Greenwood Press. pp. 60. ISBN 0–313–31822–0.

[edit] External links
Sister project Wikimedia Commons has media related to: Earthquake

[edit] Educational

* 12 of the Most Destructive Earthquakes at HowStuffWorks
* How to survive an earthquake - Guide for children and youth
* Guide to earthquakes and plate tectonics
* Earthquakes — an educational booklet by Kaye M. Shedlock & Louis C. Pakiser
* The Severity of an Earthquake
* USGS Earthquake FAQs
* IRIS Seismic Monitor - maps all earthquakes in the past five years.
* Latest Earthquakes in the World - maps all earthquakes in the past week.
* Earthquake Information from the Deep Ocean Exploration Institute, Woods Hole Oceanographic Institution
* Geo.Mtu.Edu — How to locate an earthquake's epicenter
* Photos/images of historic earthquakes
* earthquakecountry.info Answers to FAQs about Earthquakes and Earthquake Preparedness
* Interactive guide: Earthquakes - an educational presentation by Guardian Unlimited
* Geowall — an educational 3D presentation system for looking at and understanding earthquake data
* Virtual Earthquake - educational site explaining how epicenters are located and magnitude is determined
* HowStuffWorks — How Earthquakes Work
* CBC Digital Archives — Canada's Earthquakes and Tsunamis
* Earthquakes Educational Resources - dmoz

[edit] Seismological data centers

[edit] Europe

* International Seismological Centre (ISC)
* European-Mediterranean Seismological Centre (EMSC)
* Global Seismic Monitor at GFZ Potsdam
* Global Earthquake Report – chart
* Earthquakes in Iceland during the last 48 hours
* Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy
* Database of Individual Seismogenic Sources (DISS), Central Mediterranean
* Portuguese Meteorological Institute (Seismic activity during the last month)

[edit] Japan

* Earthquake Information of Japan, Japan Meteorological Agency
* International Institute of Seismology and Earthquake Engineering (IISEE)
* Building Research Institute
* Database for the damage of world earthquake, ancient period (3000 BC) to year of 2006- Building Research Institute (Japan) (建築研究所) in Japanese
* Seismic activity in last 7 days - Weathernews Inc., indicated with circled shindo (震度) scale and it's location.
o Weathernews Inc, Global web site

[edit] New Zealand

* GeoNet - New Zealand Earthquake Report (latest and recent quakes)

[edit] United States

* The U.S. National Earthquake Information Center
* Southern California Earthquake Data Center
* The Southern California Earthquake Center (SCEC)
* Putting Down Roots in Earthquake Country An Earthquake Science and Preparedness Handbook produced by SCEC
* Recent earthquakes in California and Nevada
* Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer
* Incorporated Research Institutions for Seismology (IRIS), earthquake database and software
* IRIS Seismic Monitor - world map of recent earthquakes
* SeismoArchives - seismogram archives of significant earthquakes of the world

[edit] Seismic scales

* The European Macroseismic Scale

[edit] Scientific information

* "Earthquake Magnitudes and the Gutenberg-Richter Law". SimScience. Retrieved on 2006-08-14.
* Hiroo Kanamori, Emily E. Brodsky (June 2001). "The Physics of Earthquakes". Physics Today 54 (6): 34. doi:10.1063/1.1387590, http://www.physicstoday.org/pt/vol-54/iss-6/p34.html.

[edit] Miscellaneous

* Reports on China Sichuan earthquake 12/05/2008
* Kashmir Relief & Development Foundation (KRDF)
* PBS NewsHour - Predicting Earthquakes
* USGS – Largest earthquakes in the world since 1900
* The Destruction of Earthquakes - a list of the worst earthquakes ever recorded
* Los Angeles Earthquakes plotted on a Google map
* the EM-DAT International Disaster Database
* Earthquake Newspaper Articles Archive
* Earth-quake.org
* PetQuake.org- official PETSAAF system which relies on strange or atypical animal behavior to predict earthquakes.(Link broken 03:33, 2 June 2008 (UTC))
* A series of earthquakes in southern Italy - 23 November 1980, Gesualdo
* Recent Quakes WorldWide
* Real-time earthquakes on Google Map, Australia and rest of the world
* Earthquake Information - detailed statistics and integrated with Google Maps and Google Earth
* Kharita - INGV portal for Digital Cartography - Last earthquakes recorded by INGV Italian Network (with Google Maps)
* Kharita - INGV portal for Digital Cartography - Italian Seismicity by region 1981-2006 (with Google Maps)

[show]
v • d • e
Topics in geotechnical engineering
Soils
Clay · Silt · Sand · Gravel · Peat
Soil properties
Hydraulic conductivity · Water content · Void ratio · Bulk density · Thixotropy · Reynolds' dilatancy · Angle of repose · Cohesion · Porosity · Permeability · Specific storage
Soil mechanics
Effective stress · Pore water pressure · Shear strength · Overburden pressure · Consolidation · Soil compaction · Soil classification · Shear wave · Lateral earth pressure
Geotechnical investigation
Cone penetration test · Standard penetration test · Exploration geophysics · Monitoring well · Borehole
Laboratory tests
Atterberg limits · California bearing ratio · Direct shear test · Hydrometer · Proctor compaction test · R-value · Sieve analysis · Triaxial shear test · Hydraulic conductivity tests · Water content tests
Field tests
Crosshole sonic logging · Nuclear Densometer Test
Foundations
Bearing capacity · Shallow foundation · Deep foundation · Dynamic load testing · Wave equation analysis
Retaining walls
Mechanically stabilized earth · Soil nailing · Tieback · Gabion · Slurry wall
Slope stability
Mass wasting · Landslide
Earthquakes
Soil liquefaction · Response spectrum · Seismic hazard · Ground-structure interaction
Geosynthetics
Geotextile · Geomembranes · Geosynthetic clay liner
Instrumentation for Stability Monitoring
Deformation monitoring · Automated Deformation Monitoring

Retrieved from "http://en.wikipedia.org/wiki/Earthquake"
Categories: Earthquakes | Seismology | Geological hazards | Earthquake engineering
Views

* Article
* Discussion
* Edit this page
* History

Personal tools

* Log in / create account

Navigation

* Main page
* Contents
* Featured content
* Current events
* Random article

Search

Interaction

* About Wikipedia
* Community portal
* Recent changes
* Contact Wikipedia
* Donate to Wikipedia
* Help

Toolbox

* What links here
* Related changes
* Upload file
* Special pages
* Printable version
* Permanent link
* Cite this page

Languages

* Afrikaans
* العربية
* Aragonés
* Azərbaycan
* বাংলা
* Bân-lâm-gú
* Беларуская
* Беларуская (тарашкевіца)
* Bosanski
* Brezhoneg
* Български
* Català
* Чăвашла
* Česky
* Cymraeg
* Dansk
* Deutsch
* Eesti
* Ελληνικά
* Español
* Esperanto
* Euskara
* فارسی
* Føroyskt
* Français
* Frysk
* 贛語
* Gàidhlig
* Galego
* ગુજરાતી
* 한국어
* Հայերեն
* हिन्दी
* Hrvatski
* Ido
* Bahasa Indonesia
* ᐃᓄᒃᑎᑐᑦ/inuktitut
* Íslenska
* Italiano
* עברית
* ಕನ್ನಡ
* ქართული
* Kiswahili
* Latina
* Latviešu
* Lëtzebuergesch
* Lietuvių
* Magyar
* Македонски
* മലയാളം
* मराठी
* Bahasa Melayu
* Монгол
* Nāhuatl
* Nederlands
* 日本語
* ‪Norsk (bokmål)‬
* ‪Norsk (nynorsk)‬
* Occitan
* Oromoo
* O'zbek
* Plattdüütsch
* Polski
* Português
* Ripoarisch
* Română
* Rumantsch
* Runa Simi
* Русский
* Shqip
* Sicilianu
* Simple English
* Slovenčina
* Slovenščina
* Ślůnski
* Српски / Srpski
* Srpskohrvatski / Српскохрватски
* Basa Sunda
* Suomi
* Svenska
* Tagalog
* தமிழ்
* తెలుగు
* ไทย
* Tiếng Việt
* Тоҷикӣ
* Türkçe
* Українська
* اردو
* Vèneto
* Walon
* ייִדיש
* 粵語
* 中文

Powered by MediaWiki
Wikimedia Foundation

* This page was last modified on 8 December 2008, at 04:59.
* All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a U.S. registered 501(c)(3) tax-deductible nonprofit charity.
* Privacy policy
* About Wikipedia
* Disclaimers

plate tectonic

[Collapse]
Wikipedia: Making Life Easier.
$3,491,160
Our Goal: $6 million
Donate Now »
[Expand]
Support Wikipedia: a non-profit project
Donate Now »
[Expand]
Support Wikipedia: a non-profit project — Donate Now
Plate tectonics
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Semi-protected
The tectonic plates of the world were mapped in the second half of the 20th century.

Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") describes the large scale motions of Earth's lithosphere. The theory encompasses the older concepts of continental drift, developed during the first half of the 20th century, and seafloor spreading, understood during the 1960s.

The outermost part of the Earth's interior is made up of two layers: above is the lithosphere, comprising the crust and the rigid uppermost part of the mantle. Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again due to the higher pressure.

The lithosphere is broken up into what are called tectonic plates — in the case of Earth, there are seven major and many minor plates (see list below). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 50—100 mm/a.[1]
Contents
[hide]

* 1 Synopsis of the development of the theory
* 2 Key principles
* 3 Types of plate boundaries
o 3.1 Transform (conservative) boundaries
o 3.2 Divergent (constructive) boundaries
o 3.3 Convergent (destructive) boundaries
* 4 Driving forces of plate motion
o 4.1 Friction
o 4.2 Gravitation
o 4.3 External forces
o 4.4 Relative significance of each mechanism
* 5 Major plates
* 6 Historical development of the theory
o 6.1 Continental drift
o 6.2 Floating continents
o 6.3 Plate tectonic theory
+ 6.3.1 Explanation of magnetic striping
+ 6.3.2 Subduction discovered
+ 6.3.3 Mapping with earthquakes
o 6.4 Geological paradigm shift
* 7 Biogeographic implications on biota
* 8 Plate tectonics on other planets
o 8.1 Venus
o 8.2 Mars
o 8.3 Galilean satellites
o 8.4 Titan
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Synopsis of the development of the theory
Detailed map showing the tectonic plates with their movement vectors.

In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by geosynclinal theory. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean — or, more precisely, the edges of the continental shelves — have similar shapes and seem once to have fitted together.[2] Since that time many theories were proposed to explain this apparent compatibility, but the assumption of a solid earth made the various proposals difficult to explain.[3]

The discovery of radium and its associated heating properties in 1896 prompted a re-examination of the apparent age of the Earth,[4] since this had been estimated by its cooling rate and assumption the Earth's surface radiated like a black body.[5] Those calculations implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists reasoned it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.

Plate tectonic theory arose out of the hypothesis of continental drift proposed by Alfred Wegener in 1912[6] and expanded in his 1915 book The Origin of Continents and Oceans. He suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density granite floating on a sea of more dense basalt.[7][8] But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around. Later science supported theories proposed by English geologist Arthur Holmes in 1920 that plate junctions might lie beneath the sea and Holmes' 1928 suggestion of convection currents within the mantle as the driving force.[9][10][3]

The first evidence that the lithospheric plates did move came with the discovery of variable magnetic field direction in rocks of differing ages, first revealed at a symposium in Tasmania in 1956. Initially theorized as an expansion of the global crust,[11] later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. It was at this point that Wegener's theory became generally accepted by the scientific community. Additional work on the association of seafloor spreading and magnetic field reversals by Harry Hess and Ron G. Mason[12][13][14][15] pinpointed the precise mechanism which accounted for new rock upwelling.

Following the recognition of magnetic anomalies defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a mid-ocean ridge, plate tectonics quickly became broadly accepted. Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones together with many other geologic observations soon made plate tectonics a theory with extraordinary explanatory and predictive power.

Study of the deep ocean floor was critical to development of the theory; the field of deep sea marine geology accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted by almost all scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology.

Key principles

The outer layers of the Earth are divided into lithosphere and asthenosphere. This is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature and pressure.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10-40 mm/a (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as hair grows).[16][17]

Tectonic plates consist of lithospheric mantle overlain by either of two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). Average oceanic lithosphere is typically 100 km thick[18]; its thickness is a function of its age: as time passes, it conductively cools and becomes thicker. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance oceanic lithosphere must travel before being subducted, the thickness varies ~6 km thick at mid-ocean ridges to greater than 100 km at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively.[19] Typical continental lithosphere is typically ~200 km thick[18], though this also varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ in thickness, with continental crust being considerably thicker than oceanic (35 km vs 6 km)[20]

The location where two plates meet is called a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and most widely known. These boundaries are discussed in further detail below.

Tectonic plates can include continental crust or oceanic crust, and many plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes; though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust, these are considered part of the continent when they exit the standard cycle of formation and spreading centers and subduction betneath continents. Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic").[21] As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust buoyantly projects above sea level (see isostasy for explanation of this principle).

Types of plate boundaries
Three types of plate boundary.

Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:

1. Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is one example.
2. Divergent boundaries occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries.
3. Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.

Transform (conservative) boundaries

Main article: Transform boundary

John Tuzo Wilson recognized that because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as strain. Strain is both accumulative and/or instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform faults are also found offsetting the crests of mid-ocean ridges (for example, the Mendocino Fracture Zone offshore northern California).

Divergent (constructive) boundaries
Bridge across the Álfagjá rift valley in southwest Iceland, the boundary between the Eurasian and North American continental tectonic plates.

Main article: Divergent boundary

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and subsidence).[citation needed]

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews in the Morley-Vine-Matthews hypothesis. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent (destructive) boundaries

Main article: Convergent boundary

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate.

While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts (magma) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases (consider Mount St. Helens). The melts rise to the surface and cool forming long chains of volcanoes inland from the continental shelf and parallel to it.[citation needed] The continental spine of western South America is dense with this type of volcanic mountain building from the subduction of the Nazca plate. In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as the Pacific ring of fire.

Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas and the Tibetan Plateau beyond. It may have also pushed nearby parts of the Asian continent aside to the east.[22]

When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be Japan and the Aleutian Islands in Alaska.
Oceanic / Continental

Continental / Continental

Oceanic / Oceanic

Plates may collide at an oblique angle rather than head-on to each other (e.g. one plate moving north, the other moving south-east), and this may cause strike-slip faulting along the collision zone, in addition to subduction or compression.

Not all plate boundaries are easily defined. Some are broad belts whose movements are unclear to scientists. One example would be the Mediterranean-Alpine boundary, which involves two major plates and several micro plates. The boundaries of the plates do not necessarily coincide with those of the continents. For instance, the North American Plate covers not only North America, but also far northeastern Siberia, plus a substantial portion of the Atlantic Ocean.

Driving forces of plate motion

Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. The current view, although it is still a matter of some debate, is that excess density of the oceanic lithosphere sinking in subduction zones is the most powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.[23] Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists.

Two and three-dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally heterogeneous density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeneity is mantle convection from buoyancy forces.[24] How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: friction and gravity.

Friction

Basal drag
Large scale convection currents in the upper mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.
Slab suction
Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting wherein basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Gravitation

Gravitational sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.

Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push". This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example:

1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear topographical feature that can offset or at least affect the influence of topographical ocean ridges.
2. Mantle plumes impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor.

Slab-pull
Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches.[25] There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The overall driving force for plate motion and its energy source remain subjects of ongoing research.

External forces

In a study published in the January-February 2006 issue of the Geological Society of America Bulletin, a team of Italian and U.S. scientists argued that the westward component of plates is from Earth's rotation and consequent tidal friction of the moon. As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. It has also been suggested (albeit, controversially) that this observation may also explain why Venus and Mars have no plate tectonics since Venus has no moon, and Mars' moons are too small to have significant tidal effects on Mars.[26] This is not, however, a new argument.

It was originally raised by the "father" of the plate tectonics hypothesis, Alfred Wegener. It was challenged by the physicist Harold Jeffreys who calculated that the magnitude of tidal friction required would have quickly brought the Earth's rotation to a halt long ago. Many plates are moving north and eastward, and the dominantly westward motion of the Pacific ocean basins is simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). It is argued, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.

Relative significance of each mechanism
Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. Vectors show direction and magnitude of motion.

The actual vector of a plate's motion must necessarily be a function of all the forces acting upon the plate. However, therein remains the problem regarding what degree each process contributes to the motion of each tectonic plate.

The diversity of geodynamic settings and properties of each plate must clearly result in differences in the degree to which such processes are actively driving the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving and to consider the available evidence of each driving force upon the plate as far as possible.[citation needed]

One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.[citation needed]

The driving forces of plate motion are, nevertheless, still very active subjects of on-going discussion and research in the geophysical community.

Major plates

The main plates are

* African Plate covering Africa - Continental plate
* Antarctic Plate covering Antarctica - Continental plate
* Australian Plate covering Australia - Continental plate
* Indian Plate covering Indian subcontinent and a part of Indian Ocean - Continental plate
* Eurasian Plate covering Asia and Europe - Continental plate
* North American Plate covering North America and north-east Siberia - Continental plate
* South American Plate covering South America - Continental plate
* Pacific Plate covering the Pacific Ocean - Oceanic plate

Notable minor plates include the Arabian Plate, the Caribbean Plate, the Juan de Fuca Plate, the Cocos Plate, the Nazca Plate, the Philippine Plate and the Scotia Plate.

The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangaea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

Related article

* List of tectonic plates

Plate tectonics map

Historical development of the theory

Continental drift

For more details on this topic, see Continental drift.

Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th centuries. The theory has been superseded and the concepts and data have been incorporated within plate tectonics.

By 1915, Alfred Wegener was making serious arguments for the idea in the first edition of The Origin of Continents and Oceans. In that book, he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener wasn't the first to note this (Abraham Ortelius, Francis Bacon, Benjamin Franklin, Snider-Pellegrini, Roberto Mantovani and Frank Bursley Taylor preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that propelled continental drift.

Wegener's vindication did not come until after his death in 1930. In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution’s research vessel Atlantis and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents. They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions.[27]

Beginning in the 1950s, scientists including Harry Hess, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time.

As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping.

When the rock strata of the tips of separate continents are very similar it suggests that these rocks were formed in the same way implying that they were joined initially. For instance, some parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.

Floating continents

The prevailing concept was that there were static shells of strata under the continents. It was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt. It was apparent that a layer of basalt underlies continental rocks.

However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later during study of Himalayan gravitation, and seismic studies detected corresponding density variations.

By the mid-1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg.

In 1958 the Tasmanian geologist Samuel Warren Carey published an essay The tectonic approach to continental drift in support of the expanding earth model.

Plate tectonic theory

Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist Harry Hammond Hess (Robert S. Dietz published the same idea one year earlier in Nature. However, priority belongs to Hess, since he distributed an unpublished manuscript of his 1962 article already in 1960). Hess suggested that instead of continents moving through oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In the same year, Robert R. Coats of the U.S. Geological Survey described the main features of island arc subduction in the Aleutian Islands. His paper, though little-noted (and even ridiculed) at the time, has since been called "seminal" and "prescient". In 1967, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions.

Explanation of magnetic striping
Seafloor magnetic striping.

The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:

1. at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;
2. the youngest rocks at the ridge crest always have present-day (normal) polarity;
3. stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has reversed many times.

By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field.

Subduction discovered

A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists, most notably S. Warren Carey, who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "Expanding Earth theory" hypothesis was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. Certainly there is no evidence that the moon has expanded in the past 3 billion years. Still, the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?

This question particularly intrigued Harry Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spreads away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches — very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is consumed in the trenches, new magma rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Mapping with earthquakes

During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration world wide.

Geological paradigm shift

The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) may be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?". Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box.

However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.

One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.

With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift the sea floor to great heights. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.

Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift, similar to the Great Rift Valley in northeastern Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge.

Biogeographic implications on biota

Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present day life found on different continents but having similar ancestors.[28] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora.

Plate tectonics on other planets

The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundant water.[29]

Venus

See also: Geology of Venus

Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved impact craters have been utilized as a dating method to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). Dates derived are the dominantly in the range ~500 to 750 Ma, although ages of up to ~1.2 Ga have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.

One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.[30][31] The Earth's crust is soaked with water, and water plays an important role in the development of shear zones. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or was once active on this planet.

Mars

See also: Geology of Mars

Unlike Venus, the crust of Mars has water in it and on it (mostly in the form of ice). This planet is considerably smaller than the Earth, but shows some indications that could suggest a similar style of tectonics. The gigantic volcanoes in the Tharsis area are linearly aligned like volcanic arcs on Earth; the enormous canyon Valles Marineris could have been formed by some form of crustal spreading.

As a result of observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999, large scale patterns of magnetic striping were discovered on this planet. To explain these magnetisation patterns in the Martian crust it has been proposed that a mechanism similar to plate tectonics may once have been active on the planet.[32][33] Further data from the Mars Express orbiter's High Resolution Stereo Camera in 2007 clearly showed an example in the Aeolis Mensae region.[34]

Galilean satellites

Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth.

Titan

Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe, which landed on Titan on January 14, 2005.[35]

See also

* List of plate tectonics topics
* List of tectonic plates
* List of tectonic plate interactions
* Geosyncline theory, obsolete explanation of mountain-building
* Plume tectonics, an extension of plate tectonics that attempts to explain other aspects of the field

References

1. ^ Read HH, Watson Janet (1975). Introduction to Geology. New York: Halsted. pp. 13–15.
2. ^ Kious WJ, Tilling RI. "Historical perspective". This Dynamic Earth: the Story of Plate Tectonics (Online edition ed.), U.S. Geological Survey. ISBN 0160482208. http://pubs.usgs.gov/gip/dynamic/historical.html. Retrieved on 29 January 2008. "Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods ... The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]."".
3. ^ a b Frankel Henry (1978-07). "Arthur Holmes and Continental Drift". The British Journal for the History of Science 11 (2): 130–150. http://www.jstor.org/pss/4025726.
4. ^ Joly J (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History. London: Archibald Constable. p. 36. ISBN 1402135777.
5. ^ Thomson W (1863). "On the secular cooling of the earth". Philosophical Magazine 4 (25): 1–14. doi:10.1080/14786435908238225.
6. ^ Hughes Patrick. "Alfred Wegener (1880-1930): A Geographic Jigsaw Puzzle". On the Shoulders of Giants. Earth Observatory, NASA. Retrieved on 2007-12-26. "... on January 6, 1912, Wegener ... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography."
7. ^ Alfred Wegener (1966). The Origin of Continents and Oceans, Courier Dover. pp. 246. ISBN 0486617084.
8. ^ Hughes Patrick. "Alfred Wegener (1880-1930): The Origin of Continents and Oceans". On the Shoulders of Giants. Earth Observatory, NASA. Retrieved on 2007-12-26. "By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it Pangaea (all lands), ..."
9. ^ Holmes Arthur (1928). "Radioactivity and Earth Movements". Transactions of the Geological Society of Glasgow 18: 559–606.
10. ^ Holmes Arthur (1978). Principles of Physical Geology (3rd ed.), Wiley. pp. 640–641. ISBN 0471072516.
11. ^ 1958: The tectonic approach to continental drift. In: S. W. Carey (ed.): Continental Drift – A Symposium. University of Tasmania, Hobart, 177-363 (expanding Earth from p. 311 to p. 349)
12. ^ Korgen Ben J (1995) (PDF). "A Voice From the Past: John Lyman and the Plate Tectonics Story" (PDF). Oceanography 8 (1): 19–20. http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/8_1/8.1_korgen.pdf.
13. ^ Spiess Fred, Kuperman William (2003) (PDF). "The Marine Physical Laboratory at Scripps" (PDF). Oceanography 16 (3): 45–54. http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/16_3/16.3_spiess.pdf.
14. ^ Mason RG, Raff AD (1961). "Magnetic survey off the west coast of the United States between 32°N latitude and 42°N latitude". Bulletin of the Geological Society of America 72: 1259–1266. doi:10.1130/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2.
15. ^ Raff AD, Mason RG (1961). "Magnetic survey off the west coast of the United States between 40°N latitude and 52°N latitude". Bulletin of the Geological Society of America 72: 1267–1270. doi:10.1130/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2.
16. ^ Huang Zhen Shao (1997). "Speed of the Continental Plates". The Physics Factbook.
17. ^ Hancock, Paul L; Skinner, Brian J; Dineley, David L (2000). The Oxford Companion to The Earth, Oxford University Press. ISBN 0198540396.
18. ^ a b Turcotte, D. L.; Schubert, G. (2002). "Plate Tectonics". Geodynamics (2nd edition ed.), Cambridge University Press. pp. 5. ISBN 0-521-66186-2.
19. ^ Turcotte, D. L.; Schubert, G. (2002). "Heat Transfer". Geodynamics (2nd edition ed.), Cambridge University Press. pp. 157–161. ISBN 0-521-66186-2.
20. ^ Turcotte, D. L.; Schubert, G. (2002). "Plate Tectonics". Geodynamics (2nd edition ed.), Cambridge University Press. pp. 3. ISBN 0-521-66186-2.
21. ^ Schmidt Victor A, Harbert William. "The Living Machine: Plate Tectonics". Planet Earth and the New Geosciences (third ed.). ISBN 0787242969. http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/chap03.html. Retrieved on 28 January 2008.
22. ^ Butler, Rob (October 2001). Where and how do the continents deform?, Himalayan tectonics, Dynamic Earth. School of Earth Sciences, University of Leeds. Accessed 2008-01-29.
23. ^ Pedro Mendia-Landa. "Myths and Legends on Natural Disasters: Making Sense of Our World". Retrieved on 2008-02-05.
24. ^ Tanimoto Toshiro, Lay Thorne (2000-11-07). "Mantle dynamics and seismic tomography". Proceedings of the National Academy of Science 97 (23): 12409–12410. doi:10.1073/pnas.210382197. PMID 11035784.
25. ^ Conrad CP, Lithgow-Bertelloni C (2002). "How Mantle Slabs Drive Plate Tectonics". Science 298 (5591): L45. doi:10.1126/science.1074161.
26. ^ Lovett Richard A (2006-01-24). "Moon Is Dragging Continents West, Scientist Says". National Geographic News. http://news.nationalgeographic.com/news/2006/01/0124_060124_moon.html.
27. ^ Lippsett Laurence (2001). "Maurice Ewing and the Lamont-Doherty Earth Observatory]". Living Legacies. http://www.columbia.edu/cu/alumni/Magazine/Winter2001/ewing.html. Retrieved on 4 March 2008.
28. ^ Moss SJ, Wilson MEJ (1998). "Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo" (PDF). in Hall R, Holloway JD (eds). Biogeography and Geological Evolution of SE Asia. Leiden, The Netherlands: Backhuys. pp. 133–163. ISBN 9073348978.
29. ^ Valencia Diana, O'Connell Richard J, Sasselov Dimitar D (November 2007). "Inevitability of Plate Tectonics on Super-Earths". Astrophysical Journal Letters 670 (1): L45–L48. doi:10.1086/524012. http://arxiv.org/abs/0710.0699v1.
30. ^ Bortman Henry (2004-08-26). "Was Venus alive? 'The Signs are Probably There'". Astrobiology Magazine. Retrieved on 2008-01-08.
31. ^ Kasting JF (1988). "Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus". Icarus 74 (3): 472–494. doi:10.1016/0019-1035(88)90116-9.
32. ^ Connerney JEP, Acuña MH, Wasilewski PJ, Ness NF, Rème H, Mazelle C, Vignes D, Lin RP, Mitchell DL, Cloutier PA (1999). "Magnetic Lineations in the Ancient Crust of Mars". Science 284: 794. doi:10.1126/science.284.5415.794. PMID 10221909.
33. ^ Connerney JEP, Acuña MH, Ness NF, Kletetschka G, Mitchell DL, Lin RP, Rème H (2005). "Tectonic implications of Mars crustal magnetism". Proceedings of the National Academy of Sciences 102: 14970–14975. doi:10.1073/pnas.0507469102. PMID 16217034.
34. ^ "Tectonic signatures at Aeolis Mensae". European Space Agency (2007-06-28). Retrieved on 2008-01-29.
35. ^ Soderblom Laurence A, Tomasko Martin G, Archinal Brent A, Becker Tammy L, Bushroe Michael W, Cook Debbie A, Doose Lyn R, Galuszka Donna M, Hare Trent M, Howington-Kraus Elpitha, Karkoschka Erich, Kirk Randolph L, Lunine Jonathan I, McFarlane Elisabeth A, Redding Bonnie L, Rizk Bashar, Rosiek Mark R, See Charles, Smith Peter H (2007). "Topography and geomorphology of the Huygens landing site on Titan". Planetary and Space Science 55 (13): 2015. doi:10.1016/j.pss.2007.04.015.

Further reading

* McKnight Tom (2004). Geographica: The complete illustrated Atlas of the world. New York: Barnes and Noble Books. ISBN 076075974X.
* Oreskes, Naomi (ed) (2003). Plate Tectonics: An Insider's History of the Modern Theory of the Earth, Westview. ISBN 0813341329.
* Mantle Convection in the Earth and Planets. Cambridge: Cambridge University Press. 2001. ISBN 052135367X.
* Stanley Steven M (1999). Earth System History, W.H. Freeman. pp. 211–228. ISBN 0716728826.
* Tanimoto Toshiro, Lay Thorne (2000). "Mantle dynamics and seismic tomography". Proceedings of the National Academy of Science 97: 12409. doi:10.1073/pnas.210382197. PMID 11035784.
* Thompson Graham R, Turk Jonathan (1991). Modern Physical Geology, Saunders College Publishing. ISBN 0030253985.
* Turcotte DL, Schubert G (2002). Geodynamics: Second Edition. New York: John Wiley & Sons. ISBN 0521666244.
* Winchester, Simon (2003). Krakatoa: The Day the World Exploded: August 27, 1883, HarperCollins. ISBN 0066212855.
* Atkinson L, Sancetta C (1993). "Hail and farewell". Oceanography 6 (34).
* Lyman J, Fleming RH (1940). "Composition of Seawater". J Mar Res 3: 134–146.
* Sverdrup HU, Johnson MW, Fleming RH (1942). The Oceans: Their physics, chemistry and general biology. Englewood Cliffs: Prentice-Hall. pp. 1087.
* Vine FJ, Matthews DH (1963). "Magnetic anomalies over oceanic ridges". Nature 199: 947–949. doi:10.1038/199947a0.

External links
Sister project Wikimedia Commons has media related to: Plate tectonics

* The PLATES Project, a comprehensive resource with reconstructions, movies, images, list of publications, and teaching resources, from the University of Texas Institute for Geophysics at the Jackson School of Geosciences.
* The Paleomap Project, Christopher Scotese's website with reconstructions in the past and future, paleogeographies, teaching material etc.
* Movie showing 750 million years of global tectonic activity
* Easy-to-draw illustrations for teaching plate tectonics
* An explanation of tectonic forces
* Bird, P. (2003) An updated digital model of plate boundaries, also available as a large (13 mb) PDF file
* Map of tectonic plates
* MantlePlumes.org, a website debating the existence of deep mantle plumes
* USGS site on plate motions
* The geodynamics of the North-American/Eurasian/African plate boundaries
* Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision
* "Speed of the Continental Plates". The Physics Factbook (1997).
* ImpactTectonics.org, examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets
* BBC Radio 4 In Our Time - Plate Tectonics

[show]
v • d • e
Tectonic plates
Major
African · Antarctic · Eurasian · Indian · Indo-Australian · North American · Pacific · South American

Minor
Arabian · Caribbean · Cocos · Explorer · Gorda · Jan Mayen · Juan de Fuca · Nazca · Philippine · Scotia
Other
List of tectonic plates
[show]
v • d • e
Structure of the Earth
Crust · Upper mantle · Lithosphere · Asthenosphere · Mesosphere · Mantle · Outer core · Inner core · Plate tectonics
[show]
v • d • e
Elements of Nature
Earth
History of Earth · Earth science · Structure of the Earth · Plate tectonics · Geological history of Earth · Geology
Weather
Climate · Earth's atmosphere
Life
Biosphere · Origin of life · Microbe · Flora · Plants · Fungi · Fauna · Animals · Biology · Evolutionary history of life
Environment
Wilderness · Ecology · Ecosystem
Universe
Matter · Energy · Outer space
Category · Portal

Retrieved from "http://en.wikipedia.org/wiki/Plate_tectonics"
Categories: Geophysics | Plate tectonics | Geology theories
Hidden categories: Semi-protected against vandalism | All articles with unsourced statements | Articles with unsourced statements since January 2008
Views

* Article
* Discussion
* View source
* History

Personal tools

* Log in / create account

Navigation

* Main page
* Contents
* Featured content
* Current events
* Random article

Search

Interaction

* About Wikipedia
* Community portal
* Recent changes
* Contact Wikipedia
* Donate to Wikipedia
* Help

Toolbox

* What links here
* Related changes
* Upload file
* Special pages
* Printable version
* Permanent link
* Cite this page

Languages

* Alemannisch
* العربية
* বাংলা
* Basa Banyumasan
* Беларуская
* Беларуская (тарашкевіца)
* Български
* Català
* Česky
* Cymraeg
* Dansk
* Deutsch
* Eesti
* Español
* Esperanto
* Euskara
* فارسی
* Français
* 한국어
* हिन्दी
* Hrvatski
* Bahasa Indonesia
* Íslenska
* Italiano
* עברית
* Basa Jawa
* Kiswahili
* Latviešu
* Lietuvių
* Magyar
* മലയാളം
* Bahasa Melayu
* Монгол
* Nederlands
* 日本語
* ‪Norsk (bokmål)‬
* Oromoo
* Polski
* Português
* Română
* Русский
* Simple English
* Slovenčina
* Slovenščina
* Српски / Srpski
* Suomi
* Svenska
* தமிழ்
* ไทย
* Tiếng Việt
* Türkçe
* Українська
* اردو
* 中文

Powered by MediaWiki
Wikimedia Foundation

* This page was last modified on 6 December 2008, at 12:33.
* All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a U.S. registered 501(c)(3) tax-deductible nonprofit charity.
* Privacy policy
* About Wikipedia
* Disclaimers