Plate Tectonics: how the theory explains the distribution and occurrence of earthquakes and volcanoes

Before we see how Plate Tectonics explains the distribution of Earthquakes and Volcanoes it is firstly necessary to trace the evolution of the Theory. Plate Tectonics is a composite of two earlier theories. These are:

· Theory of Continental Drift and the

· Theory of Sea Floor Spreading.

The first theory was proposed by Alfred Wegener (1880 - 1930). He was born in Berlin and received a PhD in astronomy. He later ropped astronomy to study meteorology (the new science of weather). Wegener was especially interested in Paleoclimatology. He led expeditions to Greenland in 1906/1907 to study polar air circulation. This was followed up by further expeditions in 1912-1913 and 1929-1930.

In 1910 Wegener noticed the matching coastlines of the Atlantic continents -- they looked on maps like they had once fitted together. He was not the first to notice this so had for example Francis Bacon (I561-1626) but Wegener began the first systematic study.

He spoke on the topic in January 1912, when he put forth the idea of continental displacement or what was later was called Continental Drift. In 1915 while lying wounded in hospital (World War 1) Wegener wrote his book The origin of the Continents and the Oceans. This constituted the first focused and rational argument for continental drift and veered radically from the accepted beliefs of the time.

The basic theory went as follows... all the earths continents were once joined together to form a supercontinent called Pangea….this split approx 250 m/200 m years ago in the late Paleozoic Era into 2 parts . In the north was Laurasia (which lay at the equator) and Gondwanaland (which went south to the South Pole) . The ocean between was called Tethys. These two sections later split into other parts to form today's continents. The continents arrived at their present position approx. 65million years ago during the Cretaceous.

Proof:

· Shape (morphology) of the continents.foe example West Africa and eastern South America

· Similar geology and lithology (chemical structure of rock)

· Similar paleoenvironments based on fossil evidence

The theory was rejected because:

· Wegener could not explain the process that split Pangea,

· Wegener could not explain the process that allows the continents to drift,

· German at time of world war 1, and

· academic jealously within German universities

He is tenacious and he kept printing new versions of book until 1926. He only got a chair at a university ( of Graz where he had worked since 1923 teaching meteorology and geophysics) in 1930 (shows hostility) but dies in same year in Greenland expedition.

Ideas forgotten..,,but marine research by US navy in the 50’s and 60’s turns up 3 mysteries. These are:

· little sediment on the Ocean floor

· rocks are young i.e. no billion old rocks (unlike the continents)

· there is a ridge in the middle of each of the oceans rising to an average height of 4500m above the seafloor

Data given to Harry H. Hess (1906-1969) ..Geologist in Princeton in New Jersey. He

wrote a book called The History of the Ocean Basins. It had been based on a paper that had been informally circulated in 1959. In it he proposed the Theory of Sea Floor Spreading (1962). He states that at the mid oceanic ridges the sea floor was being pulled apart ... proof basis was that one side of ridge would be identical to the other..ie a mirror image (symmetrical) .In 1969 Glomar Challenge rtook samplse (cores) from the mid Atlantic. These were identical based (in terms of symmetry) on age and paleomagneticism (i.e. magnetic poles switch over time ... find this in igneous rock as it cools…known as magnetic striping and the US Navy knew about it as early as the 1950’s)

Diagrams Magnetic Stripping

The theory of PT combines these 2 theories. It states that the Earth's outer shell, the lithosphere, long thought to be a continuous is actually broken up into a fluid mosaic of many irregular rigid segments, or plates. Comprised primarily of cool, solid rock 4 to 40 miles thick, these enormous blocks of Earths crust vary in size and shape, and have definite borders that cut through continents and oceans alike. Oceanic crust is much thinner and more dense than continental, or terrestrial crust]. The continents are made from sial ... silicon and aluminium density of 2.7 and the oceans from sima...... silicon and magnesium-...density of 2.9.grams per cubic cm

Most of the boundaries between individual plates cannot be seen, because they are hidden beneath the oceans. Yet oceanic plate boundaries can be mapped accurately from outer space by measurements from GEOSAT satellites. Earthquake and volcanic activity is concentrated near these boundaries. Tectonic plates probably developed very early in the Earth's 4.6-billion-year history, and they have been drifting about on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating.

Like many features on the Earth's surface, plates change over time. Those composed partly or entirely of oceanic lithosphere can sink under another plate, usually a lighter, mostly continental plate, and eventually disappear completely. This process is happening now off the coast of Oregon and Washington. The small Juan de Fuca Plate, a remnant of the formerly much larger oceanic Farallon Plate, will someday be entirely consumed as it continues to sink beneath the North American Plate.

There are 7/9 large plates(depends on the author) and a number of smaller plates. The main types of Plate are:

· Oceanic eg Pacific oceanic plate and

· Continental Plate e.g. Turkish-Aegean Plate.

· Most plates are comprised of both continental and oceanic crust e.g. the American plate.

Plates float on the upper mantle known as the asthenosphere (100-200km down)because the molten rock in the mantle is denser than the surface rock.

These plates also move (2 cm -9cm per year) by 2 processes

· subduction and

· the circulation of magna in the core

Diagram 1 Subduction

Diagram 2 Circulation of Magma in the Core

Earthquakes and Volcanoes

Plates do five things as they move and these account for EQ and VOL.

(especially at the plate boundaries or margins)

I Subduction....zone of convergence.....Destructive plate margin ... get EQ e.g. where African plate and Eurasian plate meet..why get RQ in Turkey and volcanoes in Italy(hot Spot)..Example continental-continental convergence

Diagram

2 Pull apart at the mid oceanic ridges... Divergence ... constructive plate margin get VOL get mid Atlantic ridge

Diagram

3 Fold mountains ... convergence can be seen as both constructive and dis. Plate margin can get EQ Everest expedition in 1996 placing sensors on the MT e.g. Mt Everest where the Indian Plate went north and collided with the Asian plate ..still moving as such Mt Ever is growing. Also explains why you get tremors

This can also be seen as an example of continental-continental convergence.

One new key word is geosyncline ... this is a depression in the ocean where the sediment builds up

Diagram

4 Hot spots ... constructive plate boundary..Get volcanic island arcs ... 2 oceanic plates collide ... Oceanic- oceanic convergence get island of Japan

Diagram

5 Move past each other at transform fault lines ... passive/conservative plate boundary ... also known as shearing get EQ once threshold is passed. E.g. San Andreas Fault Line ..at LA or San Francisco

Diagram

Conclusion

The Theory of Continental Drift has had a long and turbulent history since it was first proposed by Alfred Wegener in 1910. Vigorously challenged yet widely ignored, the theory languished for half a century, primarily due to its lack of a plausible mechanism to support the proposed drift. With the discovery of sea-floor spreading in the late 1950's and early 60's, the idea was reinvigorated, this time as the Theory of Plate Tectonics.

Plate tectonics is now almost universally accepted, its mechanisms plausible and to a degree demonstrable. However, many details of the mechanism are yet to be worked out, and many theories involving various details of plate tectonics rest on some questionable assumptions. This essay has attempted to define some of the basic principles of the mechanism, and to examine their effect on the creation of landforms.