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Tectonic Plates

a guide to tectonic plates.

Erin Stimson

on 27 June 2013

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Transcript of Tectonic Plates

by Catrin and Erin
What is a tectonic plate?
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 coming together and then separating.

Like many features on the Earth's surface, plates change over time. Those composed partly or entirely of oceanic plates 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.
Tectonic Plates
The crust of the earth is made up of 7 main tectonic plates and many other smaller ones.
A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic tectonic plate. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic tectonic plate to about 200 km or more for ancient continental tectonic plates (for example, the interior parts of North and South America).
How do these massive slabs of solid rock float despite their tremendous weight? The answer lies in the layers of the rocks. Continental crust is composed of granitic rocks which are made up of relatively lightweight minerals such as quartz and feldspar (a type of mineral that has the potential to create crystals). By contrast, oceanic crust is composed of basaltic rocks, which are much denser and heavier. The variations in plate thickness are nature's way of partly compensating for the imbalance in the weight and density of the two types of crust. Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep "roots" to support their elevations.
Floating Plates
All tectonic plates average at about 80.4672
They move about 10cm each year.

When they collide they can form mountain ranges, deep ocean trenches (eg. Mariana Trench located next to the Phillipines), volcanoes and generating tsunamis with earthquakes.
In the diagram below you can see that the continental crust is beginning to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen.

This is an example of a divergent plate boundary (where the plates move away from each other). The Atlantic Ocean was created by this process. The mid-Atlantic Ridge is an area where new sea floor is being created.

As the rift valley expands two continental plates have been constructed from the original one. The molten rock continues to push the crust apart creating new crust as it does.

As the rift valley expands, water collects forming a sea. The Mid-Atlantic Ridge is now 2,000 metres above the sea floor, which is at a depth of about 6,000 metres below sea level.

The sea floor continues to spread and the plates get bigger and bigger. This process can be seen all over the world and produces about 17 square kilometres of new plate every year.

The Diverging
Plate Boundaries
This is a convergent plate boundary, the plates move towards each other. The amount of crust on the surface of the earth remains relatively constant. Therefore, when plates diverge (separate) and form new crust in one area, the plates must converge (come together) in another area and be destroyed. An example of this is the Nazca plate being subducted (edge of a plate going under another plate) under the South American plate to form the Andes Mountain Chain. Millions of years ago India and an ancient ocean called the Tethys Ocean were sat on a tectonic plate. This plate was moving northwards towards Asia at a rate of 10 centimetres per year. The Tethys oceanic crust was being subducted under the Asian Continent. The ocean got progressively smaller until about 55 milion years ago when India 'hit' Asia. There was no more ocean left to lubricate the subduction and so the plates welled up to form the High Plateau of Tibet and the Himalayan Mountains. The continental crust under Tibet is over 70 kilometres thick. North of Katmandu, the capital of Nepal, is a deep gorge in the Himalayas. The rock here is made of schist and granite with contorted and folded layers of marine sediments.
As you can see here, the plates have collided and one is lighter than the other causing the heavier plate to sink into the magma.
Eventually, part of the tecontic plate that sunk, brakes off from the main part. It then melted and the water drained from the ocean through the trench. As the plate got lighter, it pushed up the other plate, and created the Himalayas.
Convergent Boundaries
Names Of Plates
Major Plates
Minor Plates
Eurasian Plate
African Plate
Pacific Plate
Australian-Indian Plate
North American Plate
South American Plate
Antarctican Plate
Arabian Plate
Cocos Plate
Phillipine Plate
Caribbean Plate
Nazca Plate
Juan de Fuca Plate
Thank You
We are Here
Juan de Fuca
One of the smallest tectonic plates in the world is the Juan de fuca plate which is located of the coast of the nort western coast of north america. But it hasn't always been that way before. it used to be part of the vast Farallon plate. But it was subducted under the north american plate. The plate was broken off by the ridge that was forming. Tthe plate in the near future will disappear. and this not the only plate going through the process.

When Wegener first put forward the idea in 1912 people thought he was nuts. His big problem was that he knew the continents had drifted but he couldn't explain how they drifted. The old (AND VERY WRONG!!) theory before this time was the "Contraction theory" which suggested that the planet was once a molten ball and in the process of cooling the surface cracked and folded up on itself. The big problem with this idea was that all mountain ranges should be approximately the same age, and this was known not to be true. Wegener's explanation was that as the continents moved, the leading edge of the continent would encounter resistance and thus compress and fold upwards forming mountains near the leading edges of the drifting continents. Wegener also suggested that India drifted northward into the Asia forming the Himalayas and of course Mount Everest.
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