Earth science for schools by Moorland School

Plate tectonics: The main features are:

  • The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle).
  • These plates are in constant motion travelling at a few centimetres per year.
  • The ocean floors are continually moving, spreading from the centre and sinking at the edges.
  • Convection currents beneath the plates move the plates in different directions.
  • The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.
plates

plate_colour_diagram2.gif (21726 bytes)

  • The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building.
  • Plate tectonics is a relatively new theory and it wasn't until the 1960's that Geologists, with the help of ocean surveys, began to understand what goes on beneath our feet.

VIPWhere is the Evidence for Plate Tectonics?

The continents seem to fit together like a giant jigsaw puzzle:

If you look at a map, Africa seems to snuggle nicely into the east coast of South America and the Caribbean sea. In 1912 a German Scientist called Alfred Wegener proposed that these two continents were once joined together then somehow drifted apart.  He proposed that all the continents were once stuck together as one big land mass called Pangea. He believed that Pangea was intact until about 200 million years ago

VIPClick here for some evidence of this theory

CONTINENTAL DRIFT

The idea that continents can drift about is called, not surprisingly, CONTINENTAL DRIFT.

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.

 

pangea.gif (9415 bytes)

SEA FLOOR SPREADING

It is hard to imagine that these great big solid slabs of rock could wander around the globe. Scientists needed a clue as to how the continents drifted. The discovery of the chain of mountains that lie under the oceans was the clue that they were waiting for.

VIPPLATES ARE CREATED: 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. 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.
diverge plates animation.gif (16916 bytes) 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.

 

diverge plates with opening rift valley animation.gif (18569 bytes) 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.

 

diverge3 animation  widening ocean.gif (17635 bytes) As the rift valley expands, water collects forming a sea. The Mid-Atlantic Ridge is now 2,000 metres above the adjacent sea floor, which is at a depth of about 6,000 metres below sea level.

 

diverge4 animated sequence.gif (25011 bytes) 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.

 

Click here for the Scotland story

HOW CAN WE TELL IF THE OCEAN FLOOR IS SPREADING WHEN IT MOVES SLOWER THAN A SNAIL ? VIPIt's important that we look at the evidence for sea floor spreading. So click here

Picture the following in your mind:

  1. You have a nine piece jigsaw (now there's a challenge).
  2. The piece in the middle starts to grow.
  3. It gets bigger and bigger.

What do you think will happen to the puzzle? ANSWER

Now let's think back to our plates being created at the mid-ocean ridges, it seems to be a good idea but if this is the only type of plate movement then the world would get bigger and bigger. In fact the world has remained the same size. So if plates are being created at the mid-ocean ridges then they must be being consumed somewhere else in the world.

VIPPLATES ARE DESTROYED (SUBDUCTION):
island arcs.gif (39562 bytes) 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 under the South American plate to form the Andes Mountain Chain.

Here we can see the oceanic plate moving from left to right. The top layer of the mantle and the crust (all called the lithosphere) sinks beneath the continent. A deep ocean trench is formed. Water gets carried down with the oceanic crust and the rocks begin to heat up as they travel slowly into the earth. Water is then driven off triggering the formation of pools of molten rock which slowly rises. The plate moves downwards at a rate of a few centimetres per year. The molten rock can take tens of thousands of years to then either:

  • Solidify slowly underground as intrusive igneous rock such as granite.

    or

  • Reach the surface and erupt as lava flows. Cooling rapidly to form extrusive igneous rock such as basalt.

The floor of the Easter Pacific is moving towards South America at a rate of 9 centimetres per year. It might not seem much but over the past 10 million years the Pacific crust has been subducted under South America and has sunk nearly 1000 kilometres into the Earth's interior.

Types of Convergent Boundaries

The example above showed what happened when the dense oceanic plate subducts under a lighter continental plate (ie, oceanic - continental). Two other types of subduction can take place:

island arcs animated sequence.gif (39562 bytes) When two oceanic plate meet each other (oceanic-oceanic) this often results in the formation of an island arc system. As the subducting oceanic crust melts as it goes deeper into the Earth, the newly-created magma rises to the surface and forms volcanoes. If the activity continues, the volcano may grow tall enough to breech the surface of the ocean creating an island.

The key to subduction seems to be water which acts as a kind of lubricant as the heavier plate slips underneath the lighter plate.

I must not forget to mention the Himalayas and Mount Everest because this is the third example of plate movement
India moving.jpg (28490 bytes) 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 which were deposited by the Tethys ocean over 60 million years ago.

Himalayas collision.gif (37280 bytes)

The fourth type of plate movement involves plates sliding past one another without the construction or destruction of crust. This boundary is called a conservation zone because plate is neither created nor destroyed An example of such a   boundary is the San Andreas fault in California. The force needed to move billions of tonnes of rock is unimaginable. When plates move some of the energy is released as earthquakes.

PLATE TECTONICS AND EARTH STRUCTURE WEB LINKS

Plate Tectonics   Excellent source of volcanic information.
Plate Tectonics and People    A comprehensive site dealing with the effects that plate tectonics has on humans.     ( Edited version on this site - click here)
Plate Tectonics    Excellent explanation of plate tectonics and  plate boundaries.
Plate Tectonics, TSAW Project   Good source for general information.
Earth's Interior and Plates.
Continental Drift Reconstructions
Webquest: Plate Tectonics Contains lists of questions dealing with plate tectonics.

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