Plate Tectonics

Plate Tectonics and Continental Drift

The earth's crust is broken up into a number of large parts (plates). There are also a number of smaller or minor plates.

The following activity will help you see how the plates fit together to make up the world on which we live.

Further Information - Websites

http://geologyclass.org/Tectonics%20HW.htm

http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml

http://www.learner.org/interactives/dynamicearth/drift.html

"Wegener's explanation was that as the continents moved, the leading edge of the continent would encounter resistanceand 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".

In about 1912 Alfred Wegener, a German scientist first put forward his theory. His theory stated that 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 all moving. The people thought he was nuts!

However in the 1950's and 1960's scientists began to realise that his theory was true and that the continents in fact floated on plates and the plates moved around – very slowly of course.

Evidence has been found that there are very similar plant and animal species shared by the continents. This also led scientist to believe that all the planets were at one stage very close together – they called this large land mass Pangea.

Evidence of Pangea

Scientists have found identical plant and animal fossils of the same age in rocks in Africa and South America. This strongly suggests that the two were once joined and are now found thousands of kilometres apart. For instance the fern 'Glossopteris' can be dated at 270 million years ago and can be found in banded areas of South America, Africa, India and Australia.

The plant type known as Glossopteris , once grew from the polar circle to the equator, Evidence is found across the continents. The same evidence is found for a freshwater reptile Mesosaurus which is now only found in certain areas of the coasts of Brazil and West Africa – many kilometres apart.

The types of rocks found on each continent today show similar strata and ages. Mountain ranges on the coast of West Africa disappear then suddenly reappear again on the coast of South America.

There is also evidence in the continuity of mountain chains. An example of this is the Appalachian Mountains chain which extends from the north eastern United States to the Caledonides of Ireland, Britain, Greenland, and Scandinavia.

Main Features of Tectonic Plates

1. The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle).2. The plates are in constant motion travelling at a few centimetres each year.

3. The ocean floors are continually moving, spreading from the centre and sinking at the edges.

4. Convection currents beneath the plates move the plates in different directions.

5. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.

6. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building.

7. 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.