What is plate tectonics? Plate tectonics is a theory that describes that the earth’s crust is composed of several slabs or plates made of rigid crustal materials which are in continuous movement.

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.

Where 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

CONTINENTAL DRIFT The idea that continents can drift about is called CONTINENTAL DRIFT. 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.

Sea-floor Spreading -Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart.  As the plates move apart, the rocks break and form a crack between the plates.  Earthquakes occur along the plate boundary.  Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano .

As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. 

As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor.  This chain is called an oceanic ridge.  The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. 

The ocean floor gradually extends and thus the size of these plates increases.  As these plates get bigger, others become smaller as they melt back into the Earth in the process called subduction.

Sea-floor Spreading