The earth's lithosphere is cracked into a number of large rigid slabs called plates. Where these plates meet is the plate boundary.
The plates move, floating on the soft rock, beneath the lithosphere .
Some plates carry just ocean basin but most have both continent and ocean basins.
Plates can drift apart (through sea floor spreading), push into each other or slide past each other.
Britain is on the Eurasian plate.
Most earthquakes, volcanoes and mountain forming takes place at plate boundaries.
The fastest plates e.g. Pacific Plates, move at over 10cm a year.
The North American and Eurasian plates are among the slowest movers. New York and London are moving apart by between 5-10cm each year.
One method of measuring plate speed uses laser beams bounced off a satellite.
Stripes of opposite magnetization occur in the ocean basins because periodically the earth's magnetic North Pole flips south and Earth's magnetic field switches direction. Basalt in the ocean basins records this because it contains iron compounds, which follow the magnetic field. The stripes exist because new basalt forms and pushes old basalt out. The discovery of stripes confirms that sea floor spreading is taking place.
Why do plates move? Hot, soft rock, is less dense than other rock so it rises to the earth's surface causing convection currents. This soft rock is then forced sideways when it hits the plates. As it is pushed sideways, it drags the plates along. As the magma cools near the surface, it melts again and this occurs over and over again. The result is sea floor spreading.
Radioactive materials within the Earth's crust act as a heat source to drive convection currents.
Along the mid Atlantic ridge, there are diverging plates (North American and Eurasian). There are many earthquakes along the ridge. They are moving apart and therefore creating new land. This is called a constructive boundary.
Formation of a Fault:
A fault occurs when a rock is put under enough pressure and cracks.
The fault is the movement of the rock that has been broken away.
There are different types of fault when the direction of movement is different.
When a rock is under enough stress, it stores some of the energy as strain energy.
When this energy is released, it causes an earthquake.
Formation of Folds:
Folds are formed when compressed rock deforms but unlike faults, it does not crack.
Folding normally takes place on the boundary between two converging plates.
Formation of Fold Mountains:
Mountains usually start as sedimentary rock on the bottom of the sea.
This is then squashed upwards by converging continental plates.
It then got folded, faulted, metamorphosed, and intruded by igneous plutons.
Formation of Volcanoes:
Underwater volcanoes erupt a lot at diverging boundaries.
Subduction (The driving down below of the thinner denser ocean plate below the thicker continental plate) at ocean trenches also leads to earthquakes as it is heated and softened as it rubs against the other plate.
The water is superheated and causes melting and a volcano occurs.
Continental drift
Alfred Wegener suggested that around 200 million years ago, all the continents were joined together to form one super-continent. He called the super-continent Pangaea. Then over millions of years this broke up and the continents moved apart. He called their movement continental drift.
The evidence Wegener used to prove his theory of continental drift was: 1) Fossils - Animals and plants only survive in habitats that suit them yet fossils of these have been found on different continents, often in completely different climates e.g. the small reptile Mesosaurus lived 240 million years ago and swam only in shallow waters. Its fossils have been found in Brazil and South Africa, suggesting that these were once joined. 2) Rocks - Rocks of the same age, composition and structure have been found thousands of miles apart 3) - The shapes of continents - the continents look like pieces of a jigsaw puzzle so if you move them around, they will fit together. This is known as continental fit.
Sea-floor spreading is the process whereby molten basalt is continually welling up at oceanic ridges forming new rock. At the same time the older ocean crust is moving aside, carrying the continents with it.
The evidence for sea-floor spreading is that the rock in the ocean floor is much younger than the rock in the continents and the further it is from the oceanic ridges, the older the ocean rock.
