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In 1912, a German scientist called Alfred Wegener proposed that South America and Africa were once joined together and had subsequently moved apart.
He believed that all the continents were once joined together as one big land mass called Pangaea and this was intact until about 200 million years ago.
The idea that continents are slowly shifting their positions is called continental drift.
The Earth's surface is made up of a number of large plates (like pieces of a jigsaw puzzle) that are in constant, slow motion.
The ocean floors are continually moving, spreading from the centre and sinking at the edges.
At the edges of these plates (plate boundaries) earthquakes and volcanoes occur.
Convection currents in the mantle move the plates. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.
Wegener knew the continents had drifted but he couldn't explain how they drifted. It wasn't until the 1960's that geologists used ocean surveys to explain continental drift with the theory of Plate Tectonics.
At a constructive plate boundary, two plates move apart. As the two plates move apart, magma rises up to fill the gap. This causes volcanoes. However, since the magma can escape easily at the surface the volcano does not erupt with much force.
Earthquakes are also found at constructive boundaries.
An example of a constructive boundary is the Mid-Atlantic Ridge.
The Nazca and Pacific plates are moving apart at a rate of 18cm per year while the Eurasian and North Americanplates are moving apart at a rate of 3cm per year.
To the nearest metre, how far will the Nazca and Pacific plates have moved over the next 200 years?
A destructive plate boundary is found where a continental plate meets an oceanic plate.
The oceanic plate descends under the continental plate because it is denser. As the plate descends it starts to melt due to the friction caused by the movement between the plates. This melted plate is now hot, liquid rock (magma). The magma rises through the gaps in the continental plate. If it reaches the surface, the liquid rock forms a volcano.
Collision boundaries occur when two plates of similar densities move together (i.e. a continental plate and a continental plate). This causes the material between them to buckle and rise up, forming fold mountains.
The Himalayas are an example of a chain of fold mountains. They have been formed by the African plate colliding into the Eurasian plate.