Powerful currents in the mantle cause the plates to move around constantly The plates usually move at rate of 1-10 cm a year, but this tiny movement has incredible effects where the plates meet. The towering Himalayan mountains were formed by two plates pushing into each other and the fiery volcanoes of Iceland are the result of two plates moving apart. Earthquakes, however, tend to occur at the third kind of boundary where plates slip alongside each other. This is because two plates never slide past each other smoothly. The rocks catch on each other so that the plates are still pushing. Pressure gradually builds up until the rocks can no longer stand the pressure and they suddenly give way. This movement travels to the surface in waves that shake the ground Most earthquakes are so tiny that we don't even feel them. Small earthquakes are useful because they release the dangerous pressure between the plates Every few years, however, enough pressure builds up to produce a large earthquake, often with catastrophic results for human life and buildings in the affected area. Afterwards the plates start moving once more until they get stuck again, and so the whole process starts over. Such earthquakes are nature's reminder that we don't stand on solid ground
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