Here, rock pushed far down into the Earth by the collision turns thick and sticky as it melts its way up to the surface, creating violent volcanoes like Mt Pinatubo in the Philippines. There are volcanoes beneath the sea, too, where plates pull apart, continually releasing bubbles of magma. Some in the Pacific occur far away from the edges of plates over hot spots deep inside the Earth. Islands like the Hawaiian chain are the tips of volcanoes like this. Earthquakes Earthquakes are the ground shuddering as the vast plates of the Earth's surface grind together.
Plates slide past each other all the time, but sometimes they jam together.
For years, the pressure builds up, then suddenly they lurch on again, sending shock waves in all directions. When these waves reach the surface, they create earthquakes. Body waves include primary waves which pulse through the ground like train carriages banging into each other, and secondary waves, which are like a snaking skipping rope. Places near plate edges, such as south-east Europe and the Pacific coast, suffer major quakes repeatedly. The San Andreas fault in California marks the boundary between two giant plates.