A volcano is a place where molten rock from deep inside the Earth comes out at the surface, and the hill or mountain that the molten rock builds there, layer after layer.
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Example 8.3 (Mount St. Helens, 1980)
In the spring of 1980, a bulge grew on the side of Mount St. Helens, as molten rock pushed up inside the mountain. On the morning of 18 May, a small earthquake shook the mountain, the bulge slid away in a giant landslide, and the volcano exploded sideways and upwards. The top 400 metres of the mountain were gone: the summit became a huge crater.