How do volcanoes form?
Volcanoes form where molten rock, called magma, rises from deep inside the Earth and erupts at the surface. Most occur where tectonic plates pull apart or push together.
Earth's outer shell is broken into large tectonic plates. Where plates move apart, such as along the Mid-Atlantic Ridge, magma rises to fill the gap, building new crust and volcanoes like those in Iceland. Where one plate sinks beneath another, water released from the sinking plate helps melt rock above it, feeding chains of explosive volcanoes such as those around the Pacific Ring of Fire.
Some volcanoes form far from plate edges, over hot spots, plumes of hot material rising from deep in the mantle. The Hawaiian Islands formed this way as the Pacific Plate moved slowly over a hot spot.
Different eruptions
Runny basalt lava flows gently, building wide shield volcanoes like Mauna Loa. Thick, gassy magma can explode violently, producing ash clouds and fast-moving pyroclastic flows, as at Mount Vesuvius in AD 79 and Mount St. Helens in 1980.
Did You Know?
There are about 1,350 potentially active volcanoes on land worldwide, and many more on the ocean floor, according to the US Geological Survey.