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Physics - Volcanoes

In certain places around the world, molten rock from deep in the ground leaks out. These places are volcanoes. Some volcanoes are quiet, some explosive, some in between.

Quiet volcanoes gently ooze hot, runny rock. When it flows out on the surface this rock is called lava. Very runny lava spreads out across the surrounding land. Less runny lava builds a gently sloping cone around the hole or crater that it comes from. Sticky lava builds a tall, steep-sided cone.

Violent Volcanoes

Explosive volcanoes erupt with sudden force. Hot gas trapped underground presses on the rocks above until it hurls them from the mouth of the volcano. Hot ash, cinders and molten rock shoot up into the sky and fall back like mortar shells. Explosive volcanoes build steep cones, mainly made of thick layers of ash.

A third kind of volcano sometimes quietly leaks lava, and sometimes shoots out ash and gas. Such ‘in between’ volcanoes produce cones from sandwiched layers of ash and lava.

Inactive volcanoes

In time volcanoes stop erupting. A volcano may stay quiet for centuries. But it may be merely dormant (sleeping). Hot gas could be building up below a lava plug that blocks its outlet. At last the gas pressure may suddenly blast off the volcano’s lid with an immense explosion.

About 1470 BC the Greek island of Santorini blew up with a force as great as that of hundreds of hydrogen bombs. Some volcanoes stop erupting for good. Edinburgh Castle stands on the remains of one of these extinct, or dead, volcanoes.

Where Volcanoes Grow

Like earthquakes, active volcanoes occur where the Earth’s crustal plates are separating or colliding. At such weak places in the Earth’s crust magma (molten underground rock) can well up and escape.

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