A volcano forms where molten rock, known as magma, finds a path to the surface of the Earth. Most volcanoes sit along the edges of tectonic plates, where the crust is being pulled apart or forced downward, though some rise over hot spots deep within a plate, as in Hawaii. When magma erupts it is called lava, and its behavior depends on how much gas and silica it contains. Runny lava flows gently and builds broad shield volcanoes, while thick, gas-rich magma can explode violently and hurl ash high into the atmosphere.
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