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Volcanoes are formed by the products of their own eruptions. A volcano is usually a hill or mountain built around a vent that connects with pools of molten rock way below the surface of the Earth. The word volcano also refers to the hole through which the molten rock is released.

Pushed by gas pressure, molten rock forces its way upward to break though a weak spot in the Earth's crust. When an eruption occurs, the molten rock pours out as lava flows, or blasts into the air as clouds of lava fragments. Larger pieces fall back around the vent and fragments may slide down the slope as ash flows. Some of the finer materials may be carried far off by wind and fall to the ground miles away. Even finer ash particles can be thrown high into the atmosphere and carried around the world by high altitude winds before landing somewhere.

Molten rock is called magma while below the surface of the earth. After it has erupted and exited from a volcano it is called lava. Originating many miles beneath the ground, the rising magma contains crystals, solid pieces of other rocks, and dissolved gases. It is mostly a liquid made up of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Magma also contains trace amounts of other chemicals. While cooling, the liquid magma may release crystals of these minerals while it solidifies to form igneous rock.

Lava is red-hot when it pours or shoots out of a vent. The color changes to a dark gray, black, or even red as it cools and hardens. Hot lava, rich in gas, containing high amounts of iron and magnesium is fluid and flows like hot tar. Cooler lava, low in gas but rich in silicon, sodium, and potassium flows lazily, like thick dough.

Magma contains dissolved gases, and as they rise to the surface, the restraining pressures decrease and the dissolved gases escape, sometimes explosively. If the lava is a thin liquid the gases can exit easily. If the lava is thick and pasty, the gases will build up high pressure and explode with violence. Gases in lava act like the gas in a bottle of carbonated soda. If you shake a bottle of cola and hold your thumb over the top of it, your thumb feels the pressure as the gas and drink separate. When you take your thumb off, there is an explosion of gas and liquid. The gases in lava behave the same way. Their sudden expansion creates explosions of rock, lava, dust, and ashes.

The violent separation of gas from lava may produce a bubbly rock called pumice. Some of this pumice is light and can float on water. In many eruptions, the froth is shattered into small fragments that are thrown high into the air as cinders, ash, and dust.