Volcanoes are formed when the molten rocks and gases under the earth break through the crust and erupt. A volcano can be a crack, a dome or a mountain with a crater at the top.
The molten rock under the Earth's crust is called magma. At points where the crust is weak or thin, magma pushes the ground upward to form a volcano. At the point at which the force of the magma is stronger than the crust, the magma explodes through the crust to form a volcanic eruption. The magma that flows out of a volcano is called lava. Lava usually starts flowing after the gases and ash have stopped exploding outward.
Volcanoes also throw out rocks in other forms such as ash, cinder and pumice. Volcanic mountains are formed when the lava around a vent or crack hardens and solidifies. This can takes weeks or many years. Some volcanic sites look like lakes because the huge eruptions cause the ground to sag inward.
The word "volcano" originates from Vulcan, who is the Roman god of fire said to have a forge on Vulcano, a volcanic mountain in Italy. There are 1,500 known volcanoes around the world and Hawaii's Mauna Loa is the largest volcano on earth. Indonesia has the most number of volcanoes. Most volcanoes are between 10,000 to100,000 years old.