A volcano develops where parts of the Earth's outer crust are either coming together or pulling apart. Many of these places are deep under the sea, but some are on land. The most distinctive volcanoes are cone-shaped mountains made of ash and lava, like the volcano in the picture. Volcanos can be very dangerous and erupt without much warning. The area around them can be prone to earthquakes, and water that flows from the ground near them can be very hot - called hot springs. When a volcano erupts, molten rock and ash can be thrown huge distances, and can even bury an entire town.