How a Volcano Builds a Mountain
1In some places deep beneath Earth's surface, it is hot enough to melt rock. This melted, or molten, rock is called magma. Magma is lighter than the solid rock around it, so it slowly rises toward the surface, much like a bubble rising in a glass of water. Magma also holds gases. As the magma rises and gathers in underground pockets, the gases push outward, and pressure builds.
2A volcano is a place where magma can escape through an opening in Earth's surface. The opening is called a vent. When the pressure becomes too great, the volcano erupts, which means the magma bursts or flows out through the vent. Once magma reaches the surface, it has a new name: lava. Some eruptions are quiet, with lava oozing out slowly like thick syrup. Others are explosive and blast clouds of ash, tiny bits of broken rock and glass, high into the sky.
3After lava pours out, it cools and hardens into solid rock. Ash falls back to the ground and settles around the vent. Eruption after eruption, these layers of rock and ash accumulate, or pile up, one on top of another. Gradually, over many years, the layers can build a mountain. Many volcanoes are shaped like a cone. At the top of many volcanoes is a crater, a bowl-shaped hollow around the vent.
4Some volcanoes even rise from the ocean floor. Over a very long time, the layers can grow tall enough to break through the surface of the water and form islands. The Hawaiian Islands formed this way.
5Scientists sort volcanoes into three groups. An active volcano is erupting now or has erupted in recent times. A dormant volcano is quiet for now, but it could erupt again someday, like a bear sleeping through the winter. An extinct volcano is not expected to erupt ever again, because it no longer has a supply of magma.
6Scientists who study volcanoes are called volcanologists. They measure tiny shakes in the ground and changes in the gases that drift from a vent. These clues can warn people that magma is on the move and that an eruption may be coming.