Deep beneath our feet, the Earth is extremely hot – hot enough to melt rock. This melted rock is called magma. Because magma is lighter than the solid rock around it, it slowly rises upward. Sometimes it collects in a huge underground pool called a magma chamber.
A volcano forms where magma finds a way to the surface. When the pressure in the magma chamber becomes too great, the magma bursts out through cracks or vents. Once it reaches the surface, it is called lava. Over time, layers of cooled lava and ash can build up to form a mountain.
Most volcanoes are found near the edges of tectonic plates, the giant slabs of rock that make up the Earth's outer layer. More than half of the world's active volcanoes surround the Pacific Ocean, in a horseshoe-shaped zone known as the Ring of Fire.
Scientists sort volcanoes into three groups. Active volcanoes have erupted recently and could erupt again. Dormant volcanoes have not erupted for a very long time but might wake up in the future. Extinct volcanoes are not expected to erupt ever again.
Although eruptions can be deadly, volcanoes also bring benefits. Volcanic ash breaks down into rich soil, which is why many farms are found on the slopes of old volcanoes. In some countries, such as Iceland, the heat from underground is used to warm homes and produce electricity.
