A volcano that has recently erupted and may erupt again soon is categorized as an active volcano. Dormant and extinct are also categories of volcanoes.
A volcano is a mountain that opens downward into a pool of magma. Volcanoes are formed by magma working its way to the surface from the Earth's mantle. When the magma reaches the surface, it erupts and forms lava. As the volcano erupts repeatedly, it grows. There are more than 1,500 active volcanoes on Earth; 80 or more are found under the ocean. Most active volcanoes are found in Hawaii, Alaska, California, Oregon and Washington. Cinder cones, composite volcanoes, shield volcanoes and lava volcanoes are four different types of volcanoes.