Emerald Robinson: Hi, I'm Emerald Robinson. In this "What Is" video we're going to take a closer look at volcanoes. A volcano is vent in the Earth's crust where molten rock, gas and steam erupt onto the planet's surface. Volcanoes are most common in areas of seismic activity, such as where the Earth's continental plates pull apart or collide. Volcanoes also occur far from plate lines in hotspots where the Earth's crust melts. Hawaii's volcanoes are examples of such hotspots. Volcanoes exist on all continents except Australia, and also form in ocean trenches.
The Molten rock beneath the Earth's surface is called magma. Lava refers to the magma ejected from a volcano. Magma's mineral composition determines how its lava behaves.
Mafic magma contains high concentrations of iron-rich minerals and magnesium. Mafic magma flows freely, producing lava capable of traveling some distance before cooling and solidifying. Felsic lava contains silica, and produces thicker lava. Thick lava can't travel as far.
Because Felsic lava solidifies quickly, it produces the classic cone-like volcanoes, which are called composite volcanoes. Felsic magma can also solidify beneath the Earth's surface, creating magma chambers where hot gases and magma build up. This causes composite volcanoes to explode violently.
Mafic lava produces a broad, low profile volcano called a shield volcano. Instead of explosive eruptions, shield volcano eruptions cause lava to flow in all directions. Mauna Loa, a shield volcano on the Big Island of Hawaii, is the world's largest volcano, rising 13,680 ft above sea level.
Seismologists classify volcanoes as active, dormant and extinct. Active volcanoes are either erupting or likely to erupt in the future. Dormant volcanoes have not erupted in recent history. Extinct volcanoes have exhibited no activity for long periods and are unlikely to erupt in the future. And that's the hot news on volcanoes.