A volcano is an opening in the Earth's crust where magma from the mantle reaches the surface, sometimes in a slow, dribbling fashion, called an effusive eruption, and sometimes in a violent event called an explosive eruption. Volcanoes usually occur in divergent boundaries between tectonic plates, places where the crust is weak and magma can rise to the surface due to the immense pressure of the mantle below. When magma reaches the surface, it is called lava.
Thousands of volcanoes have been recognized on Earth, and they are found on every continent and scattered across the ocean floor. Among the most famous are Mount Etna in Sicily, Mount Vesuvius in Italy, Mount Merapi in Indonesia, Sakurajima in Japan, Mauna Loa in Hawaii, Mount Rainier in Washington, USA, and Mount Erebus in Antarctica. Volcanic activity is how the world's islands get formed. Measured from the ocean floor, Mauna Loa in Hawaii is actually a taller mountain that Mt. Everest in Nepal.
The main types of volcano include the following:
- shield volcanoes, which are broad and shield-shaped, created by the slow eruption and long flow of viscous lavas;
- lava domes, formed by viscous lava that does not flow very far;
- cinder cones, which are small (98 to 1,312 feet (30 to 400 meters)), cone-shaped hills that occur on the flanks of larger volcanoes;
- stratovolcanoes, tall conical ones, like Mt. Fuji in Japan and Vesuvius in Italy;
- supervolcanoes, massive structures that explode very rarely;
- submarine volcanoes, located on the ocean floor; and
- subglacial volcanoes, located beneath continental glaciers.
Volcanoes and their eruptions have occasionally changed the course of history. In prehistory, it is thought that the eruptions of the supervolcanic Yellowstone Caldera 650,000 years ago and Lake Toba eruption 75,000 years ago almost wiped out the human species by producing especially cold winters for entire centuries. These winters would be caused by volcanic aerosols in the upper atmosphere blocking out sunlight, kickstarting a feedback process of glaciation and ushering in a minor Ice Age.