What’s a Volcano?

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Volcanoes are openings in the earth’s crust where magma reaches the surface, creating effusive or explosive eruptions. They occur at divergent tectonic plate boundaries and form islands. Major types include shield, cinder cone, stratovolcano, supervolcano, underwater, and subglacial. Eruptions have changed history, nearly wiping out humans in prehistoric times.

A volcano is an opening in the earth’s crust where magma from the mantle reaches the surface, sometimes in a slow, dripping fashion called an effusive eruption, and sometimes in a violent event called an explosive eruption. Volcanoes usually occur at divergent tectonic plate boundaries, places where the crust is weak and magma can rise to the surface due to the immense pressure of the underlying mantle. When magma reaches the surface, it is called lava.

Thousands of volcanoes have been recognized on Earth and are found on every continent and scattered across the ocean floor. Among the most famous are Mount Etna in Sicily, 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 are formed. Measured from the ocean floor, Mauna Loa in Hawaii is actually a mountain taller than Mt. Everest in Nepal.

Major volcano types include the following:
shield volcanoes, which are large, shield-shaped volcanoes created by the slow eruption and long flow of viscous lavas; lava domes, formed by viscous lava that doesn’t 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, such as Mt. Fuji in Japan and Vesuvius in Italy; supervolcanoes, massive structures that explode very rarely; underwater volcanoes, located on the ocean floor; and subglacial volcanoes, located under continental glaciers.

Thanks to
of Greenland and Antarctica, subglacial volcanoes are among the rarest, with only five known in modern times.

Volcanoes and their eruptions have occasionally changed the course of history. In prehistoric times, the eruptions of the Yellowstone supervolcanic caldera 650,000 years ago and the eruption of Lake Toba 75,000 years ago are thought to have nearly wiped out the human species by producing particularly cold winters for centuries. These winters would be caused by volcanic aerosols in the upper atmosphere blocking sunlight, kicking off a glaciation feedback process and ushering in a minor ice age.

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