A volcano is a vent in the surface of the Earth through which magma, gases and ash erupt. The name originates from the mythological Roman god of fire "Vulcan." This geographical phenomenon usually occurs at the boundaries of continental plates. The recognizable physical structure is further produced by the erupted material.
Types of Volcanoes
Generally, geologists group volcanoes into four categories: cinder cones, composite volcanoes, shield volcanoes, and lava domes.
Cinder Cone Volcano
Cinder cones are the simplest type of volcano. They are formations resulting from lava ejected from a single vent. The small fragments solidify and fall around the cent to form a circular cone around the vent, usually creating a bowl-shaped crater at the tip. Cinder cones are found in western North America and other volcanic areas of the world. In Parícutin, Mexico, a notable cinder cone started building in 1943 for nine years to a height of 1,200 feet.
Composite volcanoes, or strato-volcanoes, are steep-sided, symmetrical cones that are awesome in their grandeur. These volcanoes are built from alternating layers of lava, ash, cinders, blocks, and bombs are many times rise 8,000 feet above the ground. Lavas flows into the fissures of the volcanic cone, creating strengthening “ribs” when they solidify. Composite volcanoes constantly grow as magma from deep in the Earth’s crust rise to the surface. These volcanoes form some of the largest ones in the world, including Japan’s Mount Fuji, Ecuador’s Mount Cotopaxi, California’s Mount Shasta, Oregon’s Mount Hood, and Washington’s Mount St. Helens and Mount Rainier. As time progresses, the volcano becomes dormant and erosion eventually destroys the cone.
Shield volcanoes are formed from fluid lava flows which pour out from the central summit vent (or group of vents). The slopes of the cone are flat with profiles resembling shields, hence its name. This type of volcanoes builds up slowly when the fluid basaltic lava spread over a great range, often forming some of the largest volcanoes in the world. In northern California and Oregon, many shield volcanoes have 3 or 4 mile wide diameters and reach up to 1,500 to 2,000 feet. Kilauea and Mauna Loa volcanoes in Hawaii are two of the most active volcanoes.
Lava Dome Volcano
Lava domes are built from small masses of highly viscous lava that do not flow very far. Because of the lack of movement, the lava piles over around the vent. When the outer surface cools and cracks, the contents spill and form craggy knobs or short, steep-sided lava called “coulees.” An example of a lava dome is the Novarupta Dome formed by the 1912 eruption of the Katmai Volcano in Alaska.
Different States of Volcanos
Volcanoes can also exist in different states: active, dormant, or extinct.
Active volcanoes are not unanimously defined by the scientific community because they can have life spans varying from months to millions of years and can produce ash, cinder, or gas. As a rule of thumb, a volcano is active if it is currently erupting or showing signs of potential eruptions (earthquake activity or new gas emissions).
Dormant volcanoes are not active presently but have the potential of erupting again. Again, because active volcanoes are vaguely defined, the state of a dormant volcano is often ambiguous as well.
Finally, extinct volcanoes are unlikely to erupt again.