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6.12: Types of Volcanoes

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  • A volcano is a vent through which molten rock and gas escape from a magma chamber and they can differ in height, shape, and slope steepness. Some volcanoes are tall cones and others are just cracks in the ground. As you might expect, the shape of a volcano is related to the composition of its magma.

    Composite Volcanoes

    PictureComposite volcanoes are some of the most dangerous volcanoes on the planet. They tend to occur along oceanic-to-oceanic or oceanic-to-continental boundaries because of subduction zones. They tend to be made of felsic to intermediate rock and the viscosity of the lava means that eruptions tend to be explosive. The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a composite volcano. Viscosity also causes some eruptions to explode as ash and small rocks. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other and are sometimes called stratovolcanoes or andesite volcanoes. The result is the classic cone shape of composite volcanoes. Examples of composite volcanoes include Mount St. Helens, Mount Rainer, Mount Shasta, Mount Hood, and Mount Pinatubo.

    Sometimes composite volcanoes and other violent volcanoes can erupt so violently that they sometimes collapse in on themselves or actually blow themselves up to produce calderas. One of the most powerful volcanoes in the world—Yellowstone—is a massive caldera that has collapsed several times. Sometimes these calderas can fill up with water to produce beautiful lakes such as Mount Mazama (Crater Lake), in Oregon.

    Crater_lake_oregon

    Shield Volcanoes

    PictureShield volcanoes get their name from their shape. Although shield volcanoes are not steep, they may be very large. In fact, Mauna Loa, Hawaii is the tallest mountain in the world. From sea level, Mount Everest is the tallest, but when you consider from the ocean floor to the top of the island, Mauna Loa wins. Shield volcanoes are common at spreading centers or intraplate hot spots.The lava that creates shield volcanoes is fluid and flows easily and creates the shield shape. Shield volcanoes are built by many layers over time and the layers are usually of very similar composition. The low viscosity also means that shield eruptions are non-explosive. Eruptions tend to be mild in comparison to other volcanoes, but lava flows can destroy property and vegetation. The low viscosity magma can flow not only on the surface as lava, but also underground in lava tubes. The most well known shield volcano is Hawaii.

    Island_of_Hawai'i_-_Landsat_mosaic

    There are two types of lava flows, pahoehoe which is a ropy type of lava that flows easily (low viscosity). The other type is called aa and is a blocky type of lava and has a higher viscosity and does not like to flow well.

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    Cinder Cones

    PictureCinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than a composite volcano. Cinder cones rarely reach 300 meters in height but they have steep sides. Cinder cones grow rapidly, usually from a single eruption cycle. Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another. The rock shoots up in the air and doesn’t fall far from the vent. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit. Cinder cones are often found near larger volcanoes.

    Supervolcanoes

    PictureSupervolcano eruptions are extremely rare in Earth history. It’s a good thing because they are unimaginably large. A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km3 for Mount St. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly, supervolcanoes are the most dangerous type of volcano.Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated, however, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. This creates a huge hole or caldera into which the surface collapses.

    The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago. Yellowstone has produced many smaller (but still enormous) eruptions more recently. Fortunately, current activity at Yellowstone is limited to the region’s famous geysers.Long Valley Caldera, south of Mono Lake in California, is the second largest supervolcano in North America. Long Valley had an extremely hot and explosive rhyolite explosion about 700,000 years ago. An earthquake swarm in 1980 alerted geologists to the possibility of a future eruption, but the quakes have since calmed down.

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    A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some of the mass extinctions in our planet’s history. No one knows when the next super eruption will be.

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