Composite or Stratovolcanoes
This video of Mt. St. Helens' eruption shows how the side of the mountain bulged from the pressure of the rising magma and gases before the actual eruption. What followed was an mega-landslide where the entire north side of the mountain collapsed.
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3. Composite or stratovolcanoes (Notes)
a. Occur at subduction zones
b. Made of alternating layers of cinders & lava i. Cinders make steep (8 – 10,000 feet) ii. Lava prevents wearing away c. Explosive due to trapped gases d. Ex. Mt. Shasta, CA Mt. St. Helens |
Composite or strato-volcanoes
Some of the Earth's grandest mountains are composite volcanoes--sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.
Many of these volcanoes are subduction volcanoes and are made of thick (viscous like peanut butter) magma with lots of explosive gases dissolved in the magma. Because subduction destroys oceanic plates, the dead organisms that lie on the ocean floor melt, too. Shells and bones, when they melt, combine with the ocean water that also gets dragged down into the asthenosphere, and produce the gas, carbon dioxide. Because gas is less dense than liquid or solids, the gases push toward the surface along with the very viscous magma through cracks in the crust. This makes subduction volcanoes particularly explosive.
The actual video on the left shows how Mount St. Helens bulged and then collapsed, causing a landslide, which opened up a vent that exploded laterally (from the side).
Many of these volcanoes are subduction volcanoes and are made of thick (viscous like peanut butter) magma with lots of explosive gases dissolved in the magma. Because subduction destroys oceanic plates, the dead organisms that lie on the ocean floor melt, too. Shells and bones, when they melt, combine with the ocean water that also gets dragged down into the asthenosphere, and produce the gas, carbon dioxide. Because gas is less dense than liquid or solids, the gases push toward the surface along with the very viscous magma through cracks in the crust. This makes subduction volcanoes particularly explosive.
The actual video on the left shows how Mount St. Helens bulged and then collapsed, causing a landslide, which opened up a vent that exploded laterally (from the side).
Mt. St. Helens is a subduction volcano
Mt. St. Helens is formed where the oceanic San Juan de Fuca plate
subducts beneath the continental North American plate. As the oceanic
plate subducts and melts into magma, magma and dissolved gases rise up to form volcanic
mountains.
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The video on the lower left shows a picture of a composite volcano taken over Russian from the International Space Station.
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