Lecture 8 - Volcanoes Most volcanoes are located near plate...

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Volcanoes Most volcanoes are located near plate boundaries. 2/3 of all volcanoes are found on the ‘Ring of Fire’ surrounding the Pacific Ocean. Subduction zones and mid-ocean ridges allow molten rock to reach the surface.
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Types of Molten Rock Magma It is found deep within the crust and upper mantle. Lava It is found flowing from an erupting volcano. Essentially, lava is magma on the Earth’s surface.
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Magma The most abundant elements in magma are silicon and oxygen; when combined they are referred to as silica. Volcanic rocks are named based on the amount of silica present. Types of volcanic rocks: basalt, andesite, dacite, rhyolite low silica high silica Silicon
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Viscosity Magma also contains small amounts of gases (water vapour and carbon dioxide). Volcanoes have different shapes based on the chemistry and viscosity of their magma. Magma viscosity is determined by silica content and temperature.
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Magma Magma with high silica content: -cooler, more viscous, more gases Magma with low silica content: -hotter, less viscous, fewer gases Volcanoes with high silica magma produce the most explosive eruptions. As magma approaches the surface, the pressure lowers allowing gases to bubble up and escape.
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Magma Rhyolitic and dacitic magmas produce explosive eruptions. Basaltic and andesitic magmas produce eruptions that tend to flow rather than explode.
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Types of Volcanoes Volcanoes are classified into 4 different types. The classification is based on their shape, appearance, and style of eruptions. Types of volcanoes: -shield -composite -volcanic dome -cinder cone
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Shield Volcanoes These are the largest volcanoes on Earth and are shaped as broad arcs (like warrior shields) built from lava. They are associated with basaltic magma. Eruptions are non-explosive and consist of gentle flows.
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Shield Volcanoes Some eruptions can contain tephra . Accumulations of tephra are referred to as pyroclastic deposits . If compacted together, these deposits are called pyroclastic rock . These volcanoes are common in Hawaii, Iceland and around the Indian Ocean.
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Composite Volcanoes These volcanoes are cone-shaped and are built from a combination of lava flows and pyroclastic deposits. They are also called stratovolcanoes ; this term comes from the layers of lava and deposits. The magma is andesitic or dacitic. Eruptions are more dangerous and explosive but less frequent than shield volcanoes.
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Composite Volcanoes These volcanoes are common along the west coast from Alaska to Oregon. Mt. St. Helens is the most famous composite volcano in North America.
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Volcanic Domes These volcanoes contain highly viscous rhyolite magma. They are steep-sided mounts that form around the vents.
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Cinder Cone Volcanoes These are relatively small volcanoes composed of small pieces of tephra.
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