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Unformatted text preview: Professor Ohan Chapter 5 1- A volcano is an opening, or rupture , in a planet's surface or crust , which allows hot magma , volcanic ash and gases to escape from below the surface. 2- Volcanoes are generally found where tectonic plates are diverging or converging . A mid- oceanic ridge , for example the Mid-Atlantic Ridge , has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. 3- By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the East African Rift , the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. 4- Volcanoes can be caused by mantle plumes . These so-called hotspots , for example at Hawaii , can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system , especially on rocky planets and moons. 5- At the mid-oceanic ridges , two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. 6- The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this 7- Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma . 8- This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire . 9- Hotspots are not usually located on the ridges of tectonic plates, but above mantle plumes , where the convection of the Earth 's mantle creates a column of hot material that rises until it reaches the crust, which tends to be thinner than in other areas of the Earth ....
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- Spring '11