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Volcanos! Where do they come from?

 

Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products -- lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.

Driven by buoyancy and gas pressure the molten rock, which is lighter than the surrounding solid rock, forces its way upward and my ultimately break through zones of weaknesses in the Earth's crust. If so, an eruption begins, and the molten rock may pour from the vent as nonexplosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations of fallback fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materials may be carried by the wind only to fall to the ground many miles away. The finest ash particles may be injected miles into the atmosphere and carried many times around the world by stratospheric winds before settling out.


Volcano Mount McLoughlin, Oregon

How are volcanos and earthquakes related?

Some volcanos occur from hot spots, areas of persistant heat source in the mantle. They do not have to occur along a plate boundary in this situation.

However, there are volcanos associated with both subduction and divergent zones. As the divergent plates separate, magma can create a volcanic chain. This can be seen most easily in ocean-ocean diverging plates, commonly called a "mid-oceanic ridge."

In subduction zones, the diving crust reaches higher and higher temperatures which lead to melting. Eventually the magma reaches the surface and the lava cools. Build-up of cooled lava creates a dome or mountain appearance.



Ocean-Continent Subduction Zone

 

Earthquakes, Volcanos and Plate Boundaries


Blue lines: Plate boundaries
Yellow dots: Earthquakes
Red triangles: Active Volcanos

 

 

 

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