Volcanoes

There are three different types of volcanoes that we learned about in class. The first type of volcanoes in cinder cone or scoria cone volcanoes. These types are generally low viscous and low silica lavas. Having a low viscosity means that they tend to have fissures and tend to be pretty fast moving. Having a low silica content also makes them less explosive then other types. These volcanoes have steep cones made of pyroclastic material. Here the dominant rock is basalt. Many cinder cones tend to have a bowl shaped crater at the summit but his is not where the lava erupts from. The lava generally erupts from the sides of the volcano or through vents that release gas from deep beneath the earths crust. These types of volcanoes also have tephra or ash ejections. These ash ejections, if big enough, can block out the sun and cause damage to crops in the area. In the US there are a few examples of cinder cones. One in particular is Sunset Crater in Arizona.

Another type of volcano is a shield volcano. These are also low viscosity, low silica lavas. Again this makes them kinds just flow out and makes them less dangerous than the other explosive volcanoes. This type of volcano also has tephra ejections that make it hard for vegetation in grow around the volcano. An example of this type of volcano is Hawaii.  Here the volcanoes tend to have small craters and calderas due to the lack of explosiveness. These volcanoes also tend to be very large because the lava can flow away from them at a very fast rate.  You also tend to see lava tubes here. Lava tubes are areas where lava flows underground to the central source or volcano. Once the volcano has become dormant or the lava no longer flows from that area, there is a cave left behind. These caves are generally perfectly cylindrical due to the fast moving lava weathering away any felsic or intermediate material it comes in contact with. The ideal lava for these types to tubes is pahoehoe. This type of lava is smooth in appearance and is not as viscous as many other types of lavas. This low viscosity allows for fast flow and high temperature, allowing the lava to destroy any unlike rocks or matter in its path. This is also how these types of caves become uniformly cylindrical. The lava flows evenly through the elongated tubes at a constant rate smoothing out the edges making it almost a perfect cylinder. Less viscous lava causes the caves to have uneven ceilings and floors.

Lastly there are composite volcanoes. These then to have a higer silica content then the other making them more explosive and dangerous. The rock formed by this type of volcano is called andesite. The erruption column here tends to have lost of ash and there is a giant pyroclastic flow when this volcano erupts. An example of this type of volcano would be Mt. St. Helens and Mt. Visuvios.

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