GEOL Volcanism/Volcanoes GEOL 101
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This 3 page Class Notes was uploaded by Victoria Williams on Wednesday February 24, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 30 views. For similar materials see Introductory Geology in Geology at George Mason University.
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Date Created: 02/24/16
Volcanism Magma has three components; gas fraction (volatiles), liquid fraction (melt), and solid fraction (silicate minerals crystallizing from melt). It’s like a rock slushie. Magma Viscosity – Viscosity is the resistance of a liquid to flow. Low viscosity means the liquid flows freely, while high viscosity means the liquid flows difficultly o Factor: Composition. More silica means higher viscosity, less silica means lower viscosity. o Factor: Temperature. Cooler has a higher resistance to flow. Hotter has less resistance to flow. o Factor: Dissolved Gases. Less gas, the higher the viscosity. More gas, the lower viscosity. Volcanoes that erupt with low viscosity magma means having a more gentle eruption. With a high viscosity, it’s more explosive. Types of volcanos: o Shield volcanos – Very large, low structures that erupt with a low viscosity that is basaltic. Imagine Captain America’s shield and laying it on the ground. It’s a broad shape with a slight rise in the middle, that’s what these volcanos look like. Hawaii has many of these, like Kilauea. The melt flows from the summit caldera, a wider opening than most volcanos. Sometimes lava comes out of cracks and you can have flank eruptions. Their eruptions are not very explosive, kind of like when you pour soda and you get the little bit of fizz over. o Composite Cone/Strata Volcanoes – Moderately sized coneshaped structures that erupt higher viscosity and andesitic material (more silica than shield). They are composed of lava flows and pyroclastic material, each melt creates a new layer. These are much more explosive, which can almost destroy the entire mountain. They can be extremely dangerous, like Mount Saint Helen’s in the 80’s. o Cinder Cone – Are small coneshaped structure that erupt cinders of various compositions Source of Composite Volcano Magma – The magma in the mantle rises through the lithosphere, this magma is basaltic. But then it hits the continental crust, which is granitic. The basaltic magma heats up the granitic crust and melts it, then mixes together to make andesite magma for the composite volcano. Composite volcanoes can ONLY come from continental crust because it needs the granite composed crust to help create it. Northern California, Oregon, and Washington have the Cascades, a line of volcanoes. Right off the coast of these states there is a tectonic boundary between the continental plate and the Juan De Futa plate, which is the remnant of Farallon plate. As this plate subducts under North America, it makes the oceanic plate melt and create composite volcanos on the continental crust. Cinder Cones – Varying composition, mostly scoria: not usually explosive. These are rather small and they spit out a lot of pyroclastic material like pumice and scoria. These cones are usually just made of the cinders of the explosion, they aren’t usually layered like a strata volcano. They’re often connected to larger volcanoes. An example is Paricutin, Mexico. Eruptive Material: Lava Flows o Basaltic is the most common type of lava. Basaltic lava is of low viscosity and flows freely. Explosive eruptions of basalt are uncommon. These flows come in two forms, aa (ahah, Hawaiian name, looks chunky like the top of an apple crisp with liquid below), and pahoehoe (another Hawaiian name, it’s more of a gooey looking and it oozes more in blobs). Lava Tubes – Cavelike tunnels that once served as conduits carrying lava from an active vent to the flow’s leading edge. An example is Valentine Cave in California. When there is a tube and the surface breaks, it’s called a skylight. Volcanic Gasses – 70% water vapor, 15% carbon dioxide, 5% nitrogen, 5% sulfur dioxide, etc. The water comes from the subducting slabs of oceanic crust and comes out again in the gas. Pyroclastic Material – When lava gets projected into air, cooling (but hot) particles fall back down in various forms. Together, these forms are called pyroclastic materials. There is a chart in the book. From smallest to largest: Volcanic Ash, Lapilli (cinders, little nuggets), volcanic bombs (kind of football shaped), volcanic blocks. Pyroclastic Flows – Mixtures of hot gases with incandescent ash the flow rapidly down the slopes of volcanoes. Lahars – A debris flow composed of pyroclastic material and water. Calderas – Large craters formed by the collapse of the roof of a magma chamber. This can happen over a shield volcano but it can also form with any magma chamber because there are so many cracks in the crust. After a big eruption and all the magma is gone, there is nothing to keep the chamber up so it collapses. They sometimes fill with water, but new magma is still rising but it’ll take a while to reform. Yellowstone Caldera – A large amount of Yellowstone national park is a caldera from the huge volcano there, and there is a hot spot there right now. Fissure Eruptions – Generally associated with shield volcanoes. Comes from a crack/fissure in the earth that the lava seeps out of. Lava Domes – produced when highly viscous magma slowly rises over a period of months or years. This usually happens after a volcano erupts and then caps over again. The lava dome is an indicator that magma is rising again. Plutons – Magma chambers that have cooled and solidified after the source of magma has stopped. A bunch of the solidified magma chambers in one place can moosh together as plutons, and the sum of all of them in the area are called Batholith, sometimes form mountains. Sills are formed when magma flowed through horizontal cracks and solidified. Dikes are like sills, but vertical.
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