 Welcome to open geology. Open geology is a project to share geology with everybody. It was made by solid community college instructors. But note that the views expressed here don't necessarily reflect the views of the college. This is a Creative Commons and open source project. Today we're going to be talking about where metamorphism happens. This is chapter 6 of the opengeology.org textbook. So where does metamorphism happen? Where? Here are some notable places on our planet where you can find metamorphism occurring. Next to hot stuff, including down deep in the planet and near batholists in hot water where they're circulating hot water as in near geysers or other hydrothermal features. Under great amounts of pressure also deep within the earth or in areas where continents or plates are colliding. Fault zones where pieces of rock are grinding past each other and impact craters where you have a sudden high pressure environment. Let's examine each one a little bit more carefully. Contact metamorphism can be caused by heat intruding magma. Here is a picture of the little cottonwood stock and the host rock, the big cottonwood formation. This reddish rock has been metamorphosed or altered by the heat of the large granite pluton that existed below it. It forms a region around the pluton of altered metamorphosed rock from this contact metamorphism. Other ways you can create contact metamorphism is if you have something like a coal fire or other natural fire underneath your rock. Also if you have any kind of heat source nearby the rock you can create an altered area near that heat source, be it significant enough to metamorphose your rock. Most commonly found adjacent to igneous bodies though. Minerals, depending on the mineral, do much better in certain conditions than others. If you apply heat to those minerals what will happen is the minerals might want to form new more stable minerals that can handle those kind of heat conditions. This heat can come from the geothermal gradient which is an increase in temperature with depth which is common all over our planet. As you go down deep enough under the ground into the crest you can get pretty hot and that heat can in turn cause metamorphism from heat. Contact metamorphism which we spoke about briefly and then fiery coal seams was an interesting kind of additional one. We can create rocks like clinker which look like natural brick. What happens if you add too much heat to the rock? Well if you push a rock too far and heat it up too much you'll get liquid hot magma which is melted rock. When you add water you could change the solidus, the melting temperature of the rock and also cause it to melt. Or you could potentially be in a hot fluid situation which could metamorphose the rock in a different way. So let's talk about that. Metasomatism is when you have ion rich hot fluids moving through cracks and pore spaces in your rock. And it can take those ions, the little elements floating in the water, charged elements floating in the water and exchange those with elements that are in the rock. And this is how you get hydrothermal metamorphism. This type of metamorphism with these hot fluids can create very valuable mineral deposits. A lot of what we mine for copper and silver and gold are related to hydrothermal metamorphism. Hydrothermal, what does that mean? Hydro being the root water, thermal meaning heat so it's essentially just hot water. Another common way that rocks are metamorphosed is under pressure. And you can have your typical confining pressure where you have equal directions of pressure or pretty much equal forces of pressure in all directions. It's like diving deep into a pool you feel an equal amount of pressure in all directions. And this can happen from deep burial. Suppose you have an area where you keep getting more and more sediment piling up and the rockets vary deeper and deeper until you get into the realm that's past diagenesis and past lithification to what it would be metamorphism by pressure. When one direction of force is dominant you get differential stress and this will create some very interesting looking metamorphic rocks. This is where you have one direction stress that is stronger than the other directions and it will cause minerals to line up perpendicular to that direction of stress. So they can be as flat as possible and they arrange themselves in ways that are most advantageous for the stress that's being applied to them. What do you think the direction of stress was to create this compressed package of boxes in this picture? Note that these boxes are like the minerals that will align themselves to deal with the stress being applied to them. Which would most likely have come in from the sides to create all of these up and down boxes here, squish boxes. Most metamorphic rock is made in areas of regional metamorphism. These are large regions where you have mountain building happening and a lot of times relate it to plate tectonic boundaries. Here is a continent-continent convergent zone where you are two pieces of continental crust are colliding creating these large mountains. The root and heart of these large mountains you're creating significant amounts of single or one dominating direction of pressure differential stress. And that in turn is creating large amounts of interesting metamorphic rocks. Where else can you get metamorphism? Vault zones and these can be at depth and involve significantly high temperatures. And it can shear and spread out and pulverize your rock to such an extent that you get rocks with really tiny particles that are aligned. They have little features from shearing and being pulled apart. And a lot of times you'll see little fissures throughout the rock and you get these beautiful metamorphic rocks. Mylonites are common in these fault environments. A cool little side note of where metamorphism could potentially happen is impact craters. Of course this is not an extremely common area of metamorphism on our planet but it's definitely interesting nonetheless. Here's a large crater in Arizona created by impact. And as a meteorite strikes the planet it creates very intense amounts of pressure in an instant. And you can create things like these shatter cones and shocked quartz is another common thing that geologists look for where it's quartz that has been altered by this kind of impact. So that's it for different areas of where metamorphism can occur. Keep in mind that even though we talked about heat and pressure separately, usually in any of these metamorphosing environments you have a combination of the different metamorphosing agents. Thank you. 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