Conditions of Metamorphism

CONDITIONS OF METAMORPHISM

Metamorphism is something you can't see happening and it takes millions of years to accomplish anything when it comes to rock. Granite is extremely hard, so it won't change much. It is what's called an acid igneous rock, which will deform only if conditions are extreme. The quartz can flatten out and become more elongated. You might also see this in gabbro. When such rocks are crushed like this, you call these mylonites. You need a lot of heat to get contact metamorphism of granite but when it happens, you will have a lot of obvious changes in the crystals. If too much heat is applied on top of pressure, the granite will completely recrystallize into gneiss.

Basic or alkaline igneous rocks like gabbro and dolerites will also change under regional dynamic metamorphism that involves heat. You will also get crushing and recrystallization of the original rock's constituents. This will lead to many different types of gneisses and schists. Hornblende is one of these end-product rocks as is a rock called amphibolite.

Sedimentary rocks that are argillaceous have fine grains like clay, feldspar, mica, and quartz. Shales and other types of clays undergo contact metamorphism or heat application to make all sorts of rocks simply due to recrystallization. Heat is more likely to lead to recrystallization than pressure. Hornstone is extremely hard rock made from baking shale to the point of hardness without any recrystallization. Hornstone was used to make arrowheads. Hornstone chert is common in rocks.

If shale is very high in silica, it is called argillaceous. They get crystalized under metamorphic conditions to make a rock called novaculite. It is very hard and white; you can use it for flint napping just like hornstone. Slate itself involves a great deal of pressure and heat to be made out of shale.

Finally, you have arenaceous rocks, which are sedimentary but have the most silica in them. Sandstone is arenaceous because of its extremely high silica content. Squeeze that with pressure or add heat and you get quartzite as we just talked about. If the sandstone is not pure, you get odd schistose rocks instead of something closer to quartzite.