|MadSci Network: Earth Sciences|
Dear Christin, Metamorphism takes place under conditions of great heat and pressure deep in the Earth’s crust. There are two types of metamorphism, called “regional metamorphism” and “contact metamorphism”. It is in regional metamorphism that the role of pressure is vital. Regional metamorphism gets its name from the fact that it usually takes place over a wide area, typically an area in which a collision is taking place between the tectonic plates that comprise the Earth’s crust. As tectonic plates move very slowly across the globe they pull apart in some areas (the mid-ocean ridges) and collide in other areas. Where two continents collide the result is the formation of a new mountain range. Imagine two great crustal plates slowly pushing together, as is happening today in the Himalayan region of south Asia. Tectonic collisions generate enormous pressures, as the two plates press together and compress the rocks in the collision zone, and it is this pressure that is responsible, along with heat, for regional metamorphism. This happened in the late Palaeozoic era when Africa and North America collided and the Appalachian Mountains were formed. Regional metamorphism was responsible for the formation of a long belt of metamorphic rocks along the eastern side of the Appalachians, in what is now the piedmont between the mountains and the coastal plain from Maryland to Georgia. Rocks deep in the crust (up to 10s of kilometres deep) that are caught up in a tectonic collision will be slowly squeezed under tremendous pressure. They will also be hot because heat increases with depth in the Earth. The heat and pressure that cause metamorphism are not enough to cause rocks to melt, but are enough to cause the atoms in the crystals of the minerals that compose the rocks to become mobilized and rearrange themselves to form new minerals. The end result is a rock that has recrystallized and changed from whatever it was to start with to become a metamorphic rock, such as gneiss, schist or slate. The other type of metamorphism, contact metamorphism, takes place at much shallower depth in the crust, typically only a few kilometres at most. Magma chambers, such as beneath volcanoes, can exist for thousands or even millions of years, and the great heat contained in the molten magma will “bake” the surrounding rocks, creating an aureole of metamorphosed rock around the chamber. I hope this answers your question. Best wishes, David Scarboro
Try the links in the MadSci Library for more information on Earth Sciences.