History of Rocks
As explained in Visualizing Geology, rocks are the words that tell the story of Earth’s long history and minerals are the letters that form the words. While this report will not be discussing minerals, it will introduce the history of rocks and how they are affected by plate tectonics. The focus of this report is on two of the three types of rocks, igneous and metamorphic rocks. Plate tectonics plays a key role in the life of these types of rocks. So let’s get started by first explaining igneous rocks. Next metamorphic rocks will be explained. Lastly, plate tectonics and the affect this theory has on both types of rock.
Igneous rocks are rocks that form from magma that has been cooled and solidified. Igneous rocks have a structure that is made up of crystals. Igneous rocks are placed into to categories. As stated before, rocks are created by minerals. The process of a liquid forming into a solid is known as crystallization. The first category is extrusive or volcanic. These rocks are cooled on the top of the Earth’s surface. Extrusive rocks have microscopic crystals because it cools relatively quickly and the crystals do not have a chance to become as large as they could be. Extrusive rocks are formed by lava cooling. The second category is intrusive or plutonic. These rocks are cooled below the Earth’s surface. Intrusive rock is formed by the cooling of magma. Intrusive rocks have crystals that can be seen with the naked eye because the cooling process is relatively slow and the crystals have a chance to for. Intrusive rocks are said to be coarse grained because one can see the crystals. One will know the crystals because they are specs of colors such as gray, white, black, and pink.
Metamorphic rocks are first igneous or sedimentary rocks. When heat and pressure are applied to igneous or sedimentary rocks within the Earth, it creates metamorphic rocks. Metamorphism of rocks can happen as a result of rocks being buried by new rocks that push the rock into the Earth. It can also happen when the temperatures and pressure are extreme. The rock structure is changed by the rock being melted and the chemical changes it goes through because of the heat. How a rock turns out depends on how much pressure, how much heat, and how long the rock is exposed to these processes. Lastly, metamorphic rocks are created through contact metamorphism. Contact metamorphism is when magma comes through cracks of a formation of rock. Yes, when the rock is cooled off it becomes an igneous rock, but the rocks that the magma comes in contact with can be altered by the heat of the magma and the surrounding rock pressure.
Plate tectonics is a theory that explains what causes the motions of the Earth. The rocky outer layer is the lithosphere, which is on top of a mantle that is in constant motion and the material is extremely hot and weaker than that of the lithosphere. Below the lithosphere is the asthenosphere, which is weak do to it being located where the temperature is a degree that melting begins. The lithosphere is broken into plates. There are six large plates and several smaller ones. The plates interact with one another along their edges in three ways. The plates can diverge, converge, or slides past each other. When two plates pull apart they cause divergent margins. This occurs in continental crust and oceanic crust. An example includes the Great Rift Valley in Africa. When two plates move toward each other and collide convergent margins are formed. There are different ways a convergent margin is formed. When the crust buckles and crumples up and down into mountain ranges the lithosphere thickens creating beautiful sites such as Mount Everest and the Himalaya mountains. If both plates are oceanic, one plate will glide under the other one, going into the asthenosphere. The lifted plate creates mountains and the subducted plate eventually melts. Lastly, where two plates glide past one another transform faults are created and are more common on the ocean floor.
What causes plates to be in motion? While it is not completely understood, conduction and convection are known to play role in the process. The Earth has to release the heat that it produces. Some of the heat is released through conduction. Conduction is when heat is transferred from one thing to another or from one molecule to another. Another way heat is released is through convection. Convection is when heat moves from one place to another. Rock that is located in the mantle heats and expands, making it able to float. It will begin to move upward to the surface.
Plate tectonics plays affects many things including igneous rocks and metamorphic rocks. As stated before, igneous rocks form from the cooling of magma or lava. Extremely hot magma rises to the surface at divergent boundaries. When the magma flows to the ocean floor it forms extrusive igneous rocks. Sedimentary rock on the ocean floor gets moved downward into the mantle at convergent boundaries. The temperature of the crust will rise as it goes deeper into the mantle. The crust will melt and rise to the surface and causes an volcanic eruption, which creates igneous rocks. Metamorphic rocks form due to the active forces of the plate tectonic process. Colliding plates, subduction, and plate sliding along transform faults create stress, friction, shearing, folding, faulting, and increased heat flow. Plate tectonics deform and break down the rocks. This deformation creates openings, cracks, faults, and weak areas that allow magma to rise. The greater the tectonic forces, then the higher the pressures and temperatures affecting a rock mass, which means an increased amount of structural deformation and metamorphism.