Old Earth Ministries Online Geology Curriculum
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Geology - Chapter 7: Metamorphic Rocks
Metamorphic rocks are probably the least understood by someone not familiar with geology. These rocks form from the deformation of previously existing igneous, sedimentary, and even an older metamorphic rock. The term metamorphic comes from a Greek term that means "change form."
Monday - Read Text
Tuesday - Research
Wednesday - Quiz
Thursday - Review
Friday - Test
This lesson plan is designed so that your child can complete the chapter in five days. The only decisions you will need to make will be concerning the research task for Tuesday. It is up to you to determine if the student will simply fill in the answers, or provide a short essay answer. You will also need to determine the percentage that this research will play in the overall chapter grade, if any.
Deep under the earth's surface, the temperature and pressure that rocks are subjected to can sometimes cause the rocks to change. This change can result in the partial or complete recrystallization of the minerals in a rock, and it can also fold or twist the rock so that it appears bent from its original shape. Rocks undergoing metamorphism mostly remain solid, and do retain some characteristics of their former rock. Their final structure, texture and composition are somewhat controlled by the original rock composition. To get a good understanding of this, let's start out with an animation that shows metamorphic rock formation. Click here to watch this animation.
In the example, an igneous rock named diorite is subjected to heat and pressure. Note that the diorite has randomly oriented grains. You can see the individual grains becoming flatter as they are squeezed. The metamorphic rock formed is called gneiss. Notice that the grains are now foliated. This means that the platy (crystals squeezed flat) or elongated crystals are oriented with their axes perpendicular to the direction of the force. The resulting rock is foliated. Another term used for foliation is "banding."
One concept that is important to understand is called plastic deformation. When a rock undergoes plastic deformation, that means that it can be twisted or folded without breaking. Even though a rock may seem like a rigid object, when subjected to heat and pressure, it can bend. This occurs with many metamorphic rocks. You will study this process in more detail in a later chapter.
Because of the intense heat and pressure, much of the rocks in the earth's crust are probably metamorphic. They are formed deep beneath the earth's surface by high pressures and temperatures. They can also be formed from the intrusion of magma, which heats the surrounding rock and metamorphoses it.
How the Process Works
Metamorphism has three major forces that effect the changes to the rock. These forces are temperature, pressure, and chemistry.
As the temperature of the rock climbs, melting begins to occur, filling in the pore spaces of the rock. Typically, rocks below 200 degrees centigrade only have a small amount of pore fluid present. Between 200 and 700 degrees centigrade, the rocks become more fluid...chemical reactions occur more frequently, and new mineral assemblages appear. Above 700 degrees, the rock can take on magma-like properties, leading some rocks to appear as if they have layers of igneous and metamorphic rocks.
The next most important force acting upon rocks is pressure. This can be pressure from overlying layers of rock, or pressure from rising bodies of magma, or pressure from tectonic forces. The most significant effect of pressure is that it compacts the amount of space occupied by the rock. As mentioned previously, the pressure can produce foliation, which tells geologists the direction of force applied to produce the metamorphic rock.
The pressure and temperature rises can free many atoms and molecules from their crystal structure. These free atoms/molecules provide both a transport system for materials to move about the rock matrix, and they provide new material for crystallizing new minerals. This process is called recrystallization.
Plate Tectonics and Metamorphic Rocks
The process of forming metamorphic rocks is intimately associated with plate tectonics. Because many of these processes occur many miles beneath the surface of the earth, we cannot observe this process in action. We can, however, test how minerals respond to heat and pressure in a laboratory, and together with field observation, come up with scientific explanations for how the rocks formed.
There are two type of metamorphic processes. When a body of magma rises through the crust, it heats the surrounding rock that it passes through. This heat causes the rock to metamorphose. This type of metamorphism is called contact metamorphism. The changes may occur through the partial melting of the rock, or it may occur through chemical changes. Elements can be added or taken way by fluids entering or leaving the rock. Contact metamorphic rocks are commonly associated with batholiths, and they are commonly found around dikes and sills.
The second type of process, and much more prevalent, is called regional metamorphism. A large part of our continental crust is made up of metamorphic rocks. When tectonic plates collide, the pressure and heat from this collision produces metamorphic rocks, which can affect many cubic miles of crustal material. The colliding plates can also cause the rock layers to fold, creating interesting rock features. Examine the folded rocks in the picture. This is a small-scale picture showing an area of rock about 7 by 10 inches. Much larger scale folds can also be observed.
Like igneous rocks, metamorphic rocks are classified based on texture and composition. There are two basic types of texture for a metamorphic rock. Rocks which exhibit foliation are said to exhibit a planar texture. Planar texture can further be divided based on the type/degree of foliation. Rocks which do not show foliation are said to exhibit a granular texture. Granular texture is sometimes called non-foliated. This can be confusing, since planar texture rocks has grains that you can see. If it exhibits foliation, it is not granular texture...even if it exhibits grains.
Planar Texture Rocks
Granular Texture Rocks
As a final tool to tie this all together, please examine this excellent website with animations concerning Metamorphic Rocks and Plate Tectonics.
Research the answers to the following questions about metamorphic rocks. Your parents may have you simply answer the questions, or they may have you put it in essay form. Please follow your parents instructions.
To answer these questions, utilize a search engine to locate the best web pages, or consult a textbook/encyclopedia. You may also use the links at the bottom of this page.
Today you will complete an 10 question practice quiz. The link to the quiz will open a new window. You can come back here and check your answers. Do not click the Back button on your browser during the quiz. After the quiz, continue your research project, if necessary.
Please review the terms in bold in the text, and ensure you have completed your research work from Tuesday.
Today you will take the end of chapter test. Please close all other browser windows, and click on the link below. During the test, do not click on the Back button on your browser.
After you have completed the test, you may proceed to Chapter 8 on your next school day. Please return to the introduction page for the link to the next chapter.
Return to the Old Earth Ministries Online Geology Curriculum homepage.