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Metamorphic Rocks Metamorphic rock is the result of the transformation of an existing rock type in a process called metamorphism, which means "change in form". The existing rock, (called a protolith), is subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust. Metamorphic rocks form deep in the earth where high temperature, great pressure, and chemical reactions cause one type of rock to change into another type of rock. Metamorphic rocks begin to form at 12-16 kilometers beneath the earth's surface. They begin changing at temperatures of 100 degrees Celsius to 800 degrees Celsius. If you squeeze and heat a rock for a few million years, it can turn into a new kind of rock. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior. Extremely hot water deep underground can also cause chemical and physical changes in rock, forming metamorphic rocks. Where does the heat come from? The heat comes from magma. Where does the pressure come from? The pressure comes from layers of rock piled on top of layers and layers of rock. The layers on the bottom get squeezed. The thicker the layers, the more pressure there is. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides us with very valuable information about the temperatures and pressures that occur at great depths within the Earth's crust. Metamorphic rocks can be classified in two ways. Foliated metamorphic rocks are those which have alternating visible layers or bands. Non-foliated metamorphic rocks have no layers or bands. Some examples of foliated metamorphic rocks are gneiss and schist. Examples of non-foliated metamorphic rocks are slate, marble, and quartzite. Below is a summary of the major characteristics of metamorphic rocks. Classified by texture and composition Rarely has fossils May react with acid May have alternate bands of light and dark minerals May be composed of only one mineral, ex. marble & quartzite May have layers of visible crystals Usually made of mineral crystals of different sizes Rarely has pores or openings May have bent or curved foliation Name: Class: Date: Some examples of how metamorphic rocks were changed: Questions: 1) What does the term ‘metamorphism’ mean? 2) What is a ‘protolith’? 3) What are the two main forces that cause rocks to become metamorphic? 4) How does a studying metamorphic rock at the surface help us understand the interior of the earth? 5) List the two classifications of metamorphic rocks and give an example of each.