The Rock Cycle: Processes That Change One Rock Type into Another (page 2)

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Author: Janice VanCleave

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Lava (molten rock from within the Earth that reaches the Earth's surface) cools quickly, producing rocks with small crystals or no crystals. Igneous rocks formed by the cooling of lava are called extrusive igneous rocks. Extrusive rocks are fine-grained (having very small hard particles) or have a glassy (smooth) texture. Repeat the experiment using only small balls of clay.

Design Your Own Experiment

In nature, great pressure on rocks causes the temperature to rise. Together, the heat and pressure changes cause metamorphism, which produces metamorphic rock. Pressing the clay models of igneous rock represents the formation of foliated metamorphic rock (striped-looking metamorphic rock with grains arranged in parallel bands). Science Fair Hint: Make a display to show the clay rolls before pressure was applied to represent igneous rock and after pressure was applied in one direction to represent the resulting foliated metamorphic rock.

The Rock Cycle: Processes That Change One Rock Type into Another

  1. Metamorphism is the change in structure, appearance, and composition of a rock in the solid state within the Earth's crust as a result of changes in temperature and/or pressure. Regional metamorphism occurs when large areas of rock are changed by pressure and heat, such as in mountain building. What happens to grain arrangement during regional metamorphism when pressure is applied from one direction? Place a piece of paper on a table and position one of the clay rolls from the original experiment on the paper. Cover the clay with a second sheet of paper. Place a rolling pin on the top paper above one end of the clay roll. Pressing down firmly, roll the rolling pin across the clay roll. Repeat using the second clay roll.
    1. Rocks come from other rocks. Igneous rock forms when sedimentary or metamorphic rock melts, cools, and solidifies. Sedimentary rock is made from sediments (materials deposited by water, wind, or glaciers) of metamorphic or igneous rocks that are compacted (packed together) and cemented (stuck together). Metamorphic rock forms when igneous or sedimentary rock is changed by metamorphism. This never-ending process by which rocks change from one type to another by a series of processes involving heat, pressure, melting, cooling, and sedimentation is called the rock cycle. Draw and display a diagram similar to the one shown in Figure 13.2 to represent the rock cycle. Note that sedimentary rocks are placed at the top of the diagram because these rocks are formed when the other rocks are lifted to or near the top of the Earth's surface, while the formation of metamorphic and igneous rocks is generally below the Earth's surface.
    2. Use rock samples to prepare a display representing the rock cycle. Use found rocks or rocks purchased at rock and mineral shops or ordered from science catalogs. See Appendix 1 for a list of catalog suppliers and stores selling rocks and minerals.

Get the Facts

Thermal metamorphism includes changes due to heat. Recrystallization (enlargement of minerals) is one example of thermal metamorphism. Another example is contact metamorphism, which occurs when hot magma intrudes (penetrates into) the rock with which it comes in contact. How large an area is affected by contact metamorphism? What is a metasomatic change? How do contact and regional metamorphism compare? What are the names of different rocks before and after these metamorphic changes? To find out more about metamorphism, see John Farndon, How the Earth Works (New York: Reader's Digest Association, 1992), p. 83.

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