THE ROCK CYCLE. Name(s)

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1 THE ROCK CYCLE Name(s) Introduction There are numerous cycles that occur on the earth. For example, earlier in this course you studied the water cycle in the context of climate and weather. Over time you learned that it was energy from the sun that ultimately drives this cycle. Many other cycles that occur on earth are also driven by energy from the sun. For example, plants use energy from sunlight to construct complicated organic (carbon-containing) molecules from simple molecules (especially water and carbon dioxide) through photosynthesis. These complicated molecules are reconverted to simpler forms by the plants themselves or by creatures which eat the plants, with the resulting energy used for a variety of tasks. One such task is the building of still more complicated molecules such as proteins. These processes continue as matter and energy flow through different levels of organization in living systems. At each step energy is conserved, and the number of atoms of each type does not change. For example, there are a fixed number of carbon atoms on the earth (unless meteors provide the earth with more or we shoot some into space in rockets). An individual carbon atom may at some time be part of a carbon dioxide molecule, then part of a simple sugar in a plant, and then later a part of a complex organism such as a dinosaur or a human being. Eventually, however, complex molecules are broken down into simpler molecules, and that original carbon atom may again become part of a carbon dioxide molecule ready to be absorbed by a plant. Similar cycles occur for other types of atoms, such as for nitrogen and oxygen. (Just think - some of the atoms in your body may previously have been part of a dinosaur s body!) The concept of cycles and the continual recycling of a fixed number of resources is a very powerful and useful idea in science. The idea that the sun is the energy source for most of these cycles is similarly important. Although many of the cycles that can be studied in science are fairly complex, there are several cycles that can be studied effectively and in a concrete manner at the elementary level. The water cycle is one. The rock cycle is another one. The Rock Cycle Geologists often classify rocks according to how they are formed. This classification scheme contains three main categories of rock called sedimentary, metamorphic, and igneous. The rock cycle refers to the gradual but constant evolution of rocks as they break down and reform into new rocks within one of these three groups. The energy to drive the cycle is obtained from sunlight and from the earth s internal energy, which is thought to originate from radioactive decay in the earth s core. Sedimentary rocks are made from materials obtained from pre-existing rocks (or even animals) that have been eroded away. These sediments are deposited in areas by wind, water, or glaciers, and over time the sediments become fused together due to pressure (and often through an additional cementing process caused by chemicals dissolved in water). These rocks are often brittle, and they are easily eroded when they are exposed on the earth s surface. Examples of sedimentary rock include sandstone and limestone. When rocks are exposed to considerable heat and pressure, new crystal structures can be formed within in the rocks, or chemical changes can occur. Rocks formed in this way are called UNIT 5 INTERACTIONS AND CHANGE V - 13 CALVIN COLLEGE

2 metamorphic rock, because they occur as one type of rock changes into another. There are a great variety of metamorphic rocks, each type dependent on the starting materials and the extent to which the original rock was exposed to heat and pressure. Examples of metamorphic rock include quartzite (made from sandstone) and marble (made from limestone). A rock that is exposed to sufficient heat can melt. This liquid rock is called magma if it is under the ground, and it is called lava if it reaches the surface of the earth. When magma or lava cools and turns into a solid it is called igneous rock. Examples of igneous rock include granite, obsidian, and basalt. A Model for the Rock Cycle There are many ways to investigate rock specimens in elementary school. However, it can be difficult to envision the rock cycle, as most of these changes take place over long periods of time. There are some things a teacher can do with weathering (for instance eroding sandstone or placing other rocks in tumblers to smooth or polish them the way that real rocks can be smoothed and polished in stream beds), but there is not much a teacher can do to create metamorphic or igneous rocks in a classroom. Therefore, presented below is a simple crayon model to make these changes more concrete. (Note: this activity can also be attempted with candle wax.) 1. Take several crayons of different colors and erode them into the tiniest pieces you can. (You may use pencil sharpeners, plastic knives, cheese graters, or any other tools that are available and safe.) Try to collect your shavings onto a piece of paper or a plate. Note that the sediments do not stick together very well. You clearly have individual pieces of crayon. (You may also note that the crayon pieces have picked up excess electrostatic charge. How do you think that happened?) 2. Try to make sedimentary rocks. To do this, get a piece of aluminum foil, perhaps 15 cm square, and wrap the aluminum foil carefully around the crayon pieces to form a flat rectangle which encloses the crayon pieces. Expose the foil to high pressure. (You might try stomping on it with your heel.) After the crayon has been exposed to the pressure, carefully unwrap the aluminum foil. It is likely that the pieces of crayon will now be stuck together to form a very brittle sedimentary rock. Note that the individual components (crayon pieces) are clearly visible. Go ahead and play with your rock. It does not matter if you ruin it. 3. To make metamorphic rock you need heat as well as pressure. Fill a Styrofoam cup with hot water from the coffee pot. Tightly wrap your sedimentary rock and any remaining sediment in the aluminum foil. Make sure that your wrapping is waterproof! Hold the foil and its contents in the hot water for 20 to 30 seconds, and then expose it to high pressure as before. Finally, unwrap your rock. Describe its properties, and compare it to the sedimentary rock you made previously. UNIT 5 INTERACTIONS AND CHANGE V - 14 CALVIN COLLEGE

3 4. In order to make an igneous rock, you will first melt your metamorphic rock and then cool it. You must decide how you will cool your igneous rock. How it cools can be important in determining its final appearance (just as for real igneous rocks). Options available to you include pouring it into a beaker of ice, into a beaker of cold water, into a beaker of warm water, or onto a cupful of sand. Begin by putting the metamorphic rock (and any remaining sediments) back onto a new piece of aluminum foil shaped as a dish with sides so that when it is liquid the crayon will not run off the foil. Wear safety goggles in case of spattering. Place the foil dish on a hot plate or above a candle. Do not allow the crayon to boil. Once the crayon is melted, pour it into or onto your cooling surface. (Careful! The crayon will be very hot.). Describe your resulting igneous rock below. What color is it? Are the individual colors of the sediment evident? UNIT 5 INTERACTIONS AND CHANGE V - 15 CALVIN COLLEGE

4 Concept Maps for Cycles The rock cycle is not a simple linear progression of stages with all rocks evolving through every stage in the same sequence. At any given stage, two rocks may progress in entirely different directions. For example, one sedimentary rock may be converted into a metamorphic rock, while an identical sedimentary rock may be eroded away and turned into yet another from of sedimentary rock. Therefore, a diagram of the rock cycle must indicate a variety of paths through which a rock (and its constituent material) may pass. The same would be true for most other cycles that occur. For example, a partial diagram of the water cycle is shown below. Note that at any given stage there are a variety of directions in which the water can go. Using the arrows drawn on this diagram as prompts, describe the changes occurring at each point of the water cycle shown on this diagram. When you are finished, you will have a concept map for the water cycle. clouds snow ocean river lake ground water UNIT 5 INTERACTIONS AND CHANGE V - 16 CALVIN COLLEGE

5 Using descriptive words such as weathering, pressure, heat, and any other terms necessary, make a concept map below for the rock cycle. Note: Your diagram will look more like a web than a cycle when you are done! sedimentary rock magma sediments igneous rock metamorphic rock UNIT 5 INTERACTIONS AND CHANGE V - 17 CALVIN COLLEGE

6 A Summary of Rock Types Sedimentary Rocks A. Formed from the deposition of sediment coming from the weathering and erosion of older rocks (or animals) pressed or "glued" together. B. Characteristics: Relatively soft or brittle Often contains visible layers or bands May contain fossils C. Examples Sandstone Limestone Shale Conglomerate (separate rock particles clearly seen glued or pressed together) Igneous Rocks A. Formed from cooling and solidification of molten matter (magma or lava) (Complete melting is required prior to the formation of these types of rocks.) B. Characteristics: Typically very hard May contain crystals (size of crystals depends on rate of cooling) C. Examples: Granite Pumice Obsidian (glass-like, cooled very fast, contains no crystals) Basalt Metamorphic Rocks A. Formed from a previous rock by the application of heat and pressure (These rocks are not formed from completely molten material.) B. Characteristics: Typically harder than sedimentary rocks C. Examples: Quartzite (made from Sandstone) Marble (made from Limestone) Slate (made from shale) UNIT 5 INTERACTIONS AND CHANGE V - 18 CALVIN COLLEGE