The Rock Cycle: Metamorphic, Sedimentary, and Igneous Rocks

Transcription

1 The Rock Cycle: Metamorphic, Sedimentary, and Igneous Rocks Intended for Grade: 3 Subject: Science Description: This presentation teaches students about the properties and formation of each of the three types of rocks as they follow chocolate chips, white chocolate candy bar sections, and peanut butter chips through the rock cycle. They closely examine samples of igneous, metamorphic, and sedimentary rocks and watch a short demonstration of physical vs. chemical weathering. Objective: The student will be able examine and identify metamorphic, sedimentary, and igneous rocks and relate how internal forces (magma and volcanoes) affect the surface of the earth by creating igneous and metamorphic rocks. They will also identify and differentiate between a physical change and a chemical change. State Framework addressed: Science Framework 4b: Examine and identify rocks of different types. Science Framework 4d: Relate how internal forces affect the Earth s surface including earthquakes and volcanoes. Science Framework 5a: Identify and demonstrate chemical changes. Science Framework 5b: Identify and demonstrate physical changes. National Standard addressed: Content Standard B: Physical Science Content Standard D: Earth and Space Science NSF North Mississippi GK-8 1

2 Materials: Milk chocolate chips (1 bag for a class of 30 or less) White chocolate chips (1 bag for a class of 30 or less) Peanut Butter chips (1 bag for a class of 30 or less) Mini M&Ms (a large bag) Plastic baggies (enough for each student to have 1) Wax paper (1 roll for a class of 30 or less) Medium/large microwaveable bowl A spoon A plastic knife An apple Access to a microwave Samples of metamorphic rocks- marble, quartzite, slate, phyllite, schist, and gneiss Samples of igneous rocks- obsidian, pumice, granite, and tuff Samples of sedimentary rocks- coquina, chert, sandstone, mudstone, gypsum, limestone, shale Background: Rocks are solids composed of pieces of one or more minerals. There are three types of rocks: metamorphic, sedimentary, and igneous rocks. Metamorphic rocks are rocks transformed from one form to another by intense heat, intense pressure, or the action of hot fluids. In other words, metamorphism changes the size, shape, texture, color, and/or mineralogy of a rock, thus creating a new rock with different characteristics. Metamorphism can be simplified by thinking of ways that it occurs in your own home. Heat can metamorphose bread into toast, pressure can be used to compact an aluminum can into a flatter form, and the chemical action of hot fluids can be used to change raw vegetables into cooked ones. Since metamorphic rocks are altered forms of pre-existing rocks, every metamorphic rock has a parent rock. The parent rock is the type of rock that was metamorphosed. Parent rocks can be any of the three rock types (metamorphic rocks can be metamorphosed again into a new metamorphic rock). Here are some common parent rocks and their resulting metamorphic rocks: NSF North Mississippi GK-8 2

3 Parent Rock Type Parent Rock Limestone Shale Sandstone Crystalline Rocks Slate Phyllite Schist Intense Heat Resulting Metamorphic Rock Marble Slate Quartzite Sedimentary Igneous Intense Pressure Gneiss Metamorphic Hot Fluids Phyllite Schist Gneiss Sedimentary rocks are rocks formed when sediments are compressed together or hardened (like mud hardened by the sun that forms adobe); or when masses of intergrown minerals precipitate from water (like the rock salt that remains when ocean water is evaporated). Sediments are loose grains such as rock fragments, parts of plants or animals, and rust. Sediments and solutions are produced by chemical weathering (things decomposing or dissolving) and physical weathering (a.k.a. mechanical weathering- things being cracked, scratched, or crushed). After sediments have been produced, they are often carried around by wind and water until they are eventually deposited elsewhere in layers. The Mississippi River carries an average of 436,000 tons of sediment each day. Over the course of a year, it moves an average of 159 million tons of sediment! Sedimentary rocks form by three main processes: compaction, cementation, and precipitation. For example, sand can be compacted until it is pressurehardened into sandstone, or sandstone can form when sand grains are cemented together by chemical residues or intergrown crystals. Rock salt and gypsum are examples of sedimentary rocks that formed by precipitation during the evaporation of salt water or brine. Sedimentary rocks can be composed of the remains of organisms (shells or plant fragments), intergrown minerals that precipitated from solutions, or worn rock fragments and mineral grains. Coquina is an example of a sedimentary rock composed of shells and shell fragments, chert is an example of a sedimentary rock composed of mineral crystals, and sandstone NSF North Mississippi GK-8 3

4 is an example of a sedimentary rock composed of broken mineral grains (quartz grains). Fossils form in sedimentary rock when an animal or plant becomes buried in mud or sand. If air is excluded, as it would be in thick mud, over thousands of years the bones turn to stone by absorbing and exchanging minerals with the surrounding soil and water. The result is a fossil. Sometimes plants and animals leave an imprint, which hardens to form a fossil. From these fossils and impressions in sedimentary rock, we can tell what kind of plants and animals lived during different periods in the very ancient past. Igneous rocks are formed when molten rock (rock liquefied by intense heat and pressure) cools to a solid state as either volcanic glass or masses of intergrown mineral crystals. Molten rock below the ground is called magma and above ground is called lava. Magma is under extreme pressure (like a soda that has been shaken) and is less dense than the hardened rock surrounding it. Therefore, magma tends to rise and squeeze up (much like the blobs of lava in a lava lamp) into the Earth s crust. When the magma rises high enough, it cools and hardens into igneous rocks. Similarly, when lava is erupted from a volcano, it cools and hardens into igneous rocks. The sizes of the crystals present in igneous rocks indicate the rate at which the magma or lava cooled. Large crystals take a long time to grow, so if they are present, the rock cooled at a slow rate. Granite has visible crystals. Tiny crystals or none at all indicate that the rock cooled rapidly, allowing little or no time for crystals to form. Obsidian is an igneous rock that has no crystals because it cooled so quickly. Sometimes air bubbles are trapped in the lava while it cools. Pumice is an example of a rock that contains so many air bubbles that it can float! The formation and destruction of all three rock types can be conceptually modeled with the rock cycle. The rock cycle explains how each rock type can eventually become another rock type. For example, metamorphic rocks can be weathered into sediments, which may then be compacted and cemented together into a sedimentary rock. Metamorphic rocks may also be heated so much that they liquefy and form a body of magma. When the magma cools, an igneous rock would be formed. Finally, metamorphic rocks may be exposed to heat, pressure, or hot fluids and transform into another kind of NSF North Mississippi GK-8 4

5 metamorphic rock. The same things could happen to igneous and sedimentary rocks. Procedure: 1. Before doing anything, take a big bite out of the apple and then set the apple aside. 2. Divide the class into teams of three. 3. Begin by asking the class if they can name the three types of rocks and how they form. 4. Distribute one Ziploc bag and one 4 x8 rectangle of wax paper to each student. 5. Explain that first you will discuss metamorphic rocks. Ask the students what metamorphic rocks used to be before they were exposed to extreme heat and pressure. Allow them to ponder this question while distributing the various chips. 6. Hand out the chocolate chips, white chocolate chips, M&Ms, and peanut butter chips to the groups. In each group, give one student 1 spoonful of chocolate chips (about 15 chips), one student 1 spoonful of white chocolate chips, and one student 1 NSF North Mississippi GK-8 5

6 spoonful of peanut butter chips. Give each student several M&Ms. 7. Instruct the students to fold the piece of wax paper so that two squares are created. (Fold along the dotted line below). 8. Slide the folded wax paper into the plastic bag, placing the fold in the bottom of the bag. 9. Instruct the students to each place their chips in their plastic bag between the two halves of wax paper and then seal the bag. 10. Have the students pretend that their chips or candy bar sections are separate little rocks. Explain that they will now see how heat and pressure can make a new (metamorphic) rock out of these individual rocks. 11. Have the students place the bag of chips under their thigh. By placing their legs on the bags, they are applying heat and pressure to the rocks. Explain that it will take a while for the rocks to undergo metamorphism. It is very important that they not peak until you say so, because the rocks will cool if they remove their leg. 12. Since it will take a while (10 15 minutes) for the chips to melt and form a new metamorphic rock, use this time to discuss metamorphic rocks in more detail. Explain the material covered in the Background section of this activity. For example, the chips are the parent rocks for the new metamorphic rock that will be formed. Explain that all three types of rocks can serve as parent rocks (thus, answering the question they were pondering). 13. Have the students check their metamorphic rocks. Have them flip the bag and place it back under their thigh. 14. Show the examples of metamorphic rocks if they are available. If the parent rocks are available too, that would be even better. The rocks can be passed around the classroom from group to group and the students can closely examine the rocks with magnifying glasses. 15. Have the students check their rocks again. They should be showing signs of melting by now. Instruct the students to NSF North Mississippi GK-8 6

7 squish and squeeze the rocks and then place the bags back beneath their legs. 16. Now ask the students how sedimentary rocks form and where the sediments come from. 17. To explain the weathering process, bite the apple again (on the opposite side from the previous bite). Explain that sometimes rocks are weathered into sediments by being broken, cut, or scratched, similar to how you just broke off a piece of the apple by biting it. This is called physical weathering. 18. Turn the apple around and show the students the bite that was taken out of the apple before the activity was begun. Ask the students what they see. By now, that bite should be brown. Ask if anyone knows why the bite is turning brown? (Because it is beginning to decompose). Explain that chemical weathering occurs when things decompose or dissolve. Sometimes different minerals that compose a rock may dissolve if they come in contact with water (rain). This can create sediments and solutions, both of which are necessary for the formation of sedimentary rocks. 19. Explain that sedimentary rocks can be composed of shells or plant fragments, minerals that precipitated from solutions, or worn rock fragments and mineral grains. 20. Show examples of sedimentary rocks. Point out what they are made of and any layering that is present. Pass them around the classroom and allow the students to closely examine the rocks with magnifying glasses. 21. While the students are busy looking at sedimentary rocks, go around the room and help each group individually with the next few steps: a. Have the students carefully remove the wax paper with the metamorphic rocks inside. b. Have the student with the chocolate chip rock place the rock on the desk and remove the top piece of wax paper. c. Have the student with the white chocolate candy bar rock remove the wax paper and place the rock on top of the chocolate rock. d. Have the third student do the same, so that all three rocks are now layered on top of each other. NSF North Mississippi GK-8 7

8 e. Explain that, in reality, weathering would break down these metamorphic rocks into tiny sediments that could be blown by the wind or carried away by a stream or river. Eventually the sediments would be deposited on top of each other, forming layers. After thousands and thousands of years, the sediment would be compressed and compacted together to form a sedimentary rock. The Mississippi River carries tons of sediments through the USA and deposits them into the Gulf of Mexico. f. Allow each student to press gently on the top of the sedimentary rock with a piece of wax paper. Explain that pressure has tuned the layers of sediment into a layered sedimentary rock. g. Then remove the wax paper and slice the rock in half so the students can see the layering. 22. After every group has made a sedimentary rock, ask the students how igneous rocks form. Where does the magma come from? (Rocks deep in the earth are melted due to extreme heat). 23. With that, go around the room and collect the sedimentary rocks with wax paper and place them in the large bowl. Tell the students that their sedimentary rocks will return, but they will have been melted down into magma! (Lead the students to believe that their sedimentary rocks are being melted into magma, but instead, melt a fresh bag of chocolate chips, 2 new white chocolate candy bars, and a fresh bag of peanut butter chips. Have a helper take the bowl to the microwave and melt the rocks into a large body of magma). 24. Explain how the rate of cooling helps determine the crystal sizes present in the rock. 25. Show examples of various igneous rocks and ask the students if each rock cooled quickly or slowly (based on the crystal size). Then pass the examples around the room and allow the students to closely examine the rocks with magnifying glasses. 26. Place a large sheet of wax paper on a table in the front of the room. Spoon blobs of magma onto the paper. Allow the magma to harden into igneous rocks. NSF North Mississippi GK-8 8

9 27. Draw the rock cycle picture on the board. Have the class help you label the arrows. Reiterate that each rock type can go through changes and become the other two rock types, or even a different version of the same rock type that it started as. Then, go over what the chips started as, what they went through, and what they ended up as. Remind the students that they had to add heat and pressure to form metamorphic rocks, they had to add pressure (compaction) to form sedimentary rocks, and they had to liquefy and then allow cooling to form igneous rocks. 28. Finally, distribute the igneous rocks and let the students eat them! Allow them to come up and take a closer look at the real rock samples. Evaluation: The students participation in labeling the diagram of the rock cycle can be observed by the teacher. Extended Activities: Growing Crystals: Several times in this activity minerals that precipitate out of solution are mentioned. To help the students understand this concept, let them grow their own rock candy crystals! Here s one way to do it, although there are different recipes everywhere. This activity is from : You need 2 cups water, 5 cups sugar, pie pan, string or bamboo skewer, candy thermometer, and foil. Stretch the string across a pie pan so that it is suspended across the top. You'll have to weight the ends so that it does not touch the bottom of the pan, or you can punch holes in the edge of the pan and tie the string. You can also lay a skewer in the pan and let it hang out of the side of the pan. NSF North Mississippi GK-8 9

10 Dissolve the sugar in the water and cook until it reaches 250. This temperature is also called hard ball, and most cookbooks will tell you how to do it without a candy thermometer. Don't stir after you've dissolved the sugar. Pour the hot liquid into the pan. The string should be 3/4 inch under the surface of the sugar water. Cover the pan with foil and don't touch it for a whole week. In 7 days, lift out the string or stick and it will be covered with sugar crystals! You can also pour the liquid into a jar. Punch a bamboo skewer through a plastic coffee-can lid. Lay the lid over the jar so the skewer hangs down the middle of the liquid. Do not let the skewer touch the bottom of the jar. In 7 days, lift out the skewer and it will be covered with sugar crystals! There was so much sugar dissolved, the water couldn't hold it all. Some sugar had to undissolve and grew into sugar crystals. Sedimentary Rock Layers: to demonstrate how sedimentary rocks commonly form in layers, try this activity from This activity can be done individually, in small groups or as a demonstration by the teacher for the class. 1. Have students collect rocks, pebbles, sand, and soil at school or home. 2. Fill a jar about one-third full with the rocks, pebbles, sand, and soil. Add water and shake jar carefully. 3. Have students write a paragraph/statement on what they predict will happen and why. 4. Let the jar stand undisturbed. Materials will settle into layers with the coarser, heavier material at the bottom and the finer, lighter layers at the top. 5. Tell the students that these sedimentary layers usually form in lake beds and near running water. After a long period of time, they will turn to rock. 6. Have students measure and graph the various sizes of the layers. NSF North Mississippi GK-8 10

11 Sources: 7. Students will enjoy re-shaking the jar and watching the materials resettle. Brenan, J., and Sanford, M., 2000, Rocks and Minerals: Grades 1-3: The Education Center Inc., 48 p. Busch, R.M., ed., 2000, Laboratory Manual in Physical Geology (5 th ed.): Upper Saddle River, N.J., Prentice Hall, 276 p. Prepared by: Ashley Phillips NSF NMGK8 University of Mississippi December 2003 NSF North Mississippi GK-8 11