Classifying Rocks. Background

Transcription

1 Classifying Rocks Background Rocks on Earth can be classified into three categories based on the way they were formed. Igneous rocks are formed through volcanic action. Sedimentary rocks are formed by deposition. Metamorphic rocks are formed when existing rocks undergo a change due to extreme heat and pressure. Rocks that have a volcanic origin are classified as igneous and form when magma or lava cools and hardens. As the magma or lava cools, crystals begin to form. The size of the crystals depends upon how fast the magma or lava cools. If magma cools quickly, small crystals form and can be observed on the rock surface, for example: basalt. Lava can cool so quickly that crystals do not have time to form at all, such as with obsidian. If magma cools slowly, larger crystals form and can easily be seen on the surface of rock specimens. A common example of igneous rock that exhibits large crystal formation is granite. When Earth s materials are deposited in layers and pressed together over time, sedimentary rocks form. The formation of sedimentary rock begins with the deposition of sediments. As layers are added, the oldest or lower layers that were deposited first, experience increased pressure. The sediments and the spaces between them are crushed together in a process called compaction. During the cementation process the sediments are bound together when a solution fills in around the particles like glue. If the sediments deposited are sand particles, such as in a beach environment, sandstone forms. If the sediments are fine silt, or clay sized particles, commonly called mud such as those found in basins worldwide, shale forms. Limestone typically forms from shells and other sediments deposited in ocean environments. A quiet and undisturbed environment will sometimes lead to fossil formation within the layers of sedimentary deposition. Continue to the next page. 1

2 Background, continued Metamorphic rock forms when one type of rock changes into another due to exposure to heat and pressure often caused by movement of material deep beneath Earth s surface. This change to rock appearance and composition takes an extremely long time. Metamorphic rocks are often characterized by wavy layers of mineral crystals or by the presence of unusual minerals. Any rock can become a metamorphic rock. For example, the sedimentary rock shale forms from layers of deposited silt. When exposed to high pressure due to geologic processes, the metamorphic rock slate forms. The sedimentary rock sandstone changes to quartzite when exposed to extreme heat and pressure beneath Earth s surface. Another example of a metamorphic rock is marble which forms when limestone is exposed to extreme temperatures or pressure. Igneous basalt changes to schist in the metamorphic process. The interesting thing about the metamorphic process is that any rock, igneous, metamorphic or sedimentary, will change and form new metamorphic rock given enough time, heat and/or pressure. The rock cycle illustrates how rocks move on and under Earth s crust and change from one type of rock into another. As rocks move through the rock cycle their mineral compositions and physical structures change to reflect the processes under which they are formed. The rock cycle shows how any rock can undergo change to form any other type of rock. For example, an igneous rock formed by a volcanic lava flow can crumble to become sand, the sand can be deposited on a beach and eventually be compacted and cemented to become the sedimentary rock, sandstone. The sandstone can be buried deep into Earth s crust where extreme pressure and heat from the overlying layers change the sandstone into a metamorphic rock. Any rock can be carried so deep the temperature becomes so great the rock melts, becomes magma, and is then ready to move towards the surface to form new igneous rocks. Unlike life cycles, rocks do not have to move through each stage. An igneous rock can be changed into a metamorphic rock, a metamorphic rock can crumble to become a sedimentary rock, and sedimentary rocks can be re-melted to become magma. The pathway a rock takes on the rock cycle depends upon the movements of the rock that place the rock in the position to either crumble, be put under great pressure, or melt. Complete the Background questions in your Student Journal. 2

3 Classifying Rocks Part I: The Rock Cycle Step I: To form SEDIMENTS 1. Put on your goggles. 2. Place a sheet of paper in front of you. 3. Each group member take a wax crayon of a different color and remove the wrapper. 4. Use a pencil sharpener to WEATHER your crayon to form a pile of crayon chips and flakes on the sheet of paper. Use a plastic knife to chop the last bit of crayon as needed. Clean bits of crayon from the sharpener with a straightened paperclip. Your group will have four piles of different colored SEDIMENTS. 5. Use the knife to chop up the shavings into small bits. Step 2: The sediments are ERODED and DEPOSITED 1. Have a group member hold a plastic bag open and at a slight angle. 2. Take turns to ERODE and DEPOSIT your color of SEDIMENTS into the bag. Do this by gently pouring your color of scrapings into the bag such that layers of colors form in one corner of the bag. 3. Add a cementing solution by asking your teacher to spray one spritz of water into your bag. 4. Try not to disturb your LAYERS OF DEPOSITED SEDIMENTS as you gently press the air out of the bag while sealing it. Step 3: To form the SEDIMENTARY ROCK 1. Place the bag of LAYERED SEDIMENTS on your chair and sit on the bag of sediments to model COMPACTION and CEMENTATION of the sediments. 2. While sitting on the bag, read the Background and answer the Background questions in your Student Journal. 3. Retrieve and open the bag to observe the sedimentary rock. Break the rock in half and observe the edges. Continue to the next page. 3

4 Part I: The Rock Cycle, continued 4. Compare your rock with a sample of the sedimentary rock sandstone. Record your comparison in your Student Journal. Complete the Sedimentary Rock section for Part I in your Student Journal. 5. Place an approximately 3 cm piece of the rock on the group paper plate labeled Rock Samples and label the section sedimentary. Place the rest into the plastic bag. Step 4: To form the METAMORPHIC ROCK 1. Take the plastic bag with your sedimentary rock and reseal the bag while pushing out all excess air. 2. Apply EXTREME HEAT to your rock by dunking the bag into a cup of 45 o C tap water for 2:00 minutes. Wrap the wet bag with a paper towel. 3. Next, set the paper towel wrapped bag on the floor. Place a large, heavy book on top of the bag and take turns standing on the book to apply EXTREME PRESSURE to your rock. 4. Remove the book, open the bag and observe the metamorphic rock. 5. Break the rock in half and observe the edges. 6. Compare your rock with a sample of the metamorphic rock, schist. Record your comparison in your Student Journal. Complete the Metamorphic Rock section for Part I of your Student Journal. 7. Place an approximately 3 cm piece of the rock on the group paper plate labeled Rock Samples and label the section metamorphic. Place the remainder into the small aluminum pan. Continue to the next page. 4

5 Part I: The Rock Cycle, continued Step 5: To form the IGNEOUS ROCK 1. Put on your goggles. 2. Turn the hot plate to low heat. 3. Place the small aluminum tin with the metamorphic rock on the hot plate to MELT the rock and turn it into MAGMA. 4. While waiting, get a 500 ml glass beaker with ice. 5. When the rock completely melts to MAGMA, turn off the hot plate. Use either the tongs or a safety mitt to remove the hot tin and place the tin on top of the ice so the wax can COOL and SOLIDIFY. 6. Once solid, take your igneous rock out of the tin and observe before placing on the group paper plate labeled Rock Samples. Label the section igneous. Complete Part I in your Student Journal. 5

6 Part II: Igneous Rock Formation All igneous rocks form from cooling magma, but where and how fast the magma cools determines the type of igneous rock. Igneous rock with large and well formed crystals forms when magma wells up into the crust from the mantle but does not reach the surface. This magma is kept warm from the surrounding layers of earth and cools very slowly allowing large crystals to form. Magma that reaches Earth s surface is called lava. When lava flows out of a volcano and over the land it cools faster resulting in much smaller crystal formation. Lava that flows into water cools extremely fast and solidifies so quickly crystals are not able to form resulting in a smooth glasslike rock. You will again use a wax crayon model to observe how temperature affects the formation of the various types of igneous rocks. This activity involves using a hot plate. Follow all appropriate safety precautions such as using eye protection, tongs and safety mitts. 1. Put on your goggles. 2. Remove the paper from three crayons and place them in the three small aluminum tins. 3. Place the three small tins on a hot plate on low heat until the crayons have completely melted to form a liquid. 4. While heating the crayons, use a marker to label one disposable glass ice, and second cold water, and a third warm water. When the crayons are almost melted, send a group member to the ice and water station to obtain ice, cold water and warm water in the three glasses. 5. Once the crayons have melted, use either tongs or safety mitts to handle the hot small tin. Pour the liquid from one tin over the ice and observe. 6. Pour the liquid from the second tin into the cold water and observe. 7. Pour the liquid from the third tin into the warm water and observe. 8. Look closely for differences due to the rate of cooling of each of the igneous rocks that you created. 9. Observe the rock samples of obsidian, granite, and basalt. Rate of cooling affected these three igneous rock samples, too. Use your observations to determine the rate of cooling that formed each of the rock samples. Complete all remaining questions in your Student Journal. 6