Classifying Rocks Using a Key

Transcription

1 Classifying Rocks Using a Key Introduction Recall that a rock is a naturally occurring, solid mass of minerals or mineral-like matter. Geologists classify rocks into three major groups based on how the rocks form. Igneous rocks form when molten material lava or magma cools either on Earth s surface or underground. Extrusive rocks form as lava cools quickly at or near the Earth s surface. Extrusive rocks have either a fine-grained texture or a glassy texture. Intrusive Rocks form as magma cools slowly farther beneath the earth s surface. This slow rate of cooling allows mineral grains to grow large, and such a rock is said to have a coarse-grained texture. Sedimentary rocks form when pieces of rocks, minerals, or organic matter all of which are called sediment-are compacted and cemented. Clastic rocks are sedimentary rocks that are made of fragments of weathered Earth materials. The fragments might be fairly large,, such as pebbles; somewhat smaller, such as grains of sand; or very small, such as grains of clay. Chemical rocks are sedimentary rocks that form when minerals settle out of solution. Biochemical rocks are sedimentary rocks that form as the result of organic processes. Metamorphic rocks are rocks that form when existing rocks are subjected to changes in pressure or temperature. They can also form when they are subjected to chemical solutions. Metamorphic rocks may be foliated, which means that the components are arranged in parallel bands, or nonfoliated, which means that the rock s components are not arranged in bands. 1

2 In the investigation, you will observe rock texture, which is the shape, size, and arrangement of a rock s components. You will use rock texture and other properties to classify rocks using a key. Pre-Lab Discussion - Read the entire investigation first. 1. In your own words, describe what is meant by a rock s texture. 2. What distinguishes the two main types of igneous rocks? Explain your answer. 3. Chalk is made of tiny fragments of marine organisms. To which group of rocks does chalk belong? 4. Suppose you observe a rock with distinct bands. What type of rock might this be? Explain why. 2

3 Materials Igneous Rocks Metamorphic Rocks Sedimentary Rocks (1 M) Dilute Hydrochloric Acid (HCl) Dropper Paper Towels Hand Lens Red Pen or pencil Safety Put on safety goggles, a lab apron, and plastic disposable gloves. Take care when using chemicals such as hydrochloric acid as they may irritate the skin or stain skin or clothing. Never touch or taste a chemical unless instructed to do so. Wash your hands thoroughly after completing this investigation. Note all safety alert symbols next to the steps in the Procedure and review the meaning of each symbol by referring to the symbol guide on page xiii. Procedure 1. Put on your safety goggles, disposable gloves, and lab apron. 2. Choose one of the rock samples provided by your teacher. Observe its texture, color, crystal size, and composition with and without the hand lens. 3

4 3. Use the Key to Rock Classification (Data Table 1) to classify the sample. Begin by reading the first question. Answer Yes or No based on your observations. 4. After the words Yes and No, you will find directions to proceed to another question, or you will discover to which group of rocks your specimen belongs. If you find directions to proceed to another question, go to that question, answer it, and follow the directions. 5. Continue working through the key in this way until you come to a statement that allows you to classify your rock sample. To answer Question 8 in Data Table 1, put the rock on a paper towel and place a single drop of HCI on the rock. CAUTION: Always wear safety goggles and disposable gloves when working with chemicals. 6. In Data Table 2, record the route that you take through the key, using the numbers of the questions. For example, your route could be extrusive igneous rock In the last column of the table, write the name of the rock group of which each sample belongs. 7. When you have classified all of the samples, remove and dispose of your rubber gloves and thoroughly wash your hands with soap and water. 8. Compare your classifications with those provided by your teacher. If you made a mistake in classifying any of the samples, put the correct answer in red next to your answer 4

5 DATA TABLE 1 Key to Rock Classification 1. Does the rock contain visible connecting crystals? 2. Are all the crystals the same color and shape? 3. Are all the crystals in mixed saltand-pepper pattern 4. Does the rock contain many small holes or have a uniform dark color? 5. Is the rock glassy (does it resemble broken glass)? 6. Does the rock have flat, thin layers that can be broken apart? 7. Does the rock contain pebbles, sand, or smaller particles that are cemented together 8. Does the rock fizz when dilute HC1 is dropped on it? Yes: Go to Question 2 No: Go to Question 4 Yes: The rock is a nonfoliated metamorphic rock (possibly marble or quartzite). Yes: The rock is an intrusive Igneous rock (possibly granite or diorite). No: The rock is a foliated metamorphic rock (possibly schist or gneiss). Yes: The rock is an intrusive igneous rock (possibly granite or diorite). No: Go to Question 5 Yes: The rock is an extrusive igneous rock (obsidian). No: Go to Question 6 Yes: Yes: The rock is a foliated metamorphic rock (slate). No: Go to Question 7. Yes: The rock is a clastic sedimentary rock (possibly conglomerate sandstone or slate. No: Go to Question 8. Yes: The rock is chemical or organic sedimentary rock (limestone or chalk). No: Ask your teacher for assistance. 5

6 Observations DATA TABLE 2 Letter of Sample Route Taken Group to Which Rock Belongs Name of Rock Analysis and Conclusions 1. How difficult was it to use the key to classify your rock samples? What problems did you encounter? 2. How useful was rock color in classifying the rock samples? Explain your answer. 6

7 3. Describe the overall texture of each of the major groups of rocks intrusive, extrusive, clastic, chemical, foliated, and non-foliated. 4. Which two of the rock samples were the easiest to classify? What properties made them easy to classify? 5. Which two rock samples were the hardest to classify? Explain your answer. 7