NAME DATE PERIOD. writer and a scientist-wannabe), I think there is more work to be done in making this lab NGSS-aligned.

Transcription

1 Note to Teachers This lab, The Crystallization of Rocks from Magma, is a work in progress. I modify it every year as I learn what works, what doesn t work and what kinds of things my students tend to notice or ask. In my work with the Next Generation Science Standards (both as an NGSS writer and a scientist-wannabe), I think there is more work to be done in making this lab NGSS-aligned. I initially designed the lab to show and reinforce the idea that crystal size in igneous rock is a function of cooling rate. I will probably redesign that lab so that I am guiding students to develop and use a model to explain how igneous rocks form. The final piece (pages 7-9) in which students connect the lab experiments to an actual outcrop is an important idea in historical geology and a key way we geologists come to understand what has happened in Earth s past. Developed by M Colson page 1

2 Lab: The Crystallization of Rocks from Magma PURPOSE: In this experiment we will determine how crystal size is a function of how fast a molten material cools. We aren t using real magma! Instead, we are using a solid with a low melting point to be a stand-in for magma. The liquid thymol represents and the solid crystalline thymol represents. Molten means. STATE YOUR HYPOTHESIS: If the crystal size in igneous rocks is determined by how fast molten rock cools, then I think that when molten thymol is cooled slowly, (choose one: big, little) crystals will form. When the liquid thymol is cooled more quickly, the resulting crystals will be (choose one: big, little). MATERIALS: hot plate hot pad petri dish 4 rulers 2-90 metal scoops with thymol ice water 4 hand lens stop watch goggles 1 per student SAFETY: 1. The compound we will be using is called thymol. It is extracted from plants like thyme and bee balm. It is used in small amounts in Listerine. Thymol is not dangerous if used properly. It smells a little like icy hot. 2. DO NOT PUT YOUR HEAD OVER THE THYMOL AND INHALE DEEPLY. This will irritate your nose. 3. Thymol has a low melting temperature, about 48 C. Preheat your hot plate by laying your hand on the metal top as it heats up. Once your hand begins to be uncomfortably warm, turn the hot plate down to a lower heat setting to maintain that temperature. 4. Doing the above will keep the temperature of your hot plate as low as possible but still melt the compound. You should not see vapors or smoke rising from your metal scoop. If you do, the hot plate is too hot. 5. The metal cup should never get too hot to handle with your bare hands. 6. Do not put your fingers in your mouth, nose or eyes during lab. Wash your hands with soap and water at the end of lab today. 7. You must have your goggles on at all times. Thymol would irritate the tissues of your eyes. 8. Keep the hot plate toward the center of the lab station. Don t put pens and pencils against the hot plate. 9. Make sure the electrical cords don t dangle off the edge of the table. Plug the hot plate in above the worktop, not below the bridge that connects the lab table to the countertop. I have read and agree to follow all the safety rules set forth above. I will cooperate to the fullest extent with my teacher to insure my safety and the safety of others in the classroom. I understand that not doing so may result in my being removed from the lab. Student Signature Date Developed by M Colson page 2

3 PROCEDURE: Experiment 1 - Fast cooling in a bath of ice water. 1. Read steps 1-15 first out loud in your group. Then plug in your hot plate above the worktop. 2. Preheat the hot plate in the following manner: turn your hot plate to 5, place your palm on the hot plate. When your palm is uncomfortably warm, remove hand and turn the dial to halfway between 1 and Set one of your metal scoops with the thymol on your hot plate. Allow most of the thymol to melt. Your teacher may need to stir the thymol. 4. Get some ice water from the demonstration table. Fill your petri dish about 3/4 full of ice-cold water. 5. Remove the metal scoop from the hot plate BEFORE ALL THE CRYSTALS MELT. Begin timing the cooling now. 6. Set the metal scoop in your petri dish with the ice water. Stop timing as soon as all of the thymol is crystallized. 7. Record the length of time required for your thymol to completely crystallize in the ice water bath. 8. Dry off the bottom of your metal scoop and set it to the side. Experiment 2: Slow cooling on the hot pad. 9. Set the other metal scoop on the hot plate. Do not remelt your first batch of crystals! 10. Preheat hot plate if necessary (follow the directions in step 2). Set the other metal scoop with thymol back on your hot plate. 11. Allow most of the thymol to melt. Your teacher may need to stir the thymol. 12. Remove the metal scoop from the hot plate BEFORE ALL THE CRYSTALS MELT. Set your metal scoop on the oven mitt. 13. Begin timing the cooling now. Stop timing when all of the thymol is crystallized (no more liquid left). 14. Record the length of time required for your thymol to completely crystallize ion the pot holder. 15. Clean up your lab station: dump the water out of the petri dish. Unplug hot plate. Developed by M Colson page 3

4 OBSERVATIONS 16. Observations for the fast cooling in Experiment 1. a. The melted thymol represents. b. The thymol crystals that grow represent. c. Atoms are frozen into place in the (liquid thymol or crystalline thymol). d. Atoms are moving freely in the (liquid thymol or crystalline thymol). e. The circle below represents the metal scoop. Draw a sketch of the crystals in the circle below. Be sure to draw the interlocking nature of the crystals. f. Measure the diameter of several crystals in the watch glass. Record your measurements on the data table. 17. Observations for cooling in Experiment 2. a. Draw a sketch of the crystals in the circle below. Be sure to observe the interlocking nature of the crystals. Capture the interlocking nature of your crystals on your drawing. b. Measure the diameter of several crystals in the metal scoop. Record your measurements on the data table. Experiment 1 Cooling time Circle one: slow cooling on pot holder OR fast cooling on ice water. Experiment 2 Cooling time Circle one: slow cooling on pot holder OR fast cooling on ice water. Data Table Cooling time Length or Width of Crystal (millimeters) Measure 4 separate crystals Average crystal size Developed by M Colson page 4

5 18. Describe the differences you see between the two sets of thymol crystals. ANALYSIS and CONCLUSIONS: 1. Name the independent variable (manipulated variable). In other words, what aspect of the experiment did you change on purpose? 2. Name the dependent variable (or responding variable). In other words, what aspect of the experiment changed in response to your manipulation? 3. Construct a possible explanation for why the mass of crystals that formed in response to fast-cooling look cloudy and white, but the crystals that formed from slow cooling look clear. 4. Look carefully again at the crystals in the metal scoop that formed in response to slow cooling. Are there any regular variations in crystal size within the scoop? Describe them. Construct a possible explanation for why the crystals are smaller toward the outside of the metal scoop and bigger towards the center of the metal scoop. 5. What can you conclude about the effect of rate of cooling (or the rate of crystallization) on the size of the thymol crystals that form? Use complete sentences. Don t use the word it. 6. Look back at page one. Rewrite your initial hypothesis here. Developed by M Colson page 5

6 7. Does your conclusion support your hypothesis about the formation of igneous rocks? Explain your answer. Developed by M Colson page 6

7 APPLY YOUR CONCLUSIONS TO IGNEOUS ROCKS 1. Look up the meaning of the words intrusive igneous rock and extrusive igneous rock in your textbook. Write the definitions of these words below. extrusive igneous rock- intrusive igneous rock- 2. Now, even though you have not ever seen magma cooling underground nor have you probably seen lava cooling above ground, what can you say about the relationship between the grain size of an igneous rock and the cooling rate? Be sure to discuss the grain size of intrusive and extrusive igneous rocks. DO NOT USE THE WORD IT. Write your answer in complete sentences. 3. Recall or look at your technical descriptions of the grain sizes of these rocks. Fill in the grain size column. Rock Name grain size fine, coarse or glassy cooling rate: slow/fast/instantaneous Where did the molten rock cool? below ground, above ground, in the air intrusive or extrusive igneous rock? granite gabbro diorite rhyolite basalt andesite obsidian pumice 4. On the basis of our experiments, we can infer how fast a magma or lava cooled and therefore where an igneous rock formed based on the grain size of the rock. Fill in the last three columns of the above table. 5. You and a friend are visiting Yosemite National Park in the Sierra Nevada Mountains of California. Visit for photos. You easily recognize that the rock forming the majestic cliffs is granite. Based on our study of rocks, you know that Yosemite has not always looked this way. What can you say about how and where that granite formed and how this part of Earth has changed through time? How do you know this... be sure to refer to our thymol experiments? Answer on a separate sheet of papter. Developed by M Colson page 7

8 Geo-Puzzle - The Palisades Intrusion, New York/New Jersey Below is a sketch of a massive cliff, called the Palisades, along the Hudson River facing New York City. The cliff is about 300 meters (900 ft) from top to bottom and extends for 80 km (45 miles). It is made mainly of basaltic igneous rock. The top and bottom sandstone layers are the same. sandstone fine-grained basalt coarse-grained basalt fine-grained basalt sandstone Observations of the outcrop. 1. Color the different kinds of rock. Sandstone-yellow, fine-grained basalt-black, coarse-grained basalt-gray. 2. Where do we find, relative to each other, the coarse-grained igneous rocks and fine-grained igneous rocks? 3. Which cooled the fastest? the large-grained or fine-grained igneous rocks? How do you know this? 4. Give a possible explanation for why the fine-grained rocks are found next to the layers of sandstone rather than sandwiched between the coarse-grained igneous rock? 5. Explain why the large-grained rocks are found in the middle of the igneous rock layer? 6. Of the following two rock units, which do you think had to be in existence first: the two layers of sandstone or the layer of basaltic igneous rock? Explain your answer. Developed by M Colson page 8

9 7. Put the following story events in the order that you think they happened as the Palisades formed. Order them from what happened first (#1) to what happened last (#7) New York City was founded and began to grow. The basaltic magma (rich in iron and magnesium) began to cool quickly where the magma was in contact with the sandstone forming a fine-grained igneous rock. The Hudson River eroded a river valley, cutting through the layers of sandstone and basalt. The sand that was to become the sandstone was deposited on a delta. Basaltic magma, upwelling from the mantle, forced its way into a horizontal crack in the sandstone. Through time, the layers of delta sand lithified into solid rock layers. The magma in the interior of the layer cooled more slowly than the magma along the upper and lower edges, so large crystals grew and formed a coarsegrained igneous rock. Developed by M Colson page 9