HW6_Geology

Metamorphic Rock Inquiry Activity 7.1 Name: Course/Section: Date' Learning GOAL You will analyze and describe samples of metamorphic rock and then consider how they might be classified into logical groups. III Analyze the metamorphic rocks in Fig. A7.1.1 and actual rock samples of them, if available. Beneath or beside each picture, briefly note the most distinctive features you observe in the rock. For example, does the sample appear to be mostly or entirely composed of the same mineral? Is the rock layered (foliated)? Are the foliations planar or folded? Are different layers com- posed of different colors/types of mineral? Are some mineral grains large and others much smaller? Is there any hint of shiny mica minerals along the foliations? Figure A7.1.1 B REFLECT & DISCUSS Reflect on your observations and descriptions of metamorphic rocks in part A. Then describe how you would classify the rocks into groups. Be prepared to discuss your classification with your classmates or teacher. 197

1 2 The white mineral is quartz. What is the blue mineral? Activity 7.2 Minerals in Metamorphic Rock Name: Course/Section: Date' Learning GOAL You will be able to identify some of the more important minerals that are recognizable in hand samples of metamorphic rock. Review the descriptions of metamorphic minerals. Below each of the numbered photos in Fig. A7.2.1, identify the type of mineral shown. Be prepared to discuss your interpretations with your teacher or with others in your lab. 6 Figure A7.2.1 198

111 Analyze the foliated metamorphic rock sample in Fig. A7.3.3. 1. What mineral defines the foliation in this rock? 2. Notice that the rock consists mostly of fine-grained muscovite but also contains scattered garnet crystals. What is the name for this kind of texture? 3. What is the name of this metamorphic rock? 4. What type(s) of rock might have been the protolith for this rock? Figure A7.3.3 E Analyze the metamorphic rock sample in Fig. A7.3.4. 1. Is this rock foliated or nonfoliated (granofelsic)? What features in the photograph did you use to make your interpretation? 2. What is the name of this metamorphic rock? 3. What type(s) of rock might have been the protolith for this rock? Figure A7.3.4 F REFLECT & DISCUSS Which one of the rocks in this activity do you think has the highest metamorphic grade? Explain your answer. 200

Temperature (°C) 0 400 500 600 700 800 900 0 0 andalusite 5 2 10 1", - ; c 'ti 4 sillimanite 1523 202 E 6 kyanite 25f, 8 30`' 10 35 40 12 Figure A7.5.2 Activity 7.5 Metamorphic Grades and Facies Name: Course/Section: Date' Learning GOAL You will begin learning how to infer regional geologic history and the relationship of metamorphic facies to plate tectonics using index minerals, pressure-temperature diagrams, and geologic maps. II British geologist George Barrow mapped rocks in the Scottish Highlands that were metamorphosed by granitic igneous intru- sions. He discovered that as he walked away from the granitic intrusive igneous rock, a sequence of mineral zones generally reflected the intensity of metamorphism. He defined the following sequence of index minerals, which represent intensity of metamorphism along a gradient from low to high pressure-temperature (P-T) conditions: chlorite (lowest P-T), biotite, garnet, staurolite, kyanite, sillimanite (highest P-T) 1. Boundaries between Barrow's metamorphic zones are called isograds. On the geologic map (Fig. A7.5.1), color in the zone of maximum metamorphic intensity as indicated by Barrow's index minerals. Figure A7.5.1 III Metamorphism is typically caused by increases of both pressure and temperature. Geologists represent these relationships on pressure-tem- perature (P-T) diagrams (or phase diagrams) showing the stability of different index minerals. The minerals andalusite, kyanite, and sillimanite shown on this phase diagram (Fig. A7.5.2) are polymoiphs: minerals that have the same chemi- cal composition but different crystalline struc- ture and physical properties that can be used to distinguish them. Each polymorph is stable under pressure and temperature conditions that are different from the others. Note that any two of these minerals can occur together only under P-T conditions represented by the boundary lines in the diagram and that the three minerals can occur together only at the point where these three lines intersect: approximately 500 ° C and 4 lcilobars, which normally occurs about 15 km below Earth's surface. 1. Study the mineral zones and isograds on the two maps in Fig. A7.5.3. Which region was metamorphosed at higher pressure? How can you tell? 204

Igneous intrusions Subducting sediments are metamorphosed due to increase in pressure and temperature. Shallow crustal rocks are metamorphosed by heat emanating from a nearby magma body. Rocks buried in a large sedimentary basin may encounter low-grade metamorphic conditions near the bottom of the pile. NN N N . \ \` \ \ \ \ N N"'• s 's \ \ N \ \ \ N \ \ N NN N NN' s ' N NN• - \ N ,4 4 600°C7 900 0 0- 12CO'C Rising magma transports heat to Earth's upper crust causing an increase in the geothermal gradient. 41 .1 11111111111 . 111 •1111 ' 300° 600°C 900°C 1200°C Subsiding basin 1 - — — — -300 9 GL 2. REFLECT & DISCUSS What is the significance of the gray area of the P-T diagram in Fig. A7.5.4 that is marked by a large question mark? Hint: Where on or in Earth would you find the P-T conditions in that gray area, if anywhere? D At the time that Pentti Eskola published descriptions of these metamorphic facies in the 1920s, the plate tectonics model had not yet been developed. Geologists now realize that volcanic arcs develop at convergent plate boundaries where the oceanic edge of one plate subducts beneath the continental edge of another plate. 1. In the block diagram of a subduction zone (Fig. A7.5.5), place letters in the white circles that are linked to the starred locations on this illustration to show where Eskola's facies are most likely to occur: A = amphibolite, G = greenschist, E = eclogite, B = blueschist. Low geothermal gradients are observed in subduction zones because cold oceanic crust and overlying sediments are descending into the mantle. Figure A7.5.5 2. Discern whether the geothermal gradient (rate of change in temperature with depth) within the subduction zone is similar or dissimilar to the gradient in the middle of the volcanic arc and describe the results of your comparison in the space that follows. E REFLECT & DISCUSS The part of the Earth system comprised of solid (mostly) silicate rock begins at Earth's ground surface and extends down to the boundary between the mantle and core, about 2890 km below the surface. In a sentence or two, explain which of the three fundamental types of rock (igneous, sedimentary, metamorphic) forms most of the volume of Earth's crust and mantle. Share your explanation with your classmates. Hint: Look back at Figs. A7.5.4 and A7.5.5, and remember that very little of the volume of the crust or mantle is liquid magma. 206