Lab 6 Exercises

Part I: Metamorphic Rocks

The exercises below will guide you through the metamorphic rock samples in Rock Kits 1 and 2. Review the background information presented in Chapters 6.1 and 6.2 before you begin these exercises. You may wish to consult the Rock Classification Tables at the back of this manual as you complete the exercises below.

Tips for Classifying Metamorphic Rocks

• Your first step when examining a metamorphic rock is to determine if its texture is foliated or massive.
• If the rock is foliated, next determine the type of foliation:
  • Slaty foliations are flat, smooth surfaces along which a slate breaks. They might have a slightly shinier lustre than a shale.
  • Larger crystals of micas that define a schistose foliation give it a shiny, wavy appearance and any sheet-like or elongate minerals will be aligned in a preferred orientation.
  • Gneissic foliation, or gneissic banding, is defined by segregated bands of light-coloured quartz and feldspars and dark-coloured ferromagnesian minerals.
• If the texture is massive, test the sample with a drop of dilute HCl. Marble reacts with HCl just like its protolith – limestone!
• If the sample is massive and does not react with HCl, try testing the hardness. Quartzite is composed predominantly of quartz, giving it a hardness ~7.
• Lastly, if the metamorphic rock is foliated but has a distinctly green-colour to it, and contains abundant ferromagnesian minerals chlorite and green amphibole, it is called a greenschist.
• Figure E and Table D in the Rock Classification Tables appendix may be helpful resources as you complete these lab exercises.

1. The best way to really appreciate the metamorphic changes to a rock is to meet its parent! Examine each of the sample pairs listed below. Each pair contains a metamorphic rock and its protolith (parent rock). Identify which of the two samples is the metamorphic rock, and then carefully compare the two. In what ways are the metamorphic rock and its protolith similar? In what ways do they differ? Record your observations in the table below.

• Pair A: R221 and R361
• Pair B: R301 and R181
• Pair C: R1 and R331
• Paid D: R351 and R161

2. Examine samples R181, R301, R321, R331, and R332. These samples show the progression of changes from a protolith (shale) to a high grade metamorphic rock (gneiss). Complete the table below by recording the changes you observe in mineralogy and texture with increasing metamorphism. For the changes in mineralogy, record what new minerals you see and note any minerals that have disappeared. For the changes in texture, look for changes in grain size or the development of a foliation.

3. This progression of foliated metamorphic rocks from slate to gneiss is typical of mudrocks that are metamorphosed during regional metamorphism with a typical geothermal gradient. Using Figure 6.1.6 and Figure E in the Rock Classification Tables as a guide, complete the table below to summarize the range of temperatures and depths (pressure) responsible for the metamorphism of samples R301, R321, and R332.

4. Which two samples in your Rock Kits 1 and 2 represent non-foliated (or massive) metamorphic rocks?                                                                   

5. Do these two samples exhibit crystalline or clastic textures?                                       

6. Examine sample R351. Try to scratch this sample with the tools from your mineral ID kit. How hard is this sample?                            

7. Based on your answer above, and any other physical properties you observe, name the main mineral(s) present in this rock:                                                                                                                  

8. Examine sample R361. Try to scratch this sample with the tools from your mineral ID kit. How hard is this sample?                            

9. Place a small drop of HCl on a fresh surface of the sample. What happens?                                                                                                  

10. On the basis of these two tests name the main mineral present in this rock:                                         

11. Metamorphism may affect the texture or the mineral composition or both of these properties of the protolith. Do samples R351 and R361 have the same mineral composition as their respective sedimentary protoliths? Do they have the same texture? Explain your answers.

Part II: The Rock Cycle

The exercises below are a review of the rock cycle processes by which one type of rock is transformed into another over geological time. You will review all the rock samples in Rock Kits 1 and 2 that you have examined to date in preparation for Test 2. Review the background information presented in Chapter 6.3 before you begin these exercises. To benefit the most from these review exercises, remove all your rock samples from the Rock Kits and set the empty kits aside, so that you cannot see the names of the rocks.

All rocks are connected through the rock cycle. Any rock that you see today will at some point in the future be transformed into a different rock through the rock cycle (see Figure 6.3.1). This exercise focuses on the processes that are responsible for the transformation of one rock type into another. As a starting point, summarize the main processes involved in the formation of the three main categories of rock:

• Igneous:

• Sedimentary:

• Metamorphic:

The example presented below illustrates how you should complete question 12, by explaining the processes that formed the rocks in the second column. This example begins with granite, and the processes responsible for the formation of the granite are explained first. Then, the processes that explain how granite is transformed into quartz sandstone are outlined, and so on.

12. For each of the rocks specified below, find the corresponding rock sample from your collection of rocks. Remember, for this to be an effective review activity, do this without looking at the rock names in the kit! Complete the table with the appropriate sample number, and explain the processes from the rock cycle that have transformed the previous sample into the present sample. Begin by explaining how pebble-sized clasts of basalt formed, and then how shale could form from those basalt pebbles, and so on.

Part III: Rock Review

13. Use the flow chart below to review igneous, sedimentary, and metamorphic rocks from Labs 4-6. This review will help you prepare for Test 2. The words “fine”, “medium”, and “coarse” on the flow chart refer to grain size. Chemical formulae are listed for any monomineralic rocks. The words “light”, “intermediate”, and “dark” refer to the colour (and therefore composition) of aphanitic igneous rocks.