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 or .
- If the rock is foliated, next determine the type of foliation:
- 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 foliation give it a shiny, wavy appearance and any sheet-like or elongate minerals will be aligned in a preferred orientation.
- foliation, or gneissic banding, is defined by segregated bands of light-coloured quartz and feldspars and dark-coloured minerals.
- If the texture is massive, test the sample with a drop of dilute HCl. reacts with HCl just like its protolith – limestone!
- If the sample is massive and does not react with HCl, try testing the hardness. 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 .
- 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 (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
|Name of protolith||Name of metamorphic rock||Observations (how does the metamorphic rock differ from its protolith?)|
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.
|Changes in mineralogy||Changes in texture|
|Shale to slate
(R181 to R301/R302)
|Slate to schist
(R301/R302 to R321)
|Schist to gneiss
(R321 to R331/R332)
3. This progression of foliated metamorphic rocks from slate to gneiss is typical of mudrocks that are metamorphosed during with a typical . 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.
|Sample #||Approx. temperature (°C)||Approx. Depth (km)||Metamorphic Grade (low, intermediate, high)|
4. Which two samples in your Rock Kits 1 and 2 represent non-foliated (or ) 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:
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.
|Order||Rock Name||Sample #||Rock Cycle Processes Responsible|
|1||Granite||R1||Partial melting of pre-existing rock to generate magma. Felsic magma cools and crystallizes at depth to form granite, a felsic intrusive igneous rock.|
|2||Quartz sandstone||R161||Granite is uplifted to surface, chemically and mechanically weathered, and eroded. Sand-sized grains of quartz, feldspar and some ferromagnesian minerals are transported. Feldspars and ferromagnesian minerals chemically weather to form clay minerals and ions in solution. Quartz grains become more rounded and better sorted with transport. Eventually quartz grains are deposited in a moderate to high energy environment (depending on grain size). Grains of quartz are lithified into sandstone through burial by other sediments, compaction, and cementation by mineral(s) precipitated from a fluid.|
|3||Quartzite||R351||Quartz sandstone is metamorphosed through regional or contact metamorphism. Grains of quartz recrystallize into coarser grains to form a crystalline texture.|
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.
|Order||Rock Name||Sample #||Rock Cycle Processes Responsible|
|1||Basalt pebbles (sediment)||N/A||
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 rocks. The words “light”, “intermediate”, and “dark” refer to the colour (and therefore composition) of aphanitic igneous rocks.
the texture of a metamorphic rock with a foliation
the texture of a metamorphic rock that is not foliated
Type of foliation defined by closely spaced, flat surfaces along which a slate splits. Formed by the growth of microscopic mica minerals.
Type of foliation defined by scaly layers of visible mica minerals or other platy or elongate minerals. Rocks with this texture appear shiny or sparkly, as the light glints off cleavage planes of the aligned minerals.
Type of foliation defined by segregation bands of light and dark coloured minerals in a gneiss.
Referring to a silicate mineral that contains iron and or magnesium.
metamorphosed limestone (or dolostone) in which the calcite or dolomite has been recrystallized into larger crystals
a non-foliated metamorphic rock formed from the contact or regional metamorphism of sandstone
a foliated metamorphosed rock (typically derived from basalt) in which the green colouration is derived from either chlorite, epidote or green amphibole
the original, un-metamorphosed parent rock from which a given metamorphic rock is formed
metamorphism caused by burial of the parent rock to depths greater than 5 kilometres (typically takes place beneath mountain ranges, and extends over areas of hundreds of km2)
The rate of increase of temperature with depth in the Earth (typically around 30˚ C/km within the crust).
a rock composed of a single mineral