Geology 101 Name(s):

Lab 6: Sedimentary and metamorphic rocks Identifying sedimentary rocks Sedimentary rocks can be classified in a number of ways. For our purposes, the first division to be made is between clastic sedimentary rocks (those that are made of weathered and eroded grains) and non-clastic or other sedimentary rocks (these include sedimentary rocks of biological and chemical origin). Flow chart for identifying sedimentary rocks — If the rock is made of grains or other materials which have been deposited by wind, water or ice, or else was generated by biological or surface chemical activity, it's a sedimentary rock. First step. If the rock is made of broken up bits of rock (including extremely fine grains) → GO TO Second step alternative A. (Clastic rocks)

Else → GO TO Second step alternative B. (Chemical or biological rocks)

Second step alternative A. Consider the most common grain size in the rock from the following list. cobble or

pebble > 2 mm easily visible to naked eye; "grains" may

contain identifiable minerals sand 0.062 — 2 mm visible to naked eye silt 0.005 — 0.062

mm not visible but can be felt between fingers or across teeth

clay < 0.005 mm not visible; cannot be felt between fingers or across teeth

If the most common grain size is cobble or pebble → conglomerate If the most common grain size is sand → sandstone (arenite) If the most common mineral is quartz → quartz arenite If the rocks is medium gray to red and well-sorted → arkose

If the rock is dark-colored and has much fine grain-size material in its matrix → greywacke

If the most common grain size is silt or clay or a combination of both:

If the rock splits into irregular or regular layers → shale If the rock is massive (no layering) → mudstone The terms siltstone and claystone are also used on occasion.

Second step alternative B. Identify the most common mineral in the specimen (use mineral ID chart if necessary). If the most common mineral is quartz → chert If the most common mineral is halite → rock salt

If the most common mineral is gypsum → rock gypsum If it is black-colored, not very dense and flaky → coal

(also look for plant fibers)

If it fizzes, the most common substance is calcium carbonate, usually in the form of the mineral calcite (be careful you are not fizzing the cement)

If the rock is not very dense and pure white → chalk If the rock is made of broken-up shells → coquina If the rock is dense and white, gray or black → limestone One other thing: fossils (Latin for “dug up”) are the remains of living organisms. If the fossil is literally the body of the organism (or parts such as skeleton or shell), it is called a hard parts fossil; if the fossil merely records the shape of an organism (like a leaf impression in silt) or the passage of an organism (like preserved footprints), then it is called a trace fossil. For any sedimentary rock, if it contains any fossils, use the adjective fossiliferous in front of the rock name. Needed: sedimentary rock samples R18 – R27 (Tubs 20 – 29), R33 (Tub 35) and S1 (Tub 36) 1. Fill in the following table for clastic sedimentary rocks. Begin by determining the average grain size of the clasts in the rock (use the grain size terms in the flow chart), then the grain sorting (the choices are: well-sorted, moderately sorted, poorly sorted and unsorted) and the grain roundness (the choices are; well-rounded, sub-rounded, sub-angular and angular). See the diagrams to determine which type of rounding and sorting the grains have. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. Grain sorting: Grain rounding:

Clastic sedimentary rocks Sample # Grain

size Grain

sorting Grain

roundness Fossils Rock name

R18

R19

R20

R21

R22

2. Fill in the following table for “other” sedimentary rocks. Begin by determining the rock’s mineral composition. Then add any other details that help identify it. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. “Other” (chemical and biological origin) sedimentary rocks Sample # Mineral

composition Other

defining details

Fossils Rock name

R23

R24

R25

R26

R27

Sedimentary rock properties and depositional environments 3. Return to R18 and R19 and circle the correct answers: a. Which rock contains the most stable mineral clasts? R18 R19 (at the Earth's surface) b. Which rock is composed of rounder grains? R18 R19 c. Which rock is more well-sorted? R18 R19 d. Based on a-c, which sample was deposited furthest from its source (and thus is called mature)? R18 R19 4. The energy of the system (how much force is behind the medium of transport (air or water)) can be characterized by the size of the particles the system can carry. For instance, high-energy systems can carry large grains; low-energy systems can carry small grains. Examine and rank rocks R18, R21 and R20 in order from highest energy to lowest energy depositional system. 5. a. Some limestones (R33) are dense, fine-grained and black. So is basalt (R5). What test can you perform to tell them apart, and how does each behave in the test? b. Which rock has fossils? By the way, in general, why didn’t you worry about fossils in igneous rocks? The place in which the sediment is deposited or the organisms lived is called the depositional environment. Examples of depositional environments include terrestrial environments (like lakes, deserts and rivers), transitional environments (like beaches and tidal flats) and marine environments (like continental shelves and the abyss). Note that, over time, a beach area may be uplifted by plate tectonics so that you will find a transitional depositional environment quartz-rich sandstone deep in a mountain range!

6. In what depositional environment did rock R25 form (see diagram above)? Hint: these kinds of rocks are called evaporites. Explain how they form. 7. Look at sedimentary structure S1, which is an example of preserved ripple marks. Are they symmetrical or asymmetrical? Based on that answer and on the wavelength of the ripples, is it more likely that these ripples were originally deposited in a desert, a river, or a tidal flat? How are they preserved so that you can see them today?

Properties of metamorphic rocks Metamorphic rocks have been subjected to sufficient heat and/or pressure to melt some of their constituent minerals, but not all of them. As a result of this selective mobilization of chemicals, only certain chemical reactions can occur, and so a whole new set of metamorphic minerals are crystallized. Throw in the presence of fluids such as water and carbon dioxide (yes, at these pressures, even carbon dioxide can be a liquid), and nature has the means to create even more metamorphic minerals and therefore metamorphic rocks. Note that metamorphic rocks must be formed at depth; metamorphism is not a surface process, and so is distinguishable from mere sedimentation. Rocks that have foliation (a sort of wavy layering, though it can resemble horizontal layering) are metamorphic rocks; the foliation indicates that directional pressure was applied to the rock while the mineralogical changes were occurring. On the other hand, some metamorphic rocks are not foliated; they appear crystalline, like coarse-grained igneous rocks. These metamorphic rocks were subjected to isotropic, or nondirected, pressure. Because there are so many metamorphic minerals (of which you have seen but a few), there are all sorts of ways to name metamorphic rocks. We will concentrate on naming rocks by their metamorphic grade (that is, by the maximum degree of heat and pressure they were subjected to, and not their mineral composition), or, in some unusual cases, by their apparent composition (for instance, rocks like marble, quartzite or metaconglomerate, from which you cannot determine the metamorphic grade). The protolith of a metamorphic rock is the original rock that was metamorphosed into what you see today. As you can see from Table 6.1, the protolith’s minerals really do determine the resulting metamorphic rock’s composition. Note the differences in mineralogy even at the same grade. Table 6.1— Mineralogy of metamorphic rocks related to protolith and grade Metamor- Facies Protolith phic grade

Basalt Shale

Zeolite Calcite, chlorite, zeolite Zeolite, sodium-rich micas

Low

Greenschist Chlorite, amphibole, plagioclase, epidote

Chlorite, muscovite, plagioclase, quartz

Medium Amphibolite Amphibole, garnet, plagioclase, quartz

Garnet, biotite, muscovite, quartz

High Granulite Pyroxene, plagioclase, garnet

Biotite, orthoclase, quartz, andalusite

A metamorphic facies is a name of a set of metamorphic minerals which is uniquely created at a particular pressure and temperature. So, in addition to a metamorphic grade, a rock can belong to a particular metamorphic facies as well! Confused? You bet! However, realize that these terms all have their uses. Note that not all minerals in a given cell in the table above will show up in every specimen of that grade/facies/protolith, but all minerals in the specimen will be named in the cell! Metamorphic rock identification Needed: Samples M18 and M 19 (Tub 37), R34 through 45 (Tubs 38 – 49) 8. Some minerals are made under metamorphic conditions. You have seen a few already in Lab 3 (for instance, talc and graphite). Identify these two other metamorphic minerals: Mineral # Distinguishing features (color, cleavage,

hardness, magnetism, density, etc.) Mineral name

M-18

M-19

9. Look at rock sample R34, a regionally-metamorphosed shale. a. Name two minerals in R34 (hint: you did this already in Lab 3) b. Given that muscovite is present in R34 but hard to see, what grade of metamorphism does this mineralogy imply (use table 5.1)? c. Still using that table, what metamorphic facies is R34? d. So what is the name of the rock? To find this, see the diagram on the next page, or table 7.1 (page 145) in the text.

One other consideration: there are three different types of metamorphism, related to the particular tectonic setting of the metamorphism. As you are aware, the deeper rocks are drawn into the lithosphere, the higher the temperatures and pressures the rocks are subjected to. This is called regional metamorphism. However, there are two other sets of conditions. Convergence-type metamorphism occurs under high-pressure but low-temperature (high P, low T) conditions. Contact metamorphism occurs under high-temperature but low-pressure (high T, low P) conditions. This means that, depending on the tectonic setting, three different metamorphic rocks could arise from the same protolith. Table 6.2 summarizes these types. Table 6.2 — Mineralogy of metamorphic rocks related to protolith and grade

Protolith Meta. type

Facies Basalt Shale

Regional See table 5.1 Convergence (low grade)

Blueschist Blue amphibole, chlorite, Ca-silicates

Blue amphibole, chlorite, quartz

Convergence (high grade)

Eclogite Pyroxene, garnet, kyanite

not observed

Contact Hornfels Pyroxene, plagioclase Andalusite, biotite, orthoclase, quartz

One way that metamorphic petrologists try to quantify the conditions of metamorphism for various rocks is to draw a pressure/temperature (P/T) diagram as shown in the figure below. The field of the graph shows the ranges of various metamorphic facies. The vertical axis shows the depth of the metamorphism and the equivalent pressure in kilobars (kb). 1 bar is approximately 1 atmosphere

of pressure, and therefore 1 kb is about 1000 atmospheres of pressure. The horizontal axis shows the temperature of the metamorphism in degrees Celsius.

10. a. Use the facies from question 12c to determine the range of possible maximum pressures and the range of possible maximum temperatures at which R34 formed. Use units of °C for temperature and kbar for pressure. b. Suppose another area where the protolith was found was subjected to less than 1 kbar of pressure but the same temperature range during metamorphism. Name one mineral (besides the ones you named in part a) you would expect to find. As you have seen, some minerals are quite useful in determining the grade or type of metamorphism because they can only form under certain metamorphic conditions. These are called index minerals.

11. You are given the following information about a metamorphic rock:

Mineral composition: pyroxene, garnet, kyanite Chemical composition: silicon dioxide 50.24%, aluminum oxide 13.32%, calcium oxide 10.84%, iron oxide 9.85%, magnesium oxide 8.39%

Which type of composition is more useful in determining the grade and protolith of metamorphism and why? Or do both lists give equivalent information? 12. a. Now look at R35, which is the same metamorphic grade as R34. What are the mineralogical differences? (In other words, what minerals show up in R34 but not R35? In R35 but not R34?) b. But what is the name of this rock, anyway? Hint: kind of a trick question. 13. In fact, for many metamorphic rocks, the most common mineral in the rock is used as an adjective in front of the rock name. Fill in the appropriate mineral name for the samples below, using the suggested test given: Sample # Test Rock name R34 Cleavage _____________ schist R35 Obvious mineral _____________ schist R36 Color _____________ schist R37 Scratch _____________ schist

Protolith

Intensity of metamorphism Low grade High grade

shale slate phyllite rhyolite schist granite gneiss basalt amphibolite limestone marble sandstone quartzite conglom. metaconglomerate 14. What changes in foliation thickness and mineral grain size would you expect to see in a shale as it is subjected to greater temperatures and pressures during metamorphism? (Hint: compare, in order, R38, R39, R34, R40) 15. So fill in the following rock names, using your answer to part a and the fact that each sample represents a different metamorphic grade: Sample # Metamorphic grade Rock name R38

R39

R40

16. R41 and R42 are nonfoliated metamorphic rocks (the text calls them “granoblastic rocks”); both of these rocks achieved the same grade of regional metamorphism as R34 and R35 did. Identify the rock names using the hints suggested in the characterization column; identify their protoliths from the table on the previous page. Sample # Characterization Rock name Rock protolith R41

Glass plate

R42

Acid bottle

Plate Tectonics and Metamorphic Rocks 17. a. R43 is blueschist, a unique type of metamorphic rock that forms under conditions of high pressure and low temperature. Label the area on the cross-section on the next page where you might expect blueschist to crystallize. b. So, if you were to find blueschist as you walked along the Appalachian Trail in North Carolina, what could you infer about the history of the East Coast of the US? 18. R44 is serpentinite, which blueschist often becomes over time. A key mineral in blueschist is forsterite, a form of olivine, with the chemical formula Mg2SiO4. A key mineral in serpentinite is (surprise) serpentine (chemical formula: Mg3Si2O5(OH)4). How does serpentinite form from blueschist? (Hint: consider readily available simple molecules at metamorphic depths and the difference between the two chemical formulae) 19. a. R45 is hornfels, a unique type of metamorphic rock that forms under conditions of low pressure and high temperature. Label the area on the cross-section where you might expect hornfels to crystallize. b. What is hornfels' protolith? Or is there a unique protolith? c. Why is contact metamorphism such an appropriate term for this type of metamorphism?