Petrology Questions

Questions for lecture tests will be selected out of the questions below. New questions will be added to this page after each lecture is given.

It will certainly make your preparation for lecture tests much more efficient and effective if you prepare full answers to these questions after each lecture, before the next lecture comes. You may check with your instructor to see if your answers are sufficient or not. The second best strategy is to do all the questions that come out in a week's time during the weekend. A strategy that just will not work well at all is to study these questions only the night before the exam--there simply is too much material.

3. What is the difference between mafic magmas and felsic magmas in terms of silica content?

4. You should be prepared to contrast mafic and felsic magmas with regard to any of the factors (viscosity, temperature, silica content, and (0H) content) on the handout of the Bowens Reaction Series that I gave. Questions will be like:

5. Regarding the following other elements (expressed here as oxides), which magma type, mafic or felsic, typically has higher amounts.

7. What is the relationship between polymerization of Si-O bonds in an igneous melt and viscosity? Which kind of melt, mafic or felsic, will have more Si-O polymerization?

8. An igneous dike and a hydrothermal vein can be in size and geometry be similar. But they are very different in origin. What is the difference between a true igneous rock and rock from a hydrothermal deposit?

12. What do we think are the main reasons that magmas rise? {answer should concentrate on density of molten state verse solid state -- If this question is asked on a test, this part in […] will not be on the test question].

13. What is the difference between the way we think magmas rise in the lower crust as compared to rising though the upper part of the crust? And what is the main reason for this difference?

14. Which type of magma, mafic or felsic, is more likely to produce obsidian in a lava flow eruption? Why?

15. Regarding properties of magmas (e.g. H₂O-content, viscosity, temperature, etc.) what are the two primary factors that govern whether or not a magma is likely to erupt explosively or erupt as a lava flow? Explain the interplay of these two factors in this regard.

16. The volcanic rocks of the “Catoctin Formation” in Virginia are predominantly aphyric (that is they lack phenocrysts). Many of the lava flow rocks that compose the volcano, Mt. Shasta, in California have abundant phenocrysts. What is a difference in the volcanic processes that operated in these two areas that can be interpreted from these observations?

17. What are some differences between the rocks that solidify in the central part of a lava flow as compared to the upper part of the same flow (that is, typically)?

18. What is the origin of “vesicles” in a lava flow rock? In what part of a lava flow are these more likely to occur? [NOTE: This question has overlap with Q 17.]

20. Sills that intruded a sequence of sedimentary rocks can appear very similar in gross aspects to lava flows that erupted in a terrane of flat-lying sedimentary rocks and that were later buried by sediment. Describe some features that might be found in outcrops that would distinguish between these two situations.

21. In areas with sequences of lava flows, what parts, if any, commonly have sufficient porosity for holding and transmitting groundwater? In your answer comment on both vertical and horizontal groundwater flow with respect to lava flows.

1. Explain how viscosity and water content interact to produce explosive volcanism.

2. Why does the strength of an explosive eruption increase after the inception of the eruption in many cases?

4. Name the 5 most common constituents of pyroclastic rocks and tell briefly the origin of each.

(Your answer will explain a little about lithic fragments, crystals, glass shards & dust, pumice, and obsidian lumps. The question could be asked as “Name the 5 most common constituents of the ejecta from pyroclastic eruptions.… Or a question about each of the basic ingredients could be asked individually.)

5. In explosive volcanism, why do we usually see a large cloud above the eruption column? What is this cloud made of and how is it formed?

6. One of the deposition mechanisms associated with explosive volcanism that we discussed tends to produce a degree of size gradation in the deposit that is formed (i.e. the deposit is somewhat sorted). Which mechanism produces this, and why is there a size segregation?

7. At least two factors can affect what size of pyroclastic material will be deposited in a single layer of a pyroclastic air-fall deposit. Explain these two factors. [Note your answer should deal with eruption strength and distance from eruption center..]

8. Describe basically what happens to start an “ash flow” and what an ash flow is like. (The question here is in relation to the ash flow event itself, not to the ash-flow deposit.)

9. One of the deposition mechanisms associated with explosive volcanism commonly leaves a hard dense rock. Which mechanism produces this? Why is the resulting rock hard and dense?

10. Describe a typical single ash-flow deposit in terms of a general “stratigraphy” of the deposit.

11. What kind of deposit is more likely to lead to the frequent mudslides associated with certain volcanic terranes, air fall deposit or ash flow deposit? Why?

12. How can one estimate the total volume of material ejected from a volcano in a single eruption event?

13. What is a “eutaxitic” texture? What produces it, and what kind of volcanic eruption is it characteristic of?

14. What rock constituent is the best indicator that a particular layer of rock was of pyroclastic origin?____

15. What magma type is most likely to produce a lava dome rather than a lava flow? Why does this happen? [I mentioned the lava domes with the silicic eruptions of the Jemez Volcanics in NM. I don’t think I spoke why there are lava domes. If you can’t figure it out, there is a discussion in the text.]

1. In most suites of genetically related igneous rocks, there are common chemical trends that are systematic. For most of the igneous rock suites, as silica increases there will be a (an)…

2. What are the two main field conditions that indicate that certain rocks are probably a “suite” of genetically related igneous rocks? (We discussed two basic factors—although other factors could be brought into the discussion).

3. Within the spectrum of igneous rock compositions, which types can be found as extrusive (volcanic), and which type is, with only very rare exception, almost never found as an extrusive, almost only as plutonic? Indicate with a check the type that almost never is volcanic (suggesting no magmas of that composition).

4. There are a lot of igneous rocks (both volcanic and some plutonic rocks) that occur in southwest Colorado , in a mountain range called the "San Juans". These are dated to be from a limited time period (Miocene in age). If we want to chemically model the igneous system that formed these rocks, what would be the best thing to analyze to represent compositions of individual magmas that had occurred in this system?

5. In a volcanic rock, what would indicate to us what the early-forming minerals were in the magma that formed the rock?

6. A plutonic rock has clinopyroxene + plagioclase + olivine. In several places you see plagioclase included inside clinopyroxene crystals. What can you say about the order of crystallization in this rock? (Tell only what you can say with a measure of certainty).

7. What is the basic “rule” in applying euhedral/anhedral relations between minerals in a plutonic rock in determining the order of crystallization of the magma?

8. A particular granite has plagioclase + alkali feldspar + quartz + biotite. All the quartz is anhedral in this rock. Some biotite is found as inclusions in alkali feldspar; also there are some biotite inclusions in a few quartz grains. A few euhedral outlines of biotite are found where it is in contact with quartz. Plagioclase crystals are mostly euhedral, except where they are in contact with one another. Where alkali feldspar is in contact with plagioclase, it is anhedral, but partial euhedral outlines of alkali feldspar are found against quartz grains. What can you say about the order of crystallization in this rock? (Tell only what you can say with a measure of certainty).

NOTE: Questions like #6 & 8 if included on the test could be different, but in the same principle.

9. Which feldspar mineral is the feldspathoid, nepheline, similar to in composition? Which of the two has more SiO₂?

10. Which feldspar mineral is the feldspathoid, leucite, similar to in composition? Which of the two has more SiO₂?

11. The minerals that constitute several rocks are listed below. Which rocks of these would be classified as silica saturated, which as silica oversaturated, and which as silica undersaturated? Put saturated, undersaturated, or oversaturated in the blank for each rock. [On the test, some mineral combinations could be different].

12. If Dr. Kline brought in a rock, telling us that it has plagioclase, quartz, hornblende and olivine in it, what about his statement would make you suspicious about his ability to identify minerals?

Lecture 4. Thursday, January 26, 2006
Magma Chemistry: Melting Behavior Illustrated by Two-Component Phase Diagrams

1. Be able to answer questions about a 2-component phase diagram of a eutectic melting system such as the ones you did for the “homework” exercise.

2. What is the chemical term we use for the composition of the initial melt in an igneous system such as we discussed in class? __________________

3. Extrapolating what we learned from the 2-component phase diagram of melting behavior, if there are two rocks, each composed of 4 minerals, what statements below are probably true about melting behavior for these rocks and which statements are probably false. In this example, ignore any possible effects from solid solution. The rocks are as follows:

Rock A: 15% hornblende, 25% clinopyroxene, 40% plagioclase, 20% alkali feldspar.

Rock B: 25% hornblende, 10% clinopyroxene, 25% plagioclase, 40% alkali feldspar.

_____________If each rock is melted separately, the initial melt for both of the rocks will be the same; each will have a composition with 25% of each of the four minerals in it.

_____________ If each rock is melted separately, the initial melt for each of the rocks will be the different: Rock A will have a melt composition of 15% hornblende, 25% clinopyroxene, 40% plagioclase, 20% alkali feldspar. Rock B’s melt will be 25% hornblende, 10% clinopyroxene, 25% plagioclase, 40% alkali feldspar.

_____________ If each rock is melted separately, the initial melt for both of the rocks will be the same; however, without experimental data, we cannot say just what that composition will be.

_____________ If each rock is melted separately, the initial melt for each of the rocks will be the different; however, without experimental data, we cannot say just what the two compositions will be.

4. What kind of rock probably melts to form basaltic magmas? Do we think it fully melts or partially melts? Where in the earth do we think this melting occurs?

5. What combination of three minerals that commonly occur together in rocks of the earth has the lowest melting point? Name a couple of rock types from deep in the crust that commonly have these minerals occurring together.

6. What kind of rock probably melts to form granitic magmas? Do we think it fully melts or partially melts? Where in the earth do we think this melting occurs?

7. Granite is the rock that forms from magmas near the high silica end of the compositional spectrum of common magmas. There are a large number of granites that have very similar compositions. Is it necessary that these all formed from melting of the same rock type? Explain your answer.

1. In the IUGS system for igneous rock classification, what is the main thing about the rock that must be determined in order to come up with a rock’s name?

2. [Indirectly related to rock classification:] In igneous rock suites, two of the most important chemical trends that characterize chemical variation (as SiO₂ increases) is the trend if decreasing CaO and increasing Na₂O + K₂O. There are two minerals in igneous rocks that strongly reflect this chemical trend, and these two minerals are part of the basis of the classification system for igneous rocks. What are these two minerals?

3. Why does the IUGS igneous rock classification scheme for volcanic rocks have two composition triangles (one apex-up, and one apex-down)? [The plutonic rocks also have two similar triangles].

4. If a rock does not have any quartz or feldspathoids, from where on the IUGS diagrams do we obtain the rock’s name? (Assume the rock does have feldspars).

5. Both andesite and basalt occupy the same “pigeon hole” in the IUGS classification scheme. How can we distinguish which of the two names is appropriate for a particular rock that is classifiable in that pigeon hole? [Answer is on the handout I gave with the rectangular version of the IUGS classification scheme.]

1. It is common to see regular trends in rock composition in many suites of genetically related igneous rocks. What is the name of the most common and fundamental chemical process that is thought to occur to cause chemical variation expressed in these trends?_____________________

2. If minerals are forming (crystallizing) from an igneous melt, what must happen in order for "fractional crystallization" to occur?

3. If a mineral is "fractionated" from a melt, what is the principle that governs whether a particular element will increase in the residual melt, or whether the element will decrease in the melt?

4. There is at least one element that would be increased in a residual melt if almost any of the minerals common to mafic igneous magmas were "fractionated" from a melt. What is that element (name it as an oxide)? Also, explain why that element would increase.

5. In igneous rock suites, calcium (Ca) normally __________(increases, decreases) along with increasing SiO2, and sodium (Na) normally __________(increases, decreases).

What common igneous mineral could be fractionated from a melt and produce both these trends?

7. In igneous rock suites, magnesium (Mg) normally __________(increases, decreases) along with increasing SiO2, and iron (Fe) normally __________(increases, decreases).

Using the fractional crystallization “model” (concept), explain the common trends in magnesium and iron. Be sure to tell what mineral or minerals might be involved.

8. Considering common igneous rock suites, what happens to the ratio of Mg/(Mg+Fe) in rocks of the suite that have higher SiO₂? __________________ (increases, decreases).

Using the fractional crystallization “model” (concept), explain the common trend in magnesium to iron ratio. Be sure to tell what mineral or minerals might be involved.

9. The most common progression in magmatic activity in continental settings (based on age relations between rocks of an igneous province) is: ______

a) magmatic activity begins with mafic magmas and later felsic melts start to come in

b) magmatic activity begins with felsic magmas and later mafic melts start to come in

e) almost all magmatic activity is intermediate in composition with little or no mafic or felsic

10. Explain a little what is involved in producing the sequence you identified in question 9 above.

11. What is the concept of magma mixing for producing some of the variableness of igneous rocks in suites of genetically related igneous rocks?

12. If I showed a diagram plotting the amount of some element (e.g. CaO) against SiO₂ for one magma and another point plotting the same element vs SiO₂ for another magma, be able to indicate in principle where a point for a mixture of the two would plot (as in “magma mixing”). [This will be some were on a line of mixing between the two points.]

13. Conceptually and chemically, magma mixing seems to be a reasonable process that might act to produce some of the regular variation in the detailed chemistry of igneous rock suites. Explain some physical evidence that seems to indicate that magma mixing does indeed take place.

[I recommend that you explain by using the observation of bi-modal populations of plagioclase phenocrysts observed in some andesites as we mentioned in class. That is easier to explain than the erratic zoning we went over so quickly.]

14. What is “crustal contamination” (or “wall-rock assimilation”) and how basically is it thought to work to cause chemical variation in magmatic suites?

15. Which situation below is more likely to be affected by “wall-rock assimilation”?_____

Lecture 7: Thursday, February 9, 2006
Characteristics of Igneous Intrusive Bodies

1. What kind of magma compositions more commonly form sills when they intrude the shallow crust, mafic or felsic? Why?

2. What is the difference in shape between typical large-volume mafic plutons vs large-volume felsic plutons? (Give also the names commonly used for these two kinds of plutons).

3. Give the names of two “world-class” mafic-ultramafic lopoliths. [I could have a question, instead, where I mention the names and you are to recognize that they are famous lopoliths. Of course, I would use names that we discussed in class.]

4. What kind of observations in certain large mafic lopoliths led researchers to believe that early-formed minerals had settled out of the magma as they crystallized? Describe the evidence.

[NOTE: Your answer should include something about both sedimentary structures (such as bedding, cross-bedding, and channel structures) and sedimentary rock textures (the “cumulus” texture that looks like minerals had accumulated by sedimentary process). Note that with regard to cumulus textures, I do not expect you to be able to describe cumulus textures in detail, but I want you to know that the layered rocks in these complexes have textures that look as though minerals had accumulated by sedimentation and that this texture is called “cumulus texture”].

5. What is meant by the term “cumulus crystals” vs. “intercumulus crystals” in discussing rocks of mafic/ultramafic lopoliths. [NOTE: If I did not make it clear in class, the cumulus crystals are the ones that appear to have accumulated by sedimentation, and the intercumulus crystals are the minerals that crystallized in the interstices between the cumulus crystals.]

6. Explain how some observations in certain kinds of plutons seem to lend supporting evidence for the process of fractional crystallization in magmas. [Note: Here you could relate some of the same things as in question 4 to the concept of fractional crystallization.]

7. Tell me, in general, which part of a typical mafic lopolith commonly has the following rock types (choices are: [a] in the upper part, [b] in the lower part). [See layered complex stratigraphy handout.]

8. In what part of mafic/ultramafic igneous plutons (lower part or upper part) are rocks with minerals that have the highest temperature stability found? [Note: by highest temperature stability, we mean minerals that are stable to higher temperatures—these are also minerals that would crystallize out the earliest if a mama was cooling.]

9. An unusual rock type called “anorthosite” is common in parts of mafic lopoliths. What is this rock composed of? __________________________

10. In the geologic literature, we read of a number of “famous” batholiths, such as the Sierra Nevada batholith in California, the Elberton batholith in Georgia, the Idaho batholith in (you guessed it—Idaho!). Does each “batholith” always represent a single large pluton, that is, each representing a single magma chamber? Explain a little.

11. Describe “zoned plutons”. Be sure to indicate where the more “evolved” compositions occur in such plutons. [NOTE: the concept of “evolved” magma comes from the idea that fractionation trends drive magma compositions toward more felsic compositions—the more felsic compositions are thus considered more evolved.]

13. Describe in what way the water-content of magmas influences crystal nucleation rate and growth rate and thus grain size in plutonic rocks. I am not asking to discuss just how the water produces the effect, but basically what is the effect.

14. Why does the abundance of water that is dissolved in silicate melts commonly increase during late stages of pluton crystallization?

16. Explain what is “hydrothermal alteration”. (Be sure to include what is the source of “hydro”, what is the source of “thermal”, and what is “alteration” and why does this process happen?

Lecture 8. Thursday, February 14, 2006.
Mechanisms of magma generation and magmatic provinces:
rift magmatism, hot spots

1. We all know that adding heat (that is, increasing temperature) is a way to melt something solid. However, when considering what might produce melting in the earth, is this the only option to consider? What chemical principle for inducing melting is generally called upon for the generation of magmas in rifting situations?

2. What kind of igneous rock type is most characteristic of continental rift zones?

3. Why do we think that extensional tectonics in continental areas leads to generation of magmas?

4. How and where is melting induced to produce the characteristic rock type of continental rift zones?

5. In some continental rift zones there are rhyolitic volcanics involved. Describe a likely cause of these magmas becoming involved.

6. Most basaltic magmas in extensional zones are considered to arise from similar melting processes. Assuming most (if not all) basalts come from melting similar rocks, what is one likely cause for some basalts being more alkaline than “typical” basalts? (Here do not give your own speculation, but a process we discussed in class).

7. Some continental rift zones have an abundance of alkaline basalts. Name one famous rift zone with such rocks, and name three rock types that are thought to be derived from fractionation of alkaline basalts.

8. There are a number of isolated basins all along the east coast of North America that have (along with sedimentary rocks) basaltic lavas and gabbroic intrusions. They all have fossils dating them to Triassic and Jurassic times (i.e. Mesozoic--including dinosaur footprints and bones). To what major tectonic event are these thought to be related? [I may not have pointed this out clearly: The answer—rifting of the supercontinent, Pangea and formation of the Atlantic Ocean .]

9. When scientists of days-gone-by studied the ocean basins by dredging up samples, they noted that mostly what you got was basalt or gabbro (besides pelagic sediments). Where do we now think all this rock spread across the ocean basins originated?

10. The Bay of Islands ophiolite is considered a complete ophiolite suite. This suite of rocks includes (from top downward) pelagic sediments, basalt lava flows including pillow basalts, sheeted dike complex, gabbros, ultramafic cumulates, harzburgite (peridotite). Be able to list these different components in their correct order (top to bottom) and be able to say what each represents in relation to magmas that occur at a mid-ocean ridge system. [Something like this:]

11. In a number of orogenic belts there are bodies of mafic and/or ultramafic igneous rock that occur in fault contact with surrounding sedimentary (though sometimes metamorphosed) rocks. What are these bodies of rock now thought to be?

[You may well want to use the term “ophiolite”, but you should point out that they are considered to be slices from oceanic crust “obducted” onto the continental margin.]

12. How does the chemistry of hot-spot-generated magmas compare to magmas generated at most mid-ocean ridges?

13. Some continental rift areas have igneous rocks similar to those mentioned with hot spots in question 12. Are these generally fast rifts that lead to oceanic crust formation, or are they slow rifts that tend to not go to completion?

14. What kinds of igneous rocks are most common on typical mid-oceanic islands (= “intraplate islands” produced by “hot spots”) such as Tahiti ? List at least three rock types you would expect to find on such islands.

15. What is different about the kinds of rocks that typically can be found associated with a hot spot within a continental setting, versus a hot spot in an oceanic setting? What is a likely explanation for this difference?

16. I could give a question like: “From the list below, identify three rock types that are generally associated with ‘alkaline’ magmatism” such as at Magnet Cove, AR. You should be able to recognize such rocks as trachyte, syenite, phonolite, nepheline syenite, nephelinite, ijolite, , and carbonatite as being of the alkaline association (some of these would be included in a list with other igneous rocks).

17. What is the igneous rock “carbonatite” (i.e. what is the nature of the magma if forms from)? What other kinds of igneous rocks are commonly associated with carbonatite?

18. Erickson and Blade, the geologists who mapped the igneous complex at Magnet Cove, showed that fractionation of some common igneous minerals from a __________________ magma could produce the average composition of the igneous rocks of the complex. (Fill in the blank with the kind of magma they proposed to be the starting magma from which the rest of the rocks were derived).

19. How do Cox and VanArsdale explain the origin of the igneous rocks at Magnet Cove, Arkansas and related rocks?

Lecture 9. Thursday, February 16, 2006
Subduction zone generated magmatism

1. Besides the obvious “addition of heat”, there are other factors that could be involved in inducing melting to make igneous magmas. What is the basic chemical principle invoked to explain the initiation of melting in subduction-zone magmatism? (This question is not about the process that we think happens, but the chemical principle.)

2. In the model for generation of magmas at subduction zones that we discussed in class, we said that “hydrated oceanic crust” was an important player. What is “hydrated crust” and how does it get that way?

3. Describe the basic mechanism for producing the first melts generated in subduction zone magmatic arcs. Include just what melts, where it is, and why it melts.

4. What are the typical volcanic rock types of island arcs? Distinguish the rock types that characterize the early stages of the island arc’s history and those that are typical of more mature arcs.

5. In relation to question 4, what process or processes might be involved to produce the difference in rock types of the early stage vs the late stage of an island arc?

6. What are the typical volcanic rock types of continental magmatic arcs? Distinguish the rock types that characterize the early stages of the arc’s history and those that are typical of more mature arcs.

7. Although we think that pretty much the same kinds of processes are involved in both subduction-related island arcs and in subduction-related continental magmatic arcs, there are some differences in the kinds of rocks that characterize them. What is similar about the rock types of island arcs and continental arcs? What is the main difference?

8. Considering the differences in rock types in island arcs and continental magmatic arcs, what is a likely explanation for these differences?

Lecture 10. Tuesday, February 21, 2006.
Introduction to sedimentary rocks, siliciclastic sedimentary rocks, mudrocks

2. When we speak of clastic sedimentary rocks, what do we mean by the term “clastic”?

3. Of the common elements that compose the minerals (that in turn compose the rocks) of the earth’s crust, name 4 elements that in the weathering process tend to be carried away in solution. Where do these elements generally end up? [Note: see handout page 1.]

4. What is (in general) the origin of the clay that is such a large constituent of most “shales”? [Although more info will be forthcoming, at this point you can at least say, “From weathering of such minerals as feldspars”.]

5. What is (in general) the origin of the quartz that is so abundant in common sand deposits?

6. Place the following minerals in order of increasing resistance to chemical weathering (place the least resistant on the left to most resistant to weathering on the right):

7. Place these in the order of the most easily weathered to the least easily weathered:

8. What is the difference between a “mudstone” and a “siltstone”? [Note: see Table 14-1 in the text.]

11. Which conditions (one or more) below tend to promote chemical weathering?____________

12. Why does vegetation in the weathering environment tend to increase the weathering activity?

13. Besides Si, what other elemental constituents are characteristically common in typical clay minerals?

14. Considering clay minerals that can occur in deposits of “mud” (destined to become mudrocks), which clay mineral that we studied is most commonly associated with rather intense weathering?

16. Under the most intense weathering conditions, almost all elements are carried off in solution. Oxides and oxyhydroxides of which major element are left?

1. What is the name for the “expandable” clay mineral that we discussed in class? Explain why it expands?

2. What is an “exchangeable cation”? In montmorillonite clay, where do the exchangeable cations reside in the crystal structure? And what are the most commonly exchanged cations in montmorillonite?

3. What kinds of engineering problems are associated with soils that have large amounts of montmorillonite in them? [I forgot to mention besides shrink and swell (which can detrimentally affect foundations), these clays also tend to be rather impermeable—very poor “PERC” in determining if a septic tank can be installed or not.]

5. Name several kinds of rocks that typically will produce primarily montmorillonite clay when they weather?

6. What sedimentary formation in Arkansas has a high percentage of montmorillonite clay in it, and where does this formation typically crop out in Arkansas ?

8. Name a couple of kinds of rocks that typically would produce illite clay when they weather?

[Note: I mentioned an igneous rock that, because of its mineral content, will produce illite. However, I also spoke of how Paleozoic shales are composed primarily of illite clay.]

9. What material tends to color certain mudrocks black or dark shades of gray? And where does this material come from?

10. What material tends to color certain mudrocks various shades of brown? Name two specific minerals that are involved.

11. In what manner can Fe-oxides and oxyhydroxides be carried to the depositional site where it can be included with mud deposits to become part of a mudrock? (we covered two ways)

13. Indicate two depositional settings in the terrestrial realm where muds, possibly destined to become mudrocks, can accumulate.

14. Indicate three depositional settings in the marine realm where muds, possibly destined to become mudrocks, can accumulate.

15. In what way does the energy of the depositional environment affect the constituents that will occur in a mudrock?

16. What differences would you expect in the silt fraction of a mudrock that formed from sediment that came from a source terrane of sedimentary rocks as compared to one from sediment derived from a source terrane dominated by igneous and metamorphic rocks?

17. What differences would you expect in the clay fraction of a mudrock that formed from sediment that came from a source terrane of mafic and intermediate igneous rocks as compared to one from sediment derived from a source terrane dominated by granitic igneous rocks?

18. What differences would you expect in the clay fraction of a mudrock that formed from sediment that was deposited in a very warm and humid climate as compared to a more moderate climate?

19. What differences would you expect in the silt fraction of a mudrock that formed from sediment that was deposited in a very warm and humid climate as compared to a more moderate climate?

20. A particular sedimentary sequence has many layers of shale that are rather dark gray to black in color. What might you conclude about the deposition site in which the muds had accumulated that became the black shales? Explain a little from a chemical standpoint why the rocks are dark.

21. A particular sedimentary sequence has many layers of shale that are various shades of reddish brown in color. What might you conclude about the deposition site in which the muds had accumulated that became the “red” shales? Explain a little from a chemical standpoint why the rocks are red.

24. Explain how a study of geologic age of sediments, comparing the relative amounts of the major clay groups in rocks of different ages, supports the notion that other clays convert to illite over time.

25. Discuss how the distribution of mixed-layer clays in the stratigraphy of the Gulf of Mexico supports the notion that montmorillonite converts to illite over time.

26. How does conversion of montmorillonite to illite affect groundwater within a section of rock or sediment in which the process occurs?

Note that in a number of matters about a mudrock different situations might produce the same thing. I might put some multiple choice questions like this that could have more than one correct answer:

27. A mudrock is analyzed with XRD (X-ray diffraction) and it is found that the clay fraction is about 90% illite, with minor amounts of montmorillonite and kaolinite. What might you conclude about this rock?____

b) the sediment was derived from a source terrane rich in mafic to intermediate igneous rocks

c) the sediment was derived from a source terrane that was undergoing intense weathering conditions

1. Sandstone A is said to be “well sorted”, while sandstone B is said to be “poorly sorted”. How would you describe the difference between sandstone A and B?

2. You read an engineer’s report in which she says that the sand in a certain location was well graded. How would you describe the same phenomenon in geological terms?

3. Be prepared to tell whether a sand is (roughly speaking) poorly sorted or well sorted based on seeing a grain-size distribution curve.

4. Sandstone A is said to be a “well rounded” sandstone, while sandstone B is said to be a “poorly rounded” sandstone. What is the difference between sandstone A and B? (Hint: it has nothing to do with how much history, arts, music, philosophy, etc. the two sandstones took in college). [Note: be sure to indicate that this has to do with rounding of the grains in the sandstone. If you word your answers incorrectly it may seem like you are talking about the entire rock being rounded (like in a stream).]

6. In general, we normally associate sandstone textural features with either textural “maturity” or textural “immaturity”. In a way of generalization, indicate either “maturity” or “immaturity” for the following textural characteristics:

7. In the question above we considered sorting, rounding, and clay matrix in general with respect to textural maturity. I will describe below some sandstone textures. Tell whether it is texturally “mature”, “supermature”, “submature”, or “immature”.

8. What mineral is normally the most abundant in sandstones, and why is this so? (give two good reasons) [If this was not specifically pointed out in class, it has to do with chemical reactivity of the mineral and hardness (as in Mohs’ Scale). Quartz at the earth’s surface has a very low reactivity with water. Also it’s hardness of 7 is the hardest of all the common minerals.]

9. Which feldspar (K-spar or plagioclase) is usually the more abundant in feldspathic sandstones? Also, tell me why this feldspar is usually more abundant.

10. In the question above we deal with the fact that one of the two feldspar types is usually more abundant than the other. Think of and describe a circumstance in which the feldspar that is usually less abundant would likely be more abundant in a particular sandstone.

11. What is a common implication regarding the paleogeography interpreted by a sandstone being rich in the feldspar component?

12. What are “lithics” in sandstones? (That is, what does the term “lithics” or “lithic fragments” refer to in discussing sandstone composition?)

13. ________Of the following rock types, which one (ones) would you expect to be more likely to contribute lithics to a sandstone?

14. Of the sandstone constituents listed below, which are associated with “compositional maturity” and which with “compositional immaturity”?

15. Of the following environments of deposition, which normally has the lowest degree of textural maturity in sandstones of that environment? a) river sand bar, b) beach, c) alluvial fan

16. Of the following environments of deposition, which normally has the highest degree of textural maturity in sandstones of that environment? a) river sand bar, b) beach, c) alluvial fan

17. Of the following environments of deposition, which normally has the highest degree of compositional maturity in sandstones of that environment?

18. Of the following environments of deposition, which has the lowest degree of compositional maturity?

Lecture 13. Tuesday, March 3, 2006.
Sandstones & Conglomerates

1. Which mica is most common in sandstones?______________________________

2. What common minerals constitute the typical “heavy mineral” suite of sandstones? (name four minerals)

[Note: I forgot to tell you this, that the most commonly studied “heavy minerals” (accessory minerals) are magnetite, ilmenite, zircon, rutile, and tourmaline. Of all the accessory minerals, by far the most abundant are magnetite and ilmenite.]

3. What is a “provenance” study in sedimentary petrology?

4. We discussed in class the Dickenson and Suczek (1979) study of sandstone composition in relation to provenance in terms of tectonic setting. Regarding a continental setting, we compared sediments derived from a stable craton interior deposited on a coast line vs sediments from a nearby uplifted block of “basement” rock.

a) Which of the two provenances (craton interior or uplifted basement block) normally has more quartz?______________________________

b) Concerning the other terrane, what is the primary non-quartz component in the sands? _________________________ (choices: feldspar, lithic grains)

c) Explain the answers to (a) and (b) with a flow of logical thought why it is commonly found to be this way?

5. We discussed in class the Dickenson and Suczek (1979) study of sandstone composition in relation to provenance in terms of tectonic setting. Regarding the magmatic arc setting we considered a “well-dissected” (much eroded) magmatic arc vs an undissected arc (little erosion).

a) Which of the two provenances (dissected arc or undissected arc) normally has more lithics than feldspar?______________________________ (if neither has more, say so)

b) Which of the two provenances (dissected arc or undissected arc) normally has more feldspar than lithics?______________________________ (if neither has more, say so)

c) Explain the answers to a), and (b) with a flow of logical thought why it is commonly found to be this way?

6. What is the main difference between a breccia and a conglomerate?

7. In conglomerate textures, what component of the conglomerate does the term “framework” refer to?

8. In conglomerate textures, what component of the conglomerate does the term “matrix” refer to?

9. If a conglomerate is said to be “matrix-supported”, what does that mean? [Question could be “framework-supported”.]

10.What is the difference between a paraconglomerate and an orthoconglomerate?

11. What is the difference between a polymictic conglomerate and an oligomictic conglomerate?

12. What is a petromict conglomerate?

13. Which rock type(s) listed below would not be considered as stable conglomerate clast types? ____________

a) diorite b) gneiss c) schist d) chert e) granite f) metaquartzite g) limestone

14. What materials constitute the “matrix” of a conglomerate?

15. What factors can contribute to clast roundness?

16. On a field trip a participant notices a conglomerate with angular chert clasts and rounded clasts of granite. He comments that there must have been a nearby source terrane with chert and a source area with granite that was farther from the deposition site. Give an alternate hypothesis for the angularity/roundness of the contrasting rock types and explain why your hypothesis is possible.

17. A conglomerate has several clast types. Of them, 40% are vein quartz, 50% are metaquartzite, 7% are gneiss, and 3% are of schist. A report says that this conglomerate

1. Are beach gravels on erosional coastlines more likely to be oligomictic or petromictic? Why?

2. Are beach gravels more likely to become orthoconglomerates or paraconglomerates? Why?

[I’m not sure if it mentioned this, but the wave action would surely winnow out the finer grained stuff, leaving gravel clasts in grain to grain contact.]

4. Alluvial fan deposits more frequently have (petromictic conglomerates, oligomictic conglomerates[circle best]) than river deposits. Explain why.

5. Are conglomerates that form from glacial morraine deposits more likely to be oligomictic or petromictic? Why?

7. Of the following environments of deposition, which normally has the lowest degree of textural maturity in sandstones of that environment? a) river sand bar, b) beach, c) alluvial fan

8. Of the following environments of deposition, which normally has the highest degree of textural maturity in sandstones of that environment? a) river sand bar, b) beach, c) alluvial fan

9. Of the following environments of deposition, which normally has the highest degree of compositional maturity in sandstones of that environment?

10. Of the following environments of deposition, which has the lowest degree of compositional maturity?

11. What are the two main processes involved in the diagenesis of sands, and which one is the most important in the process of “lithification”?

12. Since sand grains themselves are not really compressible to any significant extent under typical burial depths associated with diagenesis, what happens to the sand deposit that allows it to be “compacted”?

13. What is “quartz overgrowth cement”? And how can we sometimes recognize quartz that is part of the original sand grains and distinguish it from the quartz cement?

14. Explain how in situ “pressure solutioning” in the sandstone itself could possibly be involved in creating at least some quartz cements. [A bit longer type answer. Be sure to state what chemical principle is involved (i.e. solubility of quartz at different pressures).]

15. What could you see in a sandstone that might indicate that in situ pressure solutioning was a mechanism in cementing the sand? (Hint: seen in thin section microscopy)

16. Explain a possible source of silica cement in sandstones in which there is no evidence for in situ pressure solutioning. [Hint: We discussed some possible sources of silica in sandstones of orogenic belts.]

17. What conditions are thought to bring about precipitation of opal cement in sandstones? What happens to the opal cement over long periods of time?

18. What differences in pore-water chemistry are there between conditions suitable for depositing silica cement vs calcite cement?

19. Explain how the process of hydrocarbon maturation in a source rock might work to produce porosity in an adjacent sandstone that had been previously cemented with calcite cement. [This is one I would expect you to be able to describe with a bit longer answer with logical connections.]

20. What is one possible source of hematite for making hematite cement? Is hematite cement more likely to form in a terrestrial setting or in an oceanic setting? What color does hematite cement impart to a sandstone?

21. How can clay act as a cement? What are two possible sources of clay in a sandstone that can act as a cement?

22. What is an “authigenic mineral” in the story of diagenesis? Name two minerals that can be found as authigenic minerals in sandstones.

23. If you find a sandstone with crystals of pyrite in it, what is the most likely source of the pyrite? _______

1. What is the most important factor in generating the great majority of the minerals in sediments that will become limestone?

2. ______Considering what we know about carbonate deposits in general, if you were told that the Cretaceous rocks of a particular area were a thick sequence of carbonate sedimentary rocks, what should be your first hunch as to the kind of environment that prevailed in that area during the Cretaceous?

3. ____________From the list of carbonate minerals below, what minerals have been found to be dissolved in sea water to the extent that the sea water is supersaturated in that mineral?

______________From the list of carbonate minerals below, what minerals are found in modern day carbonate sediments?

______________From the list of carbonate minerals below, what minerals are found in ancient carbonate sedimentary rocks (not including veins)?

4. Is it common to have mixtures of siliciclastic detritus (i.e. quartz sand, silt, and clay) in with abundant carbonate sediment? Why? [Note: In class I pointed out that an abundance of sea creatures that produce hard parts exists in areas w/o muddy water, so typically abundance of carbonate sediment is found in areas protected from much input of siliciclastic detritus.]

5. ______What part of a typical continental margin more commonly has carbonate sediment in the modern-day setting? [Though we did not say in class, the generalizations we considered should tell you.]

6. What kind of climate most typically can support a carbonate-depositing system? [I think I forgot to mention this, but warm climates are helpful to the growth of organisms that secrete hard parts to become constituents of carbonate sediment.]

7. Invertebrate organisms that secrete hard parts (i.e. shells, etc.) make those hard parts of what minerals? (Don’t just say “carbonates”. Be specific.)

8. What is different about the mineral(s) preserved in the ancient “rock record” of carbonates as compared to modern carbonate sediments?

1. _______Which type of organisms contribute the most to “skeletal” grains in carbonate rocks?

2. Describe the basic processes involved in generating ooids in the carbonate sedimentary environment. [a longer type answer]

3. Describe the internal structure of modern ooids. Include how ooids of round shape are related to elongate ooids.

4. What difference, if any, is there between modern ooids and those found in “the rock record”. Be as thorough as you can.

5. What are the four main types of “grains” common in carbonate sedimentary rocks? [Remember these are skeletal grains, ooids, peloids, and intraclasts (limeclasts).]

6. What are the four main types of “allochems” common in carbonate sedimentary rocks?

7. What are peloids? And how are the most common peloids generated? [Try to keep your description as professional sounding as possible—try to stay out of the gutter.]

8. What is “micritization” and what does this process have to do with some peloids?

9. What are intraclasts? Describe at least one way in which they can be generated. [a longer-type answer]

10. What this carbonate mud like on a microscopic scale? Include what mineral modern mud is made of.

11. In what way are organisms most likely involved in the generation of carbonate mud?

12. Contrast modern carbonate mud and carbonate mud in rocks that have long been lithified.

13. Some limestones are found to be “framework structures” from ancient reefs. What are twokinds of marine organisms that can produce such framework structures?

14. What part of a carbonate bank, the center or the periphery, is more commonly the site of reef structures?

15. In what sort of setting in a carbonate bank environment do oolites normally form and collect?

16. In what sort of setting in a carbonate bank environment do peloids and carbonate mud normally collect? [I mentioned the mud, but left out the peloids that can collect in the protected lagoon setting.]

I apologize that I forgot to post study questions for quite a while. I’m putting these up on April 3!

1. What kinds of features might be present in a carbonate rock that would indicate that current transport was involved in the deposition of the sediment that formed the rock at that particular spot?

2. What kinds of commonly observed features of carbonate rocks are indicative that much carbonate sediment accumulates without any significant current transport?

3. Discuss two major sources of carbonate sediment that can accumulate in deep ocean environments. [Do not just state a couple of phrases. Explain the origin of the sediment and how it gets there. This would be a matter of plankton with carbonate hard parts and sediment washed out of a carbonate bank onto the shelf beyond the bank and then transported down the continental slope by slumping and turbidity flow. Note that it would likely be finer grained constituents that are carried out onto the shelf, thus making limestones that make ti to the deep ocean by this process to most commonly be micritic. We saw this on the field trip in the rocks on the dam and Lake Ouachita .]]

4. Explain how physical conditions in the deep ocean affect the chemistry of the ocean water so that carbonate sediment tends to dissolve in deep ocean environments. [NOTE: You should comment that CO₂ is more soluble in cold conditions and conditions of high pressure, both of which occur in the deep ocean. You should also then comment that more CO₂ in solution produces more H⁺ and (HCO₃)^- (bicarbonic acid) which promotes dissolution of CaCO₃.

5. What is the CCD? Explain how rates of sediment influx and rates of dissolution are involved in this phenomenon.

6. What is the difference in the depth of the CCD in higher latitudes as compared to near the equator? Explain why. [NOTE: you do not have to tell exact depths, but the general observation that the CCD becomes shallower at higher latitudes (greater distances from the equator). It should be obvious that if cold water promotes more CO₂ in solution, that the cold conditions at great depths are present at shallower depths as you go away from the equator because of colder climate. Also the carbonate sediment production will become less and less in areas farther from the equator, also because of the colder climate.]

7. Explain how tufa deposits form at the sites of some water falls. Be sure to include the kind of terrane that is required for tufa-depositing conditions to exist and the basic chemical factor why the tufa forms in that kind of place. [Your answer should include the matter of ground water in a carbonate terrane being saturated with CaCO₃, feeding stream in the area, turbulence in the stream at the water fall promoting more of the CO₂ in solution to escape into the atmosphere, which lowers the H⁺ and (HCO₃)^- (bicarbonic acid )in the water which promotes precipitation of CaCO₃.

8. What are two things that could happen that could cause carbonate sediment to be partially dissolved after deposition (as part of the diagenetic processes)?

9. Why is it not surprising that lime sediments could be periodically exposed above sea level close to the time they are deposited?

10. When some amount of dissolution occurs in a carbonate sediment, why might it be selectively dissolving certain allochems preferentially to others?

2. Cite at least two lines of evidence that lime sediments commonly become cemented early-on, prior to deep burial.

3. There is evidence that in some situations there is some amount of compaction in lime sediment prior to cementation. What physical evidence could occur in a limestone that would indicate that the sediment of which it formed underwent significant amounts of compaction during diagenesis?

4. With lime sediments, in comparison to siliciclastic sediments, is it more common to for the sediments to be compacted by burial prior to lithification or more common for sediments to become cemented before there is a chance for compaction?

5. What is a likely explanation for the common (though not universal) observation of stylolitic surfaces parallel to bedding in limestones?

6. What kind of observations in chert-bearing limestones are evidence for the chert having come from silica actually replacing original limestone.

7. What are some possible sources of silica for generating chert replacement of limestones?

8. What evidence is there that dolostones are not from original dolomite sediment, but rather from a diagenetic replacement of calcium carbonate by dolomite? (Here I do not want the theory of how it happens, but rather evidence that indicates that some kind of diagenetic replacement process must take place).

9. What is the significance of gypsum being precipitated out of highly saline brines in the “evaporative reflux” (Sabkah) model of dolomitization?

10. Why does the Sabkah model for dolomitization involve exposure of the carbonate sediment above sea level?

11. If I give a sketch of a section of carbonate rock bedrock partially exposed above sea level, be able to draw where the boundary between fresh ground water and saline ground water will be and what its general configuration will look like. Where in the system are conditions right for dolomitizing the limestone?

12. Why does the “Dorag dolomitization” model require some portion of the carbonate environment to be exposed above sea level for a period of time? (don’t just say, “in order to get the conditions necessary for dolomitization”—explain how exposure above sea level can lead to dolomitization).

1. Name and give the chemical composition of 4 common minerals of evaporite deposits.

2. Briefly describe at least one type of modern situation where evaporite minerals are deposited.

4. Do modern evaporite deposits generally match the size and volume of the very large deposits of question 3?

5. What are the three main minerals that will precipitate out of sea water upon evaporation, and what is the sequence in which they will precipitate out?

6. Which one of the minerals in question 5 is of the greatest percentage dissolved in sea water?

7. Briefly explain at least two reasons why it is very unlikely that ancient large-volume evaporite deposits could have come from simple evaporation of single bodies of sea water. Your answer should have something about observations in actual deposits in relation to chemical considerations.

8. Besides aridity, what other factors must have been involved in deposition systems of large-volume evaporite deposits? (In class we discussed two major factors [might be considered as two aspects of one factor]. I want you to explain why these factors must be involved in models for large-scale evaporite deposits.)

9. What is a likely reason that there are extensive salt deposits in the Gulf of Mexico ? Where in the stratigraphy of this sedimentary basin were these salts deposited? (lower part, middle part, upper part) And why are they in that section of the stratigraphy?

10. What kind of environment do most bedded cherts appear to have accumulated in? Cite some evidence that suggests this kind of environment.

11. Name two important chert-rich formations (of the bedded chert type) in Arkansas . In general where in Arkansas do these formations occur?

12. What kind of geologic setting appears to have been present where the “Superior-type” banded iron formations were deposited?

13. What are the Superior-type banded iron formations like lithologically (describe what minerals are present and how the rock looks.)

14. Contrast the typical size of the Algoma-type and the Superior-type iron formations. (Which, if either, tends to form extensive deposits, and which tends to be rater limited in size?)

15. What is the kind of geological setting where “Algoma-type” banded iron formations probably formed.

16. Which kind, the Algoma or the Superior-type iron formation is the more likely to have a sulfide metallic ore deposit associated with it?

17. There are oolitic “ironstones”, such as in the Silurian Clinton Formation of the Appalachians . What is a likely origin of the hematite oolites? [We did not discuss hypothesized ideas as to the exact process, but the fact that these oolites occur along with shell fragments that are now hematite, they are most likely originally carbonate oolites (and shell fragments) that were replaced by hematite for some unknown reason.]

2. What is the primary physical condition that drives metamorphism (i.e. increasing “grades” of metamorphism correspond primarily with increases in _________________).

3. What factor in a rock is the main feature by which we recognize metamorphic grade (i.e. the degree of metamorphism)? [You’d better have picked up that the minerals present (mineral assemblage) is the main thing by which we recognize the metamorphic grade.]

4. If a suite of several interbedded rock types are all metamorphosed under the same metamorphic conditions, would you expect them to all to be affected in the same way or not? Briefly explain why.

5. What geologic process is transitional into metamorphism at the lower end of the range of conditions that produce metamorphism? What process is transitional with metamorphism at the upper end of the range of conditions that produce metamorphism?

6. If some area is said to have been affected by zeolite facies metamorphism, would you consider it to have been affected by high-grade metamorphic conditions or low-grade conditions?

7. What rock types are most likely to have zeolite minerals, produced by zeolite facies metamorphism?

8. What is a “migmatite”? (Note that if you did not get it into your notes, it is on a handout and probably in a glossary somewhere.)

10. What constituents are generally not “isochemical” in isochemical metamorphism?

11. What is meant by metasomatism? [I forgot to give you this term, I think. Metasomatism is the concept that elements migrate through rock during metamorphism, thus changing the rock’s chemical composition. At one time it was thought that metasomatism was integral in just about all metamorphism. We now recognize that probably except in certain circumstances most elements do not move very much. There does appear to be a fair amount of SiO₂ that does move, forming quartz veins in the process and lowering the silica content of rocks.]

12. Which process, metasomatism or isochemical metamorphism, probably more closely approximates the actual situation with most metamorphism?

13. In metamorphic geology, one is often concerned about the “protolith” of a certain metamorphic rock. What does the term “protolith” mean? (The term was mentioned, but I don’t know if I defined it for you clearly. Check out the text’s glossary.)

14. What are metamorphic dehydration reactions? And what are hydration reactions?

15. What kind of rock type generally begins its “metamorphic history” with dehydration reactions, and why? [I hope from the discussion it was clear that mudrocks, which have abundant clays which are (OH)-bearing minerals which become unstable under increases in temperature, are the ones.]

16. What kind of rock type generally begins its “metamorphic history” with hydration reactions, and why?

17. Which kind of reaction characterizes typical prograde metamorphism (increasing temperature), hydration or dehydration? [Remember, some study questions are very much interrelated—this one is very similar in content to #18.]

18. In general, which silicate minerals tend to be more stable with increasing temperature in the range of metamorphic conditions, minerals with structural (OH) or minerals without (OH)?

19. What is contact metamorphism? Where does it usually take place? How large an area is generally affected by contact metamorphism?

20. Name two minerals that crystallize only at relatively low pressures with high temperatures, thus making them common features of rocks affected by contact metamorphism.

1. What special process is thought to occur in the special contact metamorphic case in which a “skarn” deposit is produced? And explain why “calc-silicate” minerals are formed in this process.

2. What kind of process is responsible for standard regional metamorphism (not the special case of “low pressure” or “high pressure” metamorphism)? That is, what physically takes place in the earth that produces typical regional metamorphism? Describe in two or three sentences.

3. What kind of texture is usually present in rocks produced by regional metamorphism (it helps us recognize that the rocks were affected by regional metamorphism), and why is this texture so common in such rocks?

4. Describe a setting that can have “low-temperature / high pressure” metamorphism.

5. How is it that abnormally high pressures (relative to the temperature) are obtained to produce “low temperature/high pressure metamorphism” in the region discussed in question 4?

6. Name two possible rock types and one mineral that are commonly produced by low temperature/high pressure metamorphism and thus characterize terranes that have undergone the processes you discuss in the two questions above.

7. Describe a regional setting that can have “high-temperature / low-pressure metamorphism”. Indicate why these metamorphic conditions occur in such a setting (that is, why does the environment have low pressures [relatively speaking] while having high temperatures?).

8. If a region of metamorphic rocks has an abundance of andalusite in rocks of that region, what would that tell you about the na

Rocks of Ophiolite suite	Their significance
Pelagic sediments	Deep marine sediments deposited on oceanic crust
Pillow basalt lava flows	Extrusions from mafic magma generated beneath the MOR
Sheeted dike complex of mafic composition	Many dikes that fed the lava flows, fractures for dike intrusion formed from extension perpendicular to ridge axis.
Gabbro	Mafic magmas crystallized on the walls of sub-MOR magma chambers
Ultramafic cumulates	Collection of early formed crystals having settled from the mafic magma
Peridotite	Upper part of the mantle