IGNEOUS PETROLOGY: STUDY QUESTIONS
1. What is a basic definition of "petrology"?
2. Rearrange this sentence into a true statement:
Rocks are compounds of elements, and elements are aggregates of minerals.
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:
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Which magma type tends to have the highest silica content, mafic or felsic?
Which magma type tends to have the highest (OH) content, mafic or felsic?
Which magma type tends to have the highest viscosity, mafic or felsic?
Which magma type tends to have the highest temperature, mafic or felsic?
Be able to any mineral name into a blank in the basic Bowen’s Rxn series diagram
5. Regarding the following other elements (expressed here as oxides), which magma type, mafic
or felsic, typically has higher amounts.
 MgO
 FeO (+Fe2O3)
 CaO
 Na2O
 K2O
6. What is meant by the term "volatile" species in an igneous melt (magma)?
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?
9. What is another term for a volcanic rock? (a term given in class)
10. What is another term for an intrusive igneous rock? (a term given in class)
11. What is a “hypabyssal” igneous rock?
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. Explain how viscosity and water content interact to produce explosive volcanism.
17. Why does the strength of an explosive eruption increase after the inception of the eruption in
many cases?
18. What is “vesiculation” in a magma, and what is the basic cause?
19. Name the 5 most common constituents of pyroclastic rocks and tell briefly the origin of each.
Introduction to sedimentary rocks, siliciclastic sedimentary rocks, mudrocks
1. What is meant by the term “lithification”?
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):
Muscovite, calcic plagioclase, hornblende, biotite, quartz.
7. Place these in the order of the most easily weathered to the least easily weathered:
Sodic plagioclase, alkali feldspar, clinopyroxene, olivine, hornblende.
(Note: I could choose any of the common minerals that constitute igneous rocks).
8. What is the difference between a “mudstone” and a “siltstone”? [Note: see Table 14-1 in the
text.]
9. What is the difference between a “mud-shale” and a “mudstone”?
10. What is the difference between a “mud-shale” and a “clay-shale”?
11. Which conditions (one or more) below tend to promote chemical weathering?____________
a) dry climate b) wet climate
c) warm climate
d) cool climate
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?
15. What minerals tend to weather to form kaolinite?
16. Under the most intense weathering conditions, almost all elements are carried off in solution.
Oxides and oxyhydroxides of which major element are left?
17. What is the name for the “expandable” clay mineral that we discussed in class? Explain why
it expands?
18. 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?
19. 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.]
20. What mineral most readily produces montmorillonite clay when it weathers?
21. Name several kinds of rocks that typically will produce primarily montmorillonite clay when
they weather?
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?
22. 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?
23. Be prepared to tell whether a sand is (roughly speaking) poorly sorted or well sorted based on
seeing a grain-size distribution curve.
24. 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? 1. Which mica
is most common in sandstones?______________________________
25. What common minerals constitute the typical “heavy mineral” suite of sandstones? (name
four minerals)
26. What is the main difference between a breccia and a conglomerate?
27. In conglomerate textures, what component of the conglomerate does the term “framework”
refer to?
28. In conglomerate textures, what component of the conglomerate does the term “matrix” refer
to?
29. If a conglomerate is said to be “matrix-supported”, what does that mean? [Question could be
“framework-supported”.]
30.What is the difference between a paraconglomerate and an orthoconglomerate?
31. What is the difference between a polymictic conglomerate and an oligomictic conglomerate?
32. What is a petromict conglomerate?
33. 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
34. What materials constitute the “matrix” of a conglomerate?
35. What factors can contribute to clast roundness?
36. 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.
37. What is the most important factor in generating the great majority of the minerals in
sediments that will become limestone?
38. ______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?
a) terrestrial environment
b) shallow marine environment
c) deep marine environment
39. ____________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)?
a) aragonite
b) dolomite
c) calcite
e) magnesite
f) ankerite
g) siderite
40. Is it common to have mixtures of siliciclastic detritus (i.e. quartz sand, silt, and clay) in with
abundant carbonate sedime1.
Intro to Metamorphism
1.Just what is metamorphism?
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?
nt? Why?
6. What is meant by metasomatism?
7. Which process, metasomatism or isochemical metamorphism, probably more closely
approximates the actual situation with most metamorphism?
8. 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.)
9. What are metamorphic dehydration reactions? And what are hydration reactions?
10. 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.]
11. What kind of rock type generally begins its “metamorphic history” with hydration reactions,
and why?
12. 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.]
13. 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)?
14. 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.
15. What is the significance of a “relict” texture in a metamorphic rock? (What is meant by
“relict” texture, and what is its significance?)
16. What kind of metamorphic conditions are most likely to produce a metamorphic rock in
which relict textures are preserved?