This exposure probably representsThese questions should give you an idea of the kind of thinking I am after, and the way the questions may be structured. Answers to the questions, along with the reasoning, are given at the bottom of this page. I suggest that you try to take this exam without looking at the answers -- then check and see how you've done. When you miss a question, use my explanations to help figure out where you went wrong.
0. Glasses do not possess cleavage because
a. they are too soft b. they are too hard c. they lack defects in the crystal structure d. they have an irregular internal crystal structure e. they contain no regular planes of weaker bonding
1. Which one of the following minerals does not contain silica tetrahedra?
a. quartz b. feldspar c. olivine d. clay e. magnetite
2. Diamond and graphite are both minerals made of pure carbon, yet diamond is clear, very hard, and relatively dense (heavy), while graphite is silvery-black, very soft (pencil lead is graphite), and relatively light. How can you best explain this difference? (Mark statements true or false, then choose answer.) ____diamond is made of one isotope of carbon, while graphite consists of a different carbon isotope ____diamond and graphite have different crystal structures ____diamond forms under very high pressure and temperature, while graphite forms at much lower pressures and temperature ____graphite forms at very high pressure and temperature, while diamond forms at low pressure and temperature ____graphite is simply diamond that has been weathered at the earth's surface.
The correct matching sequence is: a. TFFFF b. FTTFF c. FTFTF d. FFFFT e. FFFFF
3. If a basalt were heated up until it melted thoroughly (no crystals left) and then was cooled very very slowly you would most likely get:
a. gabbro b. andesite porphyry c. granite d. diorite e. obsidian
4. Many granites are thought to originate by
a. partial melting of the upper mantle b. settling out of quartz and feldspar from peridotite c. deep burial, melting, and recycling of continental&endash;type material (especially sedimentary rocks) where two lithospheric plates are colliding d. melting of the tops of downgoing lithospheric slabs (in trenches) together with contamination by continental material as the melt rises through the crust e. metamorphism of andesitic rocks
5. Which mineral below is extremely unlikely to occur in a basaltic rock?
a. plagioclase b. amphibole c. pyroxene d. olivine e. quartz
6. A very angular, poorly sorted sand made of quartz and feldspar probably
a. has been recycled through the sedimentary rock-forming process several times b. was deposited in a nearshore shallow sea environment c. represents strong weathering in the source area and long transport d. may have been transported by wind (as sand dunes) e. none of the above (a-d) are likely to be true.
7. In which environment below would the sediments probably be most poorly sorted?
a. deep lake b. sand dune c. beach d. landslide e. river channel
8. In the large exposure shown below you find coarse, poorly sorted gravel grading laterally
into sand, and then into light colored laminated shale which contains gypsum crystals.
This exposure probably represents
a. alluvial fan and playa (dry lake) b. glacial outwash and lake c. river floodplain and channel d. continental shelf e. beach and lagoon
9. Which one of the sedimentary environments below would be least likely to produce cross-bedding?
a. sand dune
b. river deposit
c. turbidity current deposit
d. alluvial fan deposit
e. landslide deposit
10. A very dense, very fine-grained metamorphic rock suggests metamorphism primarily as a result of
a. high pressure and high temperature b. high pressure and lower temperature c. low pressure and low temperature d. lower pressure and high temperature e. migration of chemical fluids
11. The best evidence that a rock has a metamorphic origin is:
a. finding mica in the rock b. presence of different-colored layers in the rock c. a foliated texture d. finding garnets in the rock e. finding phenocrysts in the rock
12. Regional metamorphism
a. is often associated with mountain belts and folding
b. is generally caused by deep burial
c. affects large areas
d. all of the above (a through c) are generally associated with regional metamorphism
e. none of the above (a through c) are true; the things listed are associated with contact metamorphism, not regional metamorphism
13. An andesite metamorphosed at high temperature and pressure would probably most closely resemble a
a. granite
b. slate
c. marble
d. basalt
e. diorite
14. Where would you most expect to find large strike-slip faults?
a. mostly in the central areas of continents b. in places where lithospheric plates are sliding by one other c. in areas of crustal extension or stretching such as on mid-oceanic ridges or in uparched continental areas d. in areas of crustal shortening such as fold-mountain belts and subduction zones e. mostly in volcanic arcs
15. If you visited a large island located in a volcanic arc (such as Japan) which of the
following geologic conditions would you expect to find? (Mark true or false, then choose answer)
___very deep earthquakes
___active andesitic volcanoes
___very thin crust
___active basaltic volcanoes
___mostly very old rocks
The correct matching order is: a. TTTFT b. TTFFF c. FFTTF d. FFFTF e. TFFTT
16. In places where two oceanic-type lithospheric plates are separating we would generally expect
to find (mark statements T or F, then choose answer)
___shield volcanoes
___a volcanic arc
___creation of new ocean floor
___basalts
___very deep earthquakes
The correct matching order is a. TTTTT b. FTFTF c. TTFTT d. FTTTF e. TFTTF
17. Under the sea-floor spreading theory, the ocean floor:
a. contains the oldest rocks yet found on earth b. is very young at the mid-oceanic ridges and gets older toward the trenches c. is oldest at the mid-oceanic ridges and gets younger near the trenches d. is everywhere the same age e. is everywhere less than 50 million years old
18. If we are using radiometric dating to determine the age of a rock, how will the radiometric age be
affected if some of the daughter product has been naturally removed (for example, by weathering, or
heating, or leaching by ground water)?
a. the radiometric date will be unaffected, and we will get the rock's true age
b. the radiometric age will be older than the rock's true age
c. the radiometric age will be younger than the rock's true age
d. there's no way to tell how the radiometric age will be affected
e. the question is a lie: daughter products can never be removed from a rock except in the laboratory
19. Carbon-14 is a radioactive isotope with a half-life of approximately 5000 yr . (It's actually somewhat
greater, but we'll round it down to 5000 for this problem.) Assume that an old tire starts out containing
400,000 atoms of carbon-14. A future archaeologist finds this tire and analyzes it. It contains 25,000
atoms of carbon-14. How old is this particular tire?
a. 5,000 years
b. 10,000 years
c. 15,000 years
d. 20,000 years
e. 25,000 years
Answers (with reasoning):
0. e (cleavage occurs because the regular internal atomic structure of the mineral contains
planes of weaker bonding. Glasses have no regular internal structure, and thus no regular
planes of weaker bonding.)
1. e (all the others are silicates -- you just have to remember this)
2. b (diamond and graphite must have different crystal structures, since they have
the same composition, but different physical properties. Since diamond is dense,
it suggests that it was formed under high pressure.)
3. a (this requires that you understand igneous rock composition, classification, and the
effects of cooling rate on rock texture. Basalt and gabbro have the same mineral
composition; gabbro is coarse grained, thus it is the result of very slow cooling.)
4. c (granites are high silica -- and so are the average materials of continents;
all the other environments and processes would produce low or intermediate silica rocks)
5. e (quartz is a high-silica mineral, incompatible with the low-silica composition of a basalt)
6. e (very angular = didn't get transported very far or rolled against other grains very much;
thus can't have been transported a long distance, and can't have been deposited in
shallow nearshore environment where waves would keep rolling it against other grains.
Same is true for sand dunes.
poorly sorted = deposited quickly, or by an agent incapable of separating grain sizes;
thus unlikely to have been transported very far, can't have been deposited nearshore or
in dunes where waves or wind would winnow out the fines; also, repeated cycling through
sed rock forming process would tend to round & sort grains a lot.)
7. d (landslide just dumps stuff in a heap -- incapable of effective sorting)
8. a (alluvial fan deposits consist of poorly sorted gravels and sands typically showing lots
of channeling. Playa -- i.e. dry lake-- deposits are fine grained, well laminated or
or layered, and are likely to contain minerals precipitated from salty water, like salt
or gypsum)
9. e (cross-bedding is caused by fluid flow that deposits individual grains at ann angle to the
overall layering. Ripple marks, channeling, dune foresets and delta foresets are examples.
A landslide, dumping its material all at once, has little opportunity to create cross-bedding.)
10. b (fine grained suggests lower temperature -- higher temperature makes ions more mobile,
favors larger grains; very dense suggests high pressure -- minerals squeezed, get more
compact crystal structures on recrystallization)
11. c (foliated texture -- formed by parallel orientation of platy minerals due to recrystallization
under pressure -- is characteristic of many metamorphic rocks, Mica can be found in igneous and
sed rx, not just metamorphics. Garnets can be found in sed rocks -- incorporated from breakdown
of meta rocks. Phenocrysts are found in porphyritic igneous rocks. Sed rock can show different
colored layers too.)
12. d (regional metamorphism is metamorphism due to deep burial and heating of a large area. It is
typically associated with convergent margins, either around subduction zones, or where two
continental plates collide)
13 e (andesite and diorite are igneous rocks with the same chemical composition, but andesite cooled
quickly near the surface, and diorite cooled slowly deep beneath the surface at high temperature
and pressure. Hence if andesite is metamorphosed at high temperature and pressure, we would
expect it to most resemble a diorite.)
14. b (strike-slip motion occurs along transform boundaries -- places where plates are sliding past
one another, edge-to-edge)
15. b (deep earthquakes occur on the lower parts of downgoing slabs; an andesitic volcanic arc is
created above the downgoing slab as it melts. Crust is moderately to very thick -- one plate
is beneath another. Crust is relatively young -- all that active volcanism.)
16. e (plates being pulled apart are in tension -- new basaltic magma rises up from partial melting of upper
mantle, creating new ocean floor; may also form shield volcanoes. Oceanic crust is thin to begin with
-- new crust is probably even thinner. Thus we won't get deep earthquakes.)
17. b (new ocean floor is formed at the ridges as the plates are pulled apart; old crust is destroyed
as it is subducted at the oceanic trenches.
18. c (Radiometric dating depends upon the regular conversion of parent isotope to daughter product. The
greater the ratio of daughter to parent, the longer the radiometric clock has been ticking. If
some of the daughter product has been lost or removed, it will appear that the clock hasn't been
ticking for as long as it actually has, and the rock's radiometric age will be too young.)
19. d (To solve this, count the number of half-lives and multiply by 5,000. 400,000 -> 200,000 ->
100,000 -> 50,000 -> 25,000. That's 4 half-lives. 4 half-lives X 5,000 yr per half-life =
20,000 yr.)