EXAMPLE!!!!!!!!

EPS 101 PHYSICAL GEOLOGY SECTION 001

THIRD HOURLY EXAMINATION, Wednesday, November 26, 1997

Name: _______________________ __________________

This exam consists of only three parts! Read the instructions to each part and the following comment carefully. Numerical scores and preliminary grades will be available at the end of class on Monday, December 1, 1997. PLEASE ATTEND!! Thank you. Also, please have a well-deserved break!!!!

Part I. Relative and Absolute Geologic Time

The sketch on the attached colored sheet is a vertical cross-section through a part of Earth's continental crust with a rather interesting (depends on your point of view) geologic history. All important geologic materials, or sequences of rock types are labeled. Below, please place in succinct outline form the events producing this section of crust. NOTE: I=igneous rocks; M=metamorphic rocks; S=sedimentary rocks; F=fault!!! Also, answer the questions on the next page related to the materials in the cross section.

Chronology, Youngest Event (15 points)

Oldest Event

PLEASE NOTE THAT II GIVE PARTIAL CREDIT (LOTS) FOR THIS QUESTION, SO IF YOU CANNOT DETERMINE THE RELATIVE HISTORY OF ALL MATERIALS, AT LEAST DO SO FOR AS MANY AS YOU ARE CONFIDENT!
Questions: (20 points)

1. Rock M2is a metamorphosed granite and has been dated using U-Pb zircon methods as 490.5 +/-22.2 million years; what geologic eon does it belong to?

__________________________________________________________

2. Intrusion I3 has been dated using the K-Ar method as 35 +/-5.5 million years; what geologic era does it belong to?

__________________________________________________________

3. Rocks S1,S2,S3, and S4 are respectively conglomerate, coarse-grained arkose, thin shales, fine-grained quartz-rich sandstone, additional thin shales, and limestone. From a standpoint of depositional environments, what took place in this area during the time of deposition of these rocks ?___________________________ _______________________ __________________ ________________________________________________

___________________________________________________________. How might this have occurred? __________________________________________________________________________________________.

4. In the left part of your cross-section, sedimentary rocks S15 to S19 have been tilted and are now overlain by sedimentary rocks S1 to S4. What kind of unconformity is this? __________ _______________________________ Why must sedimentary rocks S1 to S4 be younger than sedimentary rocks S15 to S19? ______________ ______________________________________________________.

5. Volcanic rock I9 is of early Cenozoic age (after the “extinction” of the dinosaurs), and volcanic rock I14 belongs to the Paleozoic era. What is the boundary between these rocks called? ___________________________________________________________________. How does such a boundary form? ____________________________________________________________________________________.

6. Fault F1 is a moderate angle normal fault. What relationship demonstrates this? ________________ ____________________________________________________________________________________. In a relative sense, when did motion along this fault occur? __________________ ___________________ _____ _________________________________________________________.

7. Volcanic rock I15 has been dated using the U-Pb zircon method as 300+/- 4 million years. This determination essentially dates the time the rock cooled below about 800^oC. To what geologic eon does the rock belong? ______________________ Biotite phenocrysts from the rock give a K-Ar age date of about 60 million years. What geologic era is represented by this date? _____________________. Why is there a discrepancy in age determinations? ______________________________________________________________.

8. Metamorphic rock M3 is an essentially quartzite. What is one of the most abundant minerals in the rock? ______________ _____ What is a logical protolith (parent) for the rock and where would it have formed? ________________________________ _______________________________________________________.

9. Sedimentary rock S1 is a conglomerate, and contains cobbles of slate, phyllite, garnet schist, and mica-bearing gneisses. What kinds of rocks are these cobbles? _______ ____________ ____________ ___________________________________________. How would they have originated ? ______________ ________________________________________________________________________________________.

II. Short Answer Questions (40 points).

1. How can cross-cutting age relations be used to deduce or understand relative geologic ages?_____________

_________________________________________________________________ ____________________.

2. Be definition, an arkose consists of poorly sorted grains of quartz and feldspar; the grains may have originated from a myriad of sources. If someone told you that they were going to date an arkose by the potassium-argon method, would you treat them seriously. Why or why not????? _____________________________________ __________________ _____________________________________ _________________________________ __________________________________________________________ _________________________________.

3. In the context of geologic time, why is the principle of uniformitarianism important to geologists?________________________ _________________________________________________________

________________________________________________________________.

4. What are two of the “possible” causes of the Cretaceous-Tertiary extinction “event”? _____________ ____________________________________________________________________________________________ How have absolute age determinations played a role in assessing the plausibility of such possible causes? ________________________________ __________________________________________________________ __________________________________________________________________________________________

5. What is the difference between regional metamorphism and contact metamorphism? _______________ ____________________________________________________________________________________________.

6. What is the geologic time scale? ____ ___________________ _____________________________________

_______________________________________________________________________________________.

7. What is the most important kind of metamorphic reaction? ______________________________________. Of what importance is this kind of reaction in influencing the melting behavior of rocks? ___________________ __________________________________________________________________________________________?

8. In isotopic age determinations, what fundamental process is utilized to quantitatively estimate the age of a particular mineral in which the decay of an unstable isotope of some element, like potassium-40, over time has occurred? __________________________________________________________________________________ ____________________________________________________________________________________________.

9. Products of regional metamorphism typically have an internal fabric. What does this mean? ____________ ____________________________________________________________________________________________.

10. In normal faults , regardless of the absolute age of the rocks involved, it is always true that rocks that are relatively ___________________ are displaced on top of rocks that are relatively ________________.

11. Draw a simple cross section, including internal layering, depicting a moderate angle reverse fault dipping to the right.. Show the sense of offset and label the relative age of the rocks.!

12. A marble is the metamorphic equivalent of what kind of rock? _____________________________. What is the principal mineral present in a marble? _____________________________________________________.

13. Draw a cross section of a nonconformity and label the unconformity and the different kinds of rocks involved.

14. What is a strike-slip fault?________________________________ ___________________________________

________________________________________________________________ Draw a plan view of a right-lateral (dextral) strike slip fault oriented east-west. Show the sense of offset and label rock layers offset.

15. At the crest of the Sandia Mountains, upper Paleozoic sedimentary rocks rest on top of the Precambrian Sandia Granite. The age of intrusion of the granite is about 1.4 billion years. What is this relationship called?

______________ ____________________________________ __________________Explain, succinctly, how it may have developed ________ ________________________________________________________________ _____________________________________________________________. ___________________________.

16. The Earth's magnetic field is capable of and has on numerous occasions in the past reversed its polarity. How might this phenomenon be useful in determining the age of geologic materials? __________________ ______________________________ _____________________________________________________________

17. We discussed the fact that for “normal” continental lithosphere, the maximum depth of earthquake focii is less than 20 km. Thus, are most earthquakes generated in the crust or mantle? _________________________ What principal factor is responsible for the fact that most earthquakes not occur at depths below this level? ___________________________________________________ Why? _____________________________

__________________________________________________________________________________________

III. Multiple Choice (24 points). There is only one correct answer.

1. The geologic eras of the Phanerozoic eon are, from oldest to youngest,

a. Paleozoic, Cenozoic, Mesozoic

b. Paleozoic, Mesozoic, Cenozoic

c. Cenozoic, Mesozoic, Paleozoic

d. Proterozoic, Mesozoic, Cenozoic

2. A principle assumption in isotopic age determinations is:

a. the rate of decay of an unstable parent isotope remains constant over time.

b. the generation of daughter products begins as soon as the mineral is formed.

c. stable daughter products diffuse readily out of the crystals holding unstable parents.

d. atoms of unstable isotopes are characterized by an unequal number of electrons and protons.

e. virtually all minerals are capable of containing unstable isotopes of particular atoms.

3. OK, friends (I hope). I promised this one. What is the generally accepted age for the Precambrian/Cambrian boundary?

a. 65 million years

b. less than 6,600 YEARS

c. 543 (+/- a few million years

d. over 3.4 billion years

4. Which of the following is the longest (relative) unit of geologic time

a. era

b. eon

c. epoch

d. period

e. none of the above

5. In the Grand Canyon,

a. rocks of Precambrian age are immediately overlain by a thick sequence of lowermost Paleozoic carbonate rocks, indicating a major sea level drop after 570 million years ago.

b. the entire Phanerozoic section of sedimentary rocks is upside down.

c. the sequence of Phanerozic sedimentary rocks exposed records a complex series of relative sea level changes, several of which have resulted in profound disconformities marked by surface exposure in ancient times.

d. the Precambrian "basement" consists exclusively of a 2.5 billion year old gabbro.

e. the entire Mesozoic section is tilted on its side.

f. none of the above

6. A disconformity is identified when there is a significant break in time between layers of parallel strata. Which of the following probably occurred to produce a disconformity.

a. strata were exposed for a period of time, without disruption, prior to deposition of younger strata.

b. strata were tipped upside down prior to deposition of younger strata.

c. the provenance of the sedimentary rock below the unconformity must have been proximal or else grains in that rock would not be angular

d. pre-existing strata must have been deeply incised prior to deposition of younger strata.

e. the beds below the disconformity were deformed prior to deposition of overlying strata.

7. Regional metamorphism occurs

a. when rocks are heated due to the local effects of magma injection

b. when rocks are buried to sufficiently great depths, usually in response to tensional stresses.

c. when rocks are buried to sufficiently great depths, typically in association with mountain building and contractional processes, which facilitate crustal thickening.

d. when rocks are buried to temperature and pressure conditions above the liquidus.

8. Except at the magnetic equator, a compass needle at location in the northern hemisphere points to the magnetic north pole and downward from the horizontal. This phenomenon is noted as

a. magnetic declination

b. magnetic reflection

c. magnetic field reversal

d. magnetic inclination

e. normal magnetic polarity

f. none of the above

9. Our home in the Rio Grande valley is remarkable from a number of geologic perspectives. We live in the central part of a major rift, which means

a. an east-west trending zone in the North American lithosphere defined by major displacement compressional structures (i.e. reverse faults).

b. a north-south trending zone, from central Colorado down through New Mexico is currently extending, producing numerous geologically active normal faults. The probability of motion on these normal faults in the future is not zero.

c. there is sound evidence that western North America will completely split away from the rest of North America within the next five million years.

d. west dipping thrust faults are common throughout central New Mexico

10. Amphibolites are metamorphic rocks consisting essentially of amphibole and plagioclase. What is the most logical protolith?

a. rhyolite

b. halite (rock salt)

c. quartz arenite

d. basalt

e. peridotite

11. Of what significance are the terms strike and dip?

a. the terms are used to describe the orientation of lines in space.

b. the terms are used to describe the orientation of planes in space; strike being the direction of a horizontal line in the plane, and dip being the angle the plane makes with the horizontal.

c. they are terms used to describe the amount of offset along a fault.

d. they are terms used to describe the absolute age of a geologic material.

e. none of the above.

12. Strike-slip faults

a. are low-angle reverse faults

b. have mainly vertical displacement

c. have mainly horizontal movement

d. are faults on which no movement has yet occurred

e. are characterized by uplift of the footwall block.