EXAMPLE!!!!!!!!
EPS 101 PHYSICAL GEOLOGY SECTION 001
THIRD HOURLY EXAMINATION,
Monday, April 28, 1997
Name: _______________________ __________________
This
exam consists of only three parts! Read the instructions to each part and the
following comment carefully.
Numerical scores and tentative grades will be available at the end of
class on Monday, November 27, 1995. PLEASE ATTEND!! Thank you.
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 describe 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
associated questions on the next page.
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 M2 is a metamorphosed granite. It has been dated using U-Pb zircon methods
as 1,680 +/-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. Assuming that this value is the age of the
intrusion and solidification of the rock, 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. What took place in this area
at the time of deposition of these rocks ?___________________________
_______________________
__________________
___________________________________________________________. How might this have occurred? ____
__________________________________________________________________________________________.
4. In the right part of your cross-section,
sedimentary rocks S10 to S7 have been folded.
Assuming that S7 is the youngest sedimentary horizon, what kind of fold
is this? __________ _______________________________ Let us suppose, on the
other hand, that S7 is the oldest sedimentary horizon; what is the
problem?_______________ ______________________________________________________.
5. Sedimentary rock S7 is of early Cenozoic
age (after the “extinction” of the dinosaurs), while sedimentary rock S6
belongs to the Paleozoic era. What is
the boundary between these rocks called?
___________________________________________________________________. How does such a boundary form?
____________________________________________________________________________________.
6. Fault F4 is a moderate angle normal
fault. What relationship demonstrates
this? ________________
____________________________________________________________________________________. In a
relative sense, when did motion along this fault occur? __________________ ___________________ _____
_________________________________________________________.
7.
Extrusive igneous rock I2 has been dated using the U-Pb zircon method as
350+/- 4 million years. This
determination essentially dates the time the rock cooled below about 800oC. To what geologic eon does the rock
belong? ______________________ Biotite
phenocrysts from the rock give a K-Ar age date of about 70 million years. What geologic era is represented by this
date? _____________________. Why is there a discrepancy in age determinations?
______________________________________________________________.
8. Metamorphic rock M3 is an 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?
________________________________
___________________________________________________________.
II. Short Answer Questions (40 points).
1. How can cross-cutting age relations be used
to deduce or understand relative geologic ages?_____________
_________________________________________________________________
____________________.
2. By
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 applied to feldspar grains, would
you treat them seriously? Why or why
not????? _____________________________________ _________________________________
__________________________________________________________
____________________________________________________________________________________________
3. In the context of geologic time, why are
unconformities important to
geologists?________________________
_________________________________________________________
________________________________________________________________
4.
What types of evidence indicate that stress remains active within the
Earth? ___________________________
___________________________________________________________________________________________.
5. Where do deep Focus (below about 50 to 100
km depths) earthquakes occur? ____________
__________________________________________________________________________________
6. What is the geologic time scale? ____
___________________ _____________________________________
_________________________________________________________________.
7. What is the significance of the S-wave
shadow zone, beginning at about 103o away from any particular earthquake
epicenter?
____________________________________________________________________.
Why do P-waves not have any form of shadow
zone?
___________________________________________
__________________________________________________________________________________________
8. What is an earthquake? _______________ _________________________
___________________________
_________________________________________________________________
What is the relationship between earthquakes and plate boundaries?
_________________________________________________________
_________________________________________________________________________________________.
9. Explain why earthquakes occur during
faulting, but not during folding
________________________________________________________________________________
_________________________________________________________________________________.
10. In reverse 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 geologic sketch map (plan
view), using strike and dip symbols, for an anticline plunging to the
south. Label the relative age of the
rocks!!!!
12.
Explain how rock type and temperature influence the type of strain in
rocks. ______________________
___________________________________________________________________________________________
What is the geothermal gradient? Why must it decrease with depth in the
Earth?
13. Draw a cross section of an angular
unconformity and label the unconformity.
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 ________
________________________________________________________________
_____________________________________________________________. ___________________________.
What is the earth’s magnetic field, and how is
it thought to be generated?
_________________________
___________________________________________________________________________________________.
16. The Earth's magnetic field is capable of
and has on numerous occasions in the past reversed its polarity. About (!!!!) how long ago was the last
polarity reversal? ________________________________________
How might the phenomenon of polarity reversals be useful in determining the age
of geologic materials? __________________________________________________________________
________________________
____________________________________________________________
17. What are some differences between an S
and P body seismic waves?
_____________________________________________
______________________________________________
________________________________________________________________________________________
18.
We discussed the fact that for “normal” continental lithosphere, the
maximum depth of earthquake focii is about 15 (+/-) 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 youngest to oldest,
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.
Earthquake epicenters are located by
a. first arrivals of P waves from a single
station, or seismograph
b. a process of “triangulation”, involving
identifying the first arrivals from at least three stations, and making some
assumption about the actual seismic velocity from each station to the focus or
using the time difference between P and S waves to determine approximate
distances to the epicenter
c. measuring the vertical ground motion at a
minimum of three sites.
d. determining where the greatest slip along
the surface expression of the fault responsible occurred.
e. none of the above
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. Most folding results from
a. fracturing
b. compaction
c. rifting
d. convection
e. compression
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. The San Andreas fault system
a. consists of a series of strike slip faults
along which the Pacific ocean lithosphere plate is moving northward relative to
North America
b. is no longer active
c. consists of several normal faults, dipping
to the west, allowing the western margin of California to fall into the Pacific
ocean
d. consists of a series of strike-slip faults
along which the Pacific ocean lithosphere plate is moving southeastward
relative to North America
e. has resulted in numerous earthquakes with
focal depths greater than several hundred km.
10. The elastic rebound theory of faulting and
associated energy release (earthquakes)
a. requires that geologic materials rupture
with the immediate application of stresses greater than those characteristic of
surface conditions.
b. appears inappropriate for most geologic
materials.
c. explains why earthquakes occur constantly
along many faults
d. implies that for at least a period of time
geologic materials actually store energy due to applied stresses and are
strained, prior to rupture.
e. predicts that earthquakes will occur along
specific faults every hundred years.
f. all of the above
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.