Geology Reference
In-Depth Information
4. If a fl ake of biotite within a sedimentary rock (such as
a sandstone) is radiometrically dated, the date obtained
indicates when
a. _____ the biotite crystal formed;
b. _____ the sedimentary rock formed;
c. _____ the parent radioactive isotope formed;
d. _____ the daughter radioactive isotope(s) formed;
e. _____ none of these.
5. In which type of radioactive decay are two protons and
two neutrons emitted from the nucleus?
a. _____ alpha decay;
b. _____ beta decay;
c. _____ electron capture;
d. _____ fi ssion track;
e. _____ none of these.
6. What is being measured in radiometric dating is
a. _____ the time when a radioactive isotope formed;
b. _____ the time of crystallization of a mineral con-
taining an isotope;
c. _____ the amount of the parent isotope only;
d. _____ when the dated mineral became part of a
sedimentary rock;
e. _____ when the stable daughter isotope was formed.
7. How many half-lives are required to yield a min-
eral with 625,000,000 atoms of thorium 232 and
19,375,000,000 atoms of lead 235?
a. _____ l;
b. _____ 2;
c. _____ 3;
d. _____ 4;
e. _____ 5.
8. As carbon 14 decays back to nitrogen in radiocarbon
dating, what isotopic ratio decreases?
a. _____ nitrogen 14 to carbon 12;
b. _____ carbon 14 to carbon 12;
c. _____ carbon 13 to carbon 12;
d. _____ nitrogen 14 to carbon 14;
e. _____ nitrogen 14 to carbon 13.
9. Considering that the half-life of uranium 235 is
704 million years, what fraction of the original ura-
nium 235 will remain after 2,816,000,000 years?
a. _____ 1 / 2 ;
b. _____ 1 / 4 ;
c. _____ 1 / 8 ;
d. _____ 1 / 16 ;
e. _____ 1 / 32 .
10. A lithostratigraphic unit made up of two or more
formations is a
a. _____ group;
b. _____ facies;
c. _____ range zone;
d. _____ member;
e. _____ disconformity.
11. What is the difference between relative dating and
absolute dating?
12. In some places where disconformities are particularly
diffi cult to discern from a physical point of view, how
could you use the principle of fossil succession to rec-
ognize a disconformity? How could the principle of
inclusions be used to recognize a nonconformity?
13. An igneous rock was radiometrically dated using
the uranium 235 to lead 207 and potassium 40 to
argon 40 isotope pairs. The isotope pairs yielded
distinctly different ages. What possible explanation
could be offered for this result? What would you do to
rectify the discrepancy in ages?
14. Can the various principles of relative dating be used
to reconstruct the geologic history of Mars? Which
principles might not apply to interpreting the geologic
history of another planet?
15. Why were Lord Kelvin's arguments and calculations
so compelling, and what was the basic fl aw in his
assumption? What do you think the course of
geology would have been if radioactivity had not
been discovered?
16. When geologists reconstruct the geologic history of
an area, why is it important for them to differentiate
between a sill and a lava fl ow? How could you tell the
difference between a sill and a lava fl ow at an outcrop
if both structures consisted of basalt? What features
would you look for in an outcrop to positively identify
the structure as either a sill or a lava fl ow?
17. If you wanted to calculate the absolute age of an intru-
sive body, what information would you need?
18. Why do igneous rocks yield the most accurate radio-
metric dates? Why can't sedimentary rocks be dated
radiometrically? What problems are encountered in
dating metamorphic rocks?
19. What is the major difference between the carbon-14
dating technique and the techniques used for the fi ve
common, long-lived radioactive isotope pairs?
20. Given the current debate over global warming and the
many short-term consequences for humans, can you
visualize how the world might look in 100,000 years or
even 10 million years? Use what you've learned about
plate tectonics and the direction and rate of movement
of plates, as well as how plate movement and global
warming will affect ocean currents, weather patterns,
weathering rates, and other factors, to make your
prediction. Do you think such short-term changes
can be extrapolated to long-term trends in trying to
predict what Earth will be like using a geologic time
perspective?
Search WWH ::




Custom Search