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GEOLOGY 12
ANSWERS
CHAPTER 8
GEOLOGIC TIME AND THE FOSSIL RECORD
QUESTIONS FOR REVIEW
1.
Describe the significance of the Principle of Superposition and the Principle of
Original Horizontality to relative dating of sedimentary sequences.
• The Principle of Superposition introduces the idea of time sequence in sediment
deposition: the sediments at the bottom were deposited first, with successively
younger sediments laid on top. This principle by itself is limited to individual
outcrops.
• The Principle of Original Horizontality states that most sedimentary layers of rock
are deposited in a horizontal position.
2.
What is the distinction between a disconformity and an angular unconformity?
What do they have in common?
• A disconformity is an unconformity at which the bedding or strata above and below
are parallel.
• At an angular unconformity, this is not the case. The bedding or strata above and
below meet at an angle.
• Both features indicate a gap in time in the sedimentary record.
3.
Explain two ways in which you might determine the relative ages of a pluton
and surrounding sedimentary rocks.
• If the pluton is clearly discordant (i.e. dikes), it must postdate the sedimentary rocks
whose structures are crosscut.
• Likewise, if the pluton has an obvious metamorphic aureole (due to contact
metamorphism) within the sedimentary rocks, the pluton must have come later.
4.
How is the correlation of rock units made easier by the concept of faunal
succession? What is a limitation on its use?
• The usefulness of the concept of faunal succession rests in its implication that if
one finds the same fossil form in two rocks that are widely separated in space,
these rocks should be similar in age since the same creature would not have
developed at two different times in different places.
• An obvious limitation is that it cannot be used to correlate unfossiliferous rocks.
5.
Why is it important to radiometric dating that radioactive elements have
constant half-lives?
• If this were not true, then one could not use the relative amounts of parent and
daughter isotopes to calculate the age of the material since there would be no way
to know the rate(s) at which the parent had decayed into the daughter.
6.
Describe any three requirements that must be satisfied in order for a
radiometric decay scheme to be useful in dating geologic materials.
• The parent must occur in measurable abundance in the material of interest.
• Either the daughter must not be incorporated into the material, or we must have a
way to correct for the amount originally present.
• The parent must have an appropriate half-life: long enough that not all the parent
has decayed, but short enough that there has been measurable
decay/accumulation of the daughter isotope since the time of interest.
• Also, the sample or system itself must remain a closed system to parent and
daughter isotopes. For example, the release of argon gas (daughter isotope of
potassium-40) will make a rock appear younger.
7.
It has proven somewhat difficult to establish radiometric dates for the units of
the Phanerozoic time scale, because the subdivisions were defined using
sedimentary rocks. Explain. How do geologists address this problem?
• It is rarely possible to accurately date the time a sedimentary rock was deposited.
• Therefore, the rock units of various ages are dated indirectly, using either age limits
derived from correlation between type sections and dateable units elsewhere.
8.
When the geologic time scale was first established, the Precambrian was not
subdivided. Why?
• There are few fossils in Precambrian rocks. Therefore, it was not possible to
subdivide the Precambrian on the basis of appearance and disappearance of fossil
groups, as was done for the Phanerozoic.
• At the time there was no other obvious basis upon which to subdivide it in the
absence of radiometric dates. (In fact, early geologists had no idea of the great
length of Precambrian time.)
Geology 12
Chapter 8
Questions For Review
Page 2
9.
How is the time of the "Big Bang" determined?
We work backwards. Since the Big Bang, the universe has been continuously
expanding and, thus, there has been more and more distance between clusters of
galaxies. This phenomenon of galaxies moving farther away from each other is known
as the red shift.
As light from distant galaxies approach earth there is an increase of space between
earth and the galaxy. Through a calculation involving the distance of far-off clusters
and the red shift, a final estimation can be made as to how long the galaxy has been
moving away from us. In turn, this number can be used inversely to turn back the
clock to a point when the two galaxies were in the same place at the same time, or,
the moment of the Big Bang. The equation generally used to show the age of the
universe is shown here:
(distance of a particular galaxy) / (that galaxy’s velocity) = (time)
or
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4.6 x 10 cm / 1 x 10 cm/sec = 4.6 x 1017 sec
This equation, equaling 4.6 x 1017 seconds, comes out to be approximately fifteen
billion years. This calculation is almost exactly the same for every galaxy that can be
studied.
10. Briefly summarize the process by which the solar system formed. Does this
process lead to planets that are similar or quite different in composition.
Explain.
•
The PROTOPLANET HYPOTHESIS describes the steps in planet formation:
(A) The solar system begins to form as a rotating cloud, or nebulae, collapses.
(B) Instabilities in the nebulae cause dust particles to stick together. The dust
particles accrete into billions of planetesimals. The planetesimals then collide
and form protoplanets. Meanwhile, the protosun in the center of the nebular
disk becomes massive and hot enough to "turn on" by fusing hydrogen.
(C) The Sun begins to radiate energy and vaporize dust in the inner part of the
Solar System. The remaining gas is blown away to outer regions by solar
winds.
•
This hypothesis explains the fact that inner planets and outer planets have similar
composition. The inner planets are rocky and outer planets are gaseous
Geology 12
Chapter 8
Questions For Review
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11. Why is it not possible to determine the age of the earth directly using
radiometric methods? On what basis is its age estimated?
• The earth is such an active planet that its rocks constantly being reworked and
changed through the Rock Cycle so we have no rocks preserved unaltered from
the time of earth's formation.
• However, meteorites are believed to have formed from the solar nebula at the
same time and the majority of meteorites yield radiometric ages in the range of 4.5
to 4.6 billion years whic is then taken as the approximate age of the earth.
12. Has the earth always had oceans and atmosphere? Explain.
• The original atmosphere of the Earth, Venus and Mars consisted of Hydrogen and
Helium. Those light elements, however, were evaporated by solar radiation and
thus this original atmosphere escaped. The current atmospheres are therefore
secondary and evolved from other processes such as volcanic outgassing during
the process of differentiation.
• Once the Earth cooled sufficiently, sometime in the first 700 million, clouds began
to form in the atmosphere. It began to rain. And it rained and it rained and it rained.
As the original continents were formed the rain water began to accumulate in
oceans.
13. What critical change in the composition of earth's atmosphere occurred some
time after the planet's differentiation, what caused this change, and what were
its implications for the evolution of animal life on earth?
•
The early atmosphere was strongly affected by volcanic outgassing. It was
comprised of nitrogen, ammonia, methane, water vapour, carbon monoxide,
carbon dioxide, and sulphur dioxide. Before photosynthetic life, there was very
little oxygen production.
•
The precipitation of water from the atmosphere into lakes and oceans reduced
atmospheric carbon dioxide which became dissolved in the water. This left and
atmosphere dominated by nitrogen.
•
The early oceans eventually provided a suitable environment for photosynthetic
life forms (bacteria and cyanobacteria and stromatolite mounds) which appeared
3.4-3.2 bya. The first signs that free oxygen became available date from ~2.5
bya. This is the time of the earliest 'banded iron formations', which are rocks of
alternating layers of hematite or magnetite and chert. The rise of oxygen seems
to have occurred between 2.2 and 1.9 bya.
•
The increase in free-oxygen in the atmosphere gave rise to conditions suitable for
the evolution of other life forms.
Geology 12
Chapter 8
Questions For Review
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