Earth and Space Science: TEK 7a: NOTETAKING Worksheet: Answer Key
1. Scientists use __relative dating__ to determine the sequence of geologic events.
2. Scientists use __radiometric dating_ to determine the actual age of a specimen.
3. How is relative dating still used today? When specimens don’t contain radioactive isotopes.
4. From largest to smallest, what are the divisions within the geologic time scale?
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_Eon_________
Era________
_Period_____
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__Epoch______
___Age_______
5. What eon are we currently in? Phanerozoic____.
6. Steno’s Principle of Horizontality states that layers were laid down horizontally because of the force
of __gravity_________.
7. Give an example of how sedimentary layers may be laid down in a manner other than horizontal.
__sand dunes__.
8. How did Steno justify the fact that the lower layers of strata near where he lived contained no fossils?
__It was laid down before the creation of life________.
9. How can the Law of Superposition be used to determine relative ages of geological events? If it is found
in a lower strata, it is an older event_____________.
10. The Law of Lateral Continuity shows how sediments are laid down fairly uniformly, and then as the
sediments are eroded…_you can easily see how they might connect.
11. Describe three ways that cross-cutting can interrupt the geologic sequence of sedimentary layers.
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__Igneous intrusions can cut through older layers of rock_
__Animal burrows can cut across older layers of rock_
__Fractures and faults can cut across layers of rock__
12. Describe and sketch the three types of unconformities:
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Angular: Early sediment that is laid down horizontally is deformed through some process
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which makes it “angular”. Then, this deformation stops, allowing sediments to again collect
on top of the now angular sediments.
Disconformities: Harder to spot, because there is no deformation. Sedimentation stops,
followed by some sort of erosional event (flood perhaps), which then stops, allowing
sediments to again build up. The disconformity is the area where the sediments are carried
away from.
Nonconformities: unconformities that separate igneous/metamorphic regions of rock
from sedimentary. Volcanic sediment, eroded away for a period of time, and then covered
by sedimentary sediment.
13. What are inclusions, and how can they be used to determine relative age of the
sediment? _The tiny particles that make up rock, and they are older than the rock they form.
14. Index fossils are used as guides to the age of the rocks in which they are preserved. Give two
examples of index fossils.
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__Ammonite (nautilus shaped organism) Typical during Jurassic times
__Trilobite typical during the Cambrian Period.
15. Since organisms that form fossils only exist for a short time before becoming extinct, we can
presume _rocks that occur where the fossils are found are the same age.
16. How are geological biozones important to the oil and coal industries? They are based upon origin and
extinction times, so the layer that contains coal corresponds to the time the plant that formed the coal deposit was on the
Earth. These patterns are universal, and if we find coal, oil, and natural gas deposits in sediments in one location, we will find
them in approximately the same rock layers (provided they are similar biozones) all over the Earth. Most biozones are based
upon s first and last appearance of organisms (speciation and extinction events), and can be relatively dated according to those
events.