CHAPTER 6 STUDY GUIDE: THE ROCK AND FOSSIL RECORD
SECTION 1
1. Uniformitarianism and Catastrophism
are two principles that explain Earth’s
history and geologic change differently.
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2. Uniformitarianism states that the same
geologic processes shaping the Earth
today have been at work throughout
Earth’s history.
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3. Catastrophism states that all geologic
change occurs suddenly.
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4. Whereas Uniformitarianism argues that
the Earth’s mountains, canyons, and oceans
are formed by a slow, gradual process
that can still be observed today,
Catastrophism argues that the Earth’s
features are formed by rare, sudden, drastic
events called catastrophes. What Do You
Think?
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5. Modern Geology supports both
Uniformitarianism and Catastrophism. Today’s
scientists agree that most geologic change is
uniform and gradual, but they also agree that
catastrophes do occur occasionally. What
catastrophes can you think of that have changed
the Earth’s surface?
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SECTION 2: RELATIVE DATING
6. Geologists study rocks and fossils to determine
the order of events in Earth’s history.
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7. Determining whether an object or event is older
or younger than other objects or events is called
relative dating.
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8. Superposition is a principle that states that
younger rocks lie above older rocks in
undisturbed sequences. “YOUNGER OVER
OLDER” is a phrase you can use to remember
this principle.
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9. The geologic column is an ideal sequence of
rock layers that contains all the known fossils
and rock formations on Earth arranged
from oldest to youngest. Geologists rely on
the geologic column to interpret rock
sequences.
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10. Sometimes rock layers are disturbed by
features and events. Features that can disturb
rock layers are faults and intrusions. Events
that can disturb rock layers are folding and
tilting.
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11. Faults and intrusions are features that cut
across rock layers. A fault and intrusion
ARE ALWAYS YOUNGER THAN THE
LAYERS THEY CUT ACROSS.
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12. Folding and tilting are events that disturb the
horizontal layers of rock. Folding and tilting
ARE ALWAYS YOUNGER THAT THE
ROCK LAYERS THEY AFFECT.
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13. An uncomformity is a surface that represents a
missing part of the geologic column.
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14. Unconformities form where rock layers are
missing and they create a gap in the rock-layer
sequences.
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15. Unconformities are created by nondeposition
of sediments and/or erosion of sediments.
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16. There are three types of unconformities:
disconformity, nonconformity, and angular
unconformity. They each represent missing
time - time that was not recorded in layers of
rock.
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17. A disconformity exits where part of a
sequence of parallel rock layers is missing. A
disconformity is the most common type of
unconformity and the most difficult to see.
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18. A nonconformity exists where sedimentary
rock layers lie on top of an eroded surface of
non-layered igneous or metamorphic rock.
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19. An angular unconformity exists between
horizontal rock layers and rock layers that
are tilted or folded. (The tilted or folded layers
were eroded before horizontal layers formed
above them.)
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SECTION 3: ABSOLUTE DATING
20. Absolute Dating is the process of establishing
the age of an object, such as a fossil or rock
layer, by determining the number of years it has
existed. How does this differ from relative
dating?
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21. Isotopes are atoms of the same element that
have the same number of protons but have
different numbers of neutrons.
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22. Unstable isotopes are radioactive.
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23. Radioactive isotopes tend to break down into
stable isotopes of other elements in a process
called radioactive decay.
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24. If you know the rate of decay for an element in
a rock, you can figure out the age of the rock.
Determining the absolute age of a sample
based on the ratio of parent material to
daughter material is called radiometric
dating.
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25. The time it takes for one-half of a radioactive
sample to decay is called a half-life.
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26. There are three types of radiometric dating:
(1) Uranium-Lead Method, (2) PotassiumArgon Method, (3) Carbon-14 Method.
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27. Scientist use different radiometric dating
techniques based on the estimated age of an
object. For example: Uranium-238 has a halflife of 4.5 billion years, so it can be used to date
rocks more than 10 million years old. However,
Carbon-14 has a half-life of only 5,730 years, so
it is mainly used for dating things that lived
within the last 50,000 years. Potassium-40 has
half-life of 1.3 billion years, so it dates rocks
older than 100,000 years.
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SECTION 4: FOSSILS
28. A fossil is any naturally preserved evidence of
life.
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29. Usually fossils are preserved in rock when the
soft, fleshy parts of their bodies decay and the
hard parts are quickly buried in sediment.
However, fossils may be preserved in other
ways. Read on to discover other ways fossils
are preserved.
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30. Fossils may be preserved through mineral
replacement, such as permineralization and
petrification. Permineralization occurs when
minerals fill in the pore spaces of an
organism’s tissues. Petrification occurs when
the organism’s tissues are completely replaced
by minerals.
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31. Many insect fossils, such as mosquitoes, are
preserved in amber, which is hardened tree sap.
Some organisms are preserved through
mummification, which occurs when the body of
the organism dries out before it can decay. Other
fossils may preserved in tar pits and in ice.
Some of the best fossils are frozen specimens.
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32. A trace fossil is any naturally preserved evidence
of an animal’s activity. Examples include
tracks, burrows, and coprolites.
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33. Coprolites are preserved feces or dung from
animals. Coprolites provide valuable information
about the habits and diets of the animals that left
them.
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34. Molds and Casts are also considered fossils. A
mold is a cavity in the ground or rock where a
plant or animal was buried. A cast forms when
sediment fills a mold and becomes a rock. A cast
shows what the outside of the organism looked
like.
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35. By examining fossils scientists can find out what
was happening in the environment when the
sediments surrounding the fossils were deposited.
For example, What do fossils of marine organisms
on the top of a mountain tell us?
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36. Scientists can interpret how plants and animals
have changed over time by studying fossils from
different parts of the geologic column.
For example, we know that fish existed before
amphibians because fish were found in a lower
layer of rock. How do we know that amphibians
existed before reptiles?
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37. Index fossils are fossils that lived during a
relatively short period of time. Index fossils help
scientists date a rock layer without directly using
radiometric dating.
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SECTION FIVE: GEOLOGIC TIME SCALE
38. The geologic time scale, divides the Earth’s 4.6
billion year history into distinct intervals of time.
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39. The largest divisions of geologic time are the
eons. Eons are divided into eras. Eras are
divided into periods. Periods are divided into
epochs. Epochs are the shortest divisions of
geologic time.
40. The Paleozoic Era: Paleozoic means “old life.”
The first land-dwellers – plants and amphibians
were evident during this era.
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41. The Mesozoic Era: Mesozoic means “middle
life.” This era is known as the Age of Reptiles.
Dinosaurs inhabited the land and water.
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42. The Cenozoic Era: Cenozoic means “recent
life.” The Cenozoic Era began 65 million years
ago and continues to the present. This era is
known as the Age of Mammals. Which era do we
live in?
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43. Scientist estimate the Earth to be 4.6 billion years
old. If the Earth’s history is put on a scale of only
12 hours, human civilization would have been
around for less than one second.
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