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The Geologic Time Scale
Objectives
• Describe the geologic time scale.
• Distinguish among the following geologic time
scale divisions: eon, era, period, and epoch.
Vocabulary
– geologic time scale
– eon
– era
– period
– epoch
The Geologic Time Scale
The Geologic Time Scale
• By studying the characteristics of rocks and the
fossils within them, geologists can interpret the
environments in which the rocks were deposited,
reconstruct Earth’s history, and possibly predict
events or conditions in the future.
The Geologic Time Scale
The Rock Record
• Geologists have divided the history of Earth
into time units based upon the fossils
contained within the rocks.
• The geologic time scale is a record
of Earth’s history from its origin
4.6 billion years ago to the present.
• This scale allows the correlation of geologic
events, environmental changes, and the
development of life-forms that are preserved
in the rock record.
The Geologic Time Scale
The Rock Record
The Geologic Time Scale
Geologic Time
• The oldest division of time is at the bottom of
the geologic time scale.
• The time scale is divided into units called eons,
eras, periods, and epochs.
– An eon, measured in billions of years, is the longest
time unit of the geologic time scale.
– An era, defined by the differences in life-forms found in
rock and measured in hundreds of millions to billions of
years, is the second-longest span of time on the
geologic time scale.
The Geologic Time Scale
Geologic Time
• Precambrian Time, which makes up
approximately 90 percent of geologic time, is
divided into the Archean and Proterozoic Eons.
• The end of the Proterozoic, the more recent of
the two, is marked by the first appearance of
organisms with hard parts.
The Geologic Time Scale
Geologic Time
Plants and Animals Evolve
– During the Paleozoic Era, the oceans became full of a
wide diversity of plants and animals; land plants
appeared and were followed by land animals.
– The Mesozoic Era is known for the emergence and
evolution of dinosaurs, reef-building corals, large
predatory reptiles, and flowering plants and trees.
– During the Cenozoic Era, mammals increased both in
number and diversity, human ancestors developed, and
grasses and flowering plants expanded on land.
The Geologic Time Scale
Geologic Time
Periods of Geologic Time
– Periods, usually measured in terms of tens of millions
of years to hundreds of millions of years, are defined by
the life-forms that were abundant or became extinct
during the time in which specific rocks were deposited.
– The Cenozoic is divided into three periods: the
Paleogene, Neogene, and Quaternary.
– The boundaries between the periods of the Cenozoic
are not marked by extinction events.
The Geologic Time Scale
Geologic Time
Epochs of Geologic Time
– Epochs are smaller divisions of geologic time and are
usually measured in millions of years to tens of millions
of years.
– The Cenozoic Periods have been further divided into
epochs including the Paleocene and the Oligocene.
– Different groups of organisms have been used to
distinguish the various epochs.
– Regardless of how a geologic period was defined, each
unit contains specific characteristics that set it apart
from the rest of geologic history.
The Geologic Time Scale
Section Assessment
1. Match the following terms with their definitions.
___
C eon
___
A period
___
D era
___
B epoch
A. time periods defined by the life forms
that were present; usually measured
in terms of tens of millions to
hundreds of millions of years
B. smaller divisions of time; usually
measured in millions to tens of
millions of years
C. the longest period of time; measured
in billions of years
D. second longest period of time;
measured in hundreds of millions to
billions of years
The Geologic Time Scale
Section Assessment
2. How does the geologic time scale correspond
with the rock record?
Moving upward on the scale, each division is
younger, just as the rock layers in the rock
record grow younger as you move upward.
The Geologic Time Scale
Section Assessment
3. Identify whether the following statements are
true or false.
______
true Precambrian Time represents 90 percent of
geologic time.
______
false Human ancestors developed during the
Paleozoic Era.
______
false The boundaries between the periods of the
Cenozoic are marked by mass extinctions.
______
true Organisms in Precambrian Time had soft
bodies with no shells or skeletons.
Relative-Age Dating of Rocks
Objectives
• Apply the principles for determining relative age to
interpret rock sequences.
• Describe an unconformity and how it is formed within
the rock record.
Vocabulary
– uniformitarianism
– cross-cutting relationships
– original horizontality
– unconformity
– superposition
– correlation
Relative-Age Dating of Rocks
Relative-Age Dating of Rocks
• The principle of uniformitarianism states that
the forces that continually change the surface
features of Earth today have been occurring
since Earth formed.
• Only the rate, intensity, and scale with which the
forces occur have changed.
• The resulting sediments and rocks all record an
environment and fossils within the rocks preserve
evidence of the life-forms that lived during the
time of deposition.
Relative-Age Dating of Rocks
Principles for Determining Relative Age
• The concept of relative-age dating places the
ages of rocks and the events that formed them
in order, but without exact dates.
• This is done by comparing one event or rock
layer to another.
Relative-Age Dating of Rocks
Principles for Determining Relative Age
Geologic Principles
– The principle of original
horizontality states that
sedimentary rocks are
deposited in horizontal or
nearly horizontal layers.
– The principle of superposition
states that in an undisturbed
rock sequence, the oldest rocks
are at the bottom and each
successive layer is younger than
the layer beneath.
Relative-Age Dating of Rocks
Principles for Determining Relative Age
Geologic Principles
– The principle of cross-cutting
relationships states that an
intrusion or a fault is younger
than the rock it cuts across.
Relative-Age Dating of Rocks
Principles for Determining Relative Age
Inclusions
– Relative age also can be determined where an
overlying rock layer contains particles of rock material
from the layer beneath it.
– These particles, called inclusions, indicate that the
rocks in the lower layer are older than those on top.
Relative-Age Dating of Rocks
Other Means of Determining Relative Age
• The fact that Earth is constantly changing makes
it difficult to find an undisturbed sequence of
rock layers.
• An unconformity is a gap in the rock record
usually caused by an erosional surface becoming
buried by the deposition of younger rocks.
Relative-Age Dating of Rocks
Other Means of Determining Relative Age
• The unconformity is called a disconformity when
horizontal sedimentary rocks overlie horizontal
sedimentary rocks.
Relative-Age Dating of Rocks
Other Means of Determining Relative Age
• A different type of unconformity exists when
sedimentary rocks overlie nonsedimentary rocks.
• The contact point
between the
nonsedimentary and
sedimentary rock is
called a nonconformity.
Relative-Age Dating of Rocks
Other Means of Determining Relative Age
• An angular unconformity is created when
horizontal sedimentary rocks are uplifted and
tilted, are exposed to erosional processes, and
deposition resumes.
Relative-Age Dating of Rocks
Other Means of Determining Relative Age
Correlation of Rock Strata
– Correlation is the matching of outcrops of one
geographic region to another.
– Geologists examine rocks for distinctive fossils and
unique rock or mineral features to help correlate the
rock layers.
– Correlation allows geologists to accurately locate that
same rock layer in another location.
Relative-Age Dating of Rocks
Section Assessment
1. Match the following terms with their definitions.
___
A original horizontality
___
C superposition
A. principle which states that
sedimentary rocks are
deposited in horizontal layers
___
B unconformity
B. a gap in the rock record
___
D correlation
C. principle which states that
oldest rocks are at the
bottom and that each
successive layer is younger
D. matching of outcrops from
one geographic region
to another
Relative-Age Dating of Rocks
Section Assessment
2. What is the principle of uniformitarianism?
The principle of uniformitarianism states that the
forces that change the surface of Earth that are
occurring today have been occurring since
Earth formed.
Relative-Age Dating of Rocks
Section Assessment
3. Identify whether the following statements are
true or false.
______
false Relative-age dating allows geologists to
determine the age of rock formations.
______
true A limestone layer is older than a shale layer
that is above it.
______
false Rock layers are often found undisturbed if you
dig deep enough.
______
false The grains in a rock layer can be from a
younger layer of rock.
Absolute-Age Dating of Rocks
Objectives
• Explain the several different methods used by scientists
to determine absolute age.
• Describe how objects are dated by the use of certain
radioactive elements.
• Explain how annual tree rings and glacial varves are
used to date geologic events.
Vocabulary
– radioactive decay
– dendrochronology
– radiometric dating
– varve
– half-life
– key bed
Absolute-Age Dating of Rocks
Absolute-Age Dating of Rocks
• Absolute-age dating enables scientists to
determine the actual age of a rock, fossil, or other
object using the decay rate of radioactive isotopes.
– Radioactive substances emit nuclear particles
at a constant rate regardless of any physical or
environmental changes.
– The original radioactive element is referred to as the
“parent,” and the new element is referred to as the
“daughter.”
– As the numbers of protons and neutrons change with
each nuclear emission, the element is converted to a
different element.
Absolute-Age Dating of Rocks
Absolute-Age Dating of Rocks
• Radioactive decay is the emission of radioactive
particles and the resulting change into other
elements over time.
Absolute-Age Dating of Rocks
Use of Radioactive Isotopes
• In a process called radiometric dating, scientists
attempt to determine the ratio of parent nuclei to
daughter nuclei within a given sample of a rock or
fossil to determine its absolute age.
• Because it often takes a long time for the entire
amount of an isotope to decay, geologists use
the half-life of an isotope.
Absolute-Age Dating of Rocks
Use of Radioactive Isotopes
• Half-life is the length of time it takes for one-half of
the original amount of an isotope to decay.
Absolute-Age Dating of Rocks
Use of Radioactive Isotopes
Carbon-14
– Carbon-14 (C-14) is a radioactive isotope that is
commonly used to determine the absolute age of
an object, especially one that is of organic origin.
– C-14 is accurate
for dating objects
up to 75 000
years old.
– For the dating of
a particularly old
rock sample, a radioactive isotope with a longer
half-life must be used.
Absolute-Age Dating of Rocks
Other Ways to Determine Age
• Naturally occurring materials, such as trees,
lake-bottom sediment, and volcanic ash can also
be used to help geologists determine the age of
an object or event.
Absolute-Age Dating of Rocks
Other Ways to Determine Age
Tree Rings
– The age of a tree can be determined by counting
the number of annual tree rings in a cross section
of the tree.
– The widths of tree rings are directly related to the
climatic conditions during growth periods.
– Dendrochronology is the science of comparing annual
growth rings in trees to date events and changes in past
environments.
Absolute-Age Dating of Rocks
Other Ways to Determine Age
Seasonal Climatic Changes
– About 11 000 years ago, continental glaciers covered
the northern part of the United States.
– Varves are bands of alternating light- and dark-colored
sediments of sand, clay, and silt found in lakes that
resulted from summer and winter runoff from glaciers.
– Varves from different lakes can be compared to
determine the ages of glacial lake sediments from
about 15 000 to 12 000 years ago.
Absolute-Age Dating of Rocks
Other Ways to Determine Age
Distinctive Sediment Layers
– When a widespread sediment layer is formed by an
instantaneous or short-lived event, geologists may be
able to determine the time of the event through
radiometric dating.
– A key bed is a layer which has been dated and acts as
a time marker, which can be used to correlate rock
layers across large areas.
Absolute-Age Dating of Rocks
Section Assessment
1. Match the following terms with their definitions.
___
B half-life
___
D dendrochronology
___
A varve
___
C key bed
A. alternating light- and darkcolored sedimentary deposits
in glacial lakes
B. the time period until the ratio of
parent-to-daughter atoms is
equal
C. a widespread layer that has
been accurately dated
D. the science of comparing
annual growth rings in trees
to date events and
environmental changes
Absolute-Age Dating of Rocks
Section Assessment
2. How old is an object of organic origin if it has
25 percent of carbon-14 remaining? Why?
The object is 11 460 years old. To reach the
25 percent level, the carbon-14 has experienced
2 half-life cycles of 5730 years each.
Absolute-Age Dating of Rocks
Section Assessment
3. Why would rubidium-87 with a half-life of
48.6 billion years probably not be useful in
dating an object that is 100 000 years old?
With such a long half-life, the ratio of
parent-to-daughter atoms would be too
small to measure.
Remains of Organisms in the Rock Record
Objectives
• Define fossil.
• Explain several methods by which fossils can be preserved.
• Describe the characteristics of an index fossil.
• Discuss how fossils can be used to interpret Earth’s past
physical and environmental history.
Vocabulary
– fossil
– permineralization
– evolution
– index fossil
– original preservation
– mold
– altered hard part
– cast
Remains of Organisms in the Rock Record
Remains of Organisms in the Rock Record
• Fossils are the evidence or remains of
once-living plants or animals.
• The fossil record provides evidence of evolution.
• Evolution is an adaptive change in the DNA of
populations as a result of mutation and/or
environmental change.
• Fossils preserved in the rock record also provide
information about past environmental conditions
and can be used to correlate rock layers from one
area to another.
Remains of Organisms in the Rock Record
Types of Fossils
• Fossils with original preservation are the soft
and hard parts of plant and animal remains that
have not undergone any kind of change since
the organisms’ deaths.
• Such fossils are uncommon because their
preservation requires extraordinary circumstances
such as freezing, drying out, or oxygen-free
environments.
Remains of Organisms in the Rock Record
Types of Fossils
Altered Hard Parts
– Altered hard parts are fossils in which all the organic
material has been removed and the hard parts of a
plant or animal have been changed either by mineral
replacement or by recrystallization.
– Permineralization is the process by which pore
spaces in a fossil are filled in with mineral substances.
– During the process of recrystallization, the exterior of
the shell or a bone remains the same, but the shell
microstructures are destroyed.
Remains of Organisms in the Rock Record
Types of Fossils
Altered Hard Parts
Remains of Organisms in the Rock Record
Types of Fossils
Index Fossils
– Index fossils are remains of plants or animals that can
be used by geologists to correlate rock layers over large
geographic areas or to date a particular rock layer.
– An index fossil is easily recognized, abundant, and
widely distributed geographically and must also have
lived during a short period of time.
Remains of Organisms in the Rock Record
Types of Fossils
Molds and Casts
– A mold is formed when the original shell parts of an
organism within a sedimentary rock are weathered and
eroded.
– A hollowed-out impression, or mold, of the shells is left
in their place.
– A cast of an organism is created if the cavity later
becomes filled with minerals or sediment.
Remains of Organisms in the Rock Record
Types of Fossils
Molds and Casts
Remains of Organisms in the Rock Record
Types of Fossils
Indirect Evidence of Past Life
– Trace fossils are indirect evidence of plant and
animal life.
– Trace fossils can provide information about how an
organism lived, how it moved, or how it obtained food.
Remains of Organisms in the Rock Record
Why study fossils?
• The study of fossils allows scientists to interpret
and describe Earth’s history.
• Fossils may help scientists find patterns and
cycles that can be used to predict future
phenomena, such as climatic changes.
• The study of fossils allows geologists to locate
energy resources.
Remains of Organisms in the Rock Record
Section Assessment
1. Match the following terms with their definitions.
___
C original
preservation
___
A altered hard part
___
B permineralization
___
D cast
A. fossils in which all organic
material has been removed and
bones or shells have been
structurally changed
B. process by which pore spaces
are filled in with mineral
substances
C. fossils in which soft and hard
parts of an organism have not
undergone any kind of change
D. formed when a mold becomes
filled with minerals or sediments
Remains of Organisms in the Rock Record
Section Assessment
2. What conditions are necessary to allow original
preservation?
Original preservation requires extraordinary
circumstances such as freezing, drying out, or
oxygen-free environments.
Remains of Organisms in the Rock Record
Section Assessment
3. How might petroleum geologists use fossils?
Petroleum geologists use certain index
microfossils to determine whether oil might be
present at a particular site. These fossils provide
information about the ages of rocks and, in some
cases, information that indicates whether the
temperature and pressure conditions needed to
form oil or gas were present in those layers.
Chapter Resources Menu
Study Guide
Section 21.1
Section 21.2
Section 21.3
Section 21.4
Chapter Assessment
Image Bank
Section 21.1 Study Guide
Section 21.1 Main Ideas
• Geologists have separated Earth’s history into divisions
based upon the fossil record.
• The divisions of the geologic time scale, in descending
order and decreasing length of time spans, are eons,
eras, periods, and epochs.
Section 21.2 Study Guide
Section 21.2 Main Ideas
• The principles of uniformitarianism, original horizontality,
superposition, and cross-cutting relationships are used to
interpret Earth’s rock record and, thus, to describe the
planet’s history.
• Unconformities caused by weathering and erosion or by
periods of nondeposition mark missing layers in the rock
record.
Section 21.3 Study Guide
Section 21.3 Main Ideas
• Absolute-age dating measures the actual age of an
object such as a mineral, rock, or fossil.
• Radioactive decay is the emission of particles from a
radioactive atom. The decay rate can be used to
determine the age of a rock or fossil. The time it takes a
radioactive element to decay to 50 percent of its original
mass is known as its half-life.
• Tree rings and varves can also determine the dates of
events and changes in the environment. Volcanic ash
and meteorite-impact debris create key beds that mark
the time of the event.
Section 21.4 Study Guide
Section 21.4 Main Ideas
• The remains and evidence of plants and animals that
once lived on Earth are called fossils.
• Fossils preserved in the rock record provide information
about past environmental conditions, evolutionary
changes in life-forms, and help geologists to correlate
rock layers from one area to another.
Chapter Assessment
Multiple Choice
1. Which of the following is not an eon?
a. Phanerozoic
c. Cenozoic
b. Proterozoic
d. Archean
The Cenozoic Era is the most recent period in the
Phanerozoic Eon. The Cenozoic was preceded by the
Mesozoic and Paleozoic Eras which form the rest of the
Phanerozoic Eon. The Phanerozoic Eon was preceded
by the Proterozoic and Archean Eons.
Chapter Assessment
Multiple Choice
2. A gap in the rock record is known as a(n) ____.
a. unconformity
c. superposition
b. correlation
d. inclusion
Unconformities can be classified as disconformities or
nonconformities. They are the result of deposition on top
of an active erosional surface.
Chapter Assessment
Multiple Choice
3. Approximately how old is a wood sample that
has 3.2 percent of its C-14 remaining?
a. 11 000 years
c. 24 000 years
b. 16 000 years
d. 28 000 years
After five half-lives of 5730 years (28 650 years total),
around 3.2 percent of the C-14 would remain.
Chapter Assessment
Multiple Choice
4. A change in populations as a result of
environmental change is known as ____.
a. permineralization
c. altered hard parts
b. fossilization
d. evolution
Fossils provide evidence of the past existence of a wide
variety of life-forms, most of which have become extinct.
The fossil record also provides evidence that populations
have undergone change through time in response to
changes in their environments.
Chapter Assessment
Multiple Choice
5. On the geologic time scale, which division is
defined by the life forms that were abundant
or became extinct?
a. period
c. eon
b. era
d. epoch
The order of the divisions of geologic time classification
beginning with the largest is eon, era, period, and epoch.
Chapter Assessment
Short Answer
6. What does the principle of superposition state?
The principle of superposition states that in
an undisturbed rock sequence, the oldest
rocks are at the bottom and each successive
layer is younger than the layer beneath.
Chapter Assessment
Short Answer
7. What does the principle of cross-cutting
relationships state?
The principle of cross-cutting relationships
states that an intrusion or a fault is younger
than the rock it cuts across.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
true Fallout from a major volcanic eruption can form
a key bed.
______
true Homo sapiens evolved in the past .01 M.Y.B.P.
______
false Varves are direct evidence of past life-forms.
______
false Dendrochronology is limited to the age of the
oldest living tree.
______
true The first fish appeared in the Paleozoic Era.
______
true All organisms contain carbon.
Image Bank
Chapter 21 Images
Image Bank
Chapter 21 Images
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