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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 – era
time scale – period
– Eon
– 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 Early Earth
HOW OLD IS EARTH?
We know that Earth must be at least as old as the
oldest rocks in the crust.
– The age of the oldest rocks on Earth is between
3.96 to 3.8 billion years.
– Evidence of 4.1- to 4.2-billion-year-old crust
exists in the mineral zircon that is contained in
metamorphosed sedimentary rocks in Australia.
The Early Earth
EARTH’S “BIRTH”
For about the first 4 billion years of Earth’s
4.6-billion-year existence, most of the life-forms that
inhabited Earth were unicellular organisms. It wasn’t
until 2.1 BYA eukaryotic algae was 1st appeared.
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 -
• geologic events
• environmental changes
• development of life-forms 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 secondlongest span of time on the geologic time scale.
The Geologic Time Scale
GEOLOGIC TIME
Precambrian Time - 90 % of geologic time, is
divided into the Archean and Proterozoic Eons.
Proterozoic (end), 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, followed by land animals.
– The Mesozoic Era, 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 (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:
 Paleogene
 Neogene
 Quaternary.
The Geologic Time Scale
GEOLOGIC TIME
Epochs of Geologic Time
– Epochs are smaller divisions(usually measured in
millions of years to tens of millions of years).
– The Cenozoic Periods - further divided into
epochs - Paleocene & Oligocene.
– Different groups of organisms are in the various
epochs.
– Each unit has 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.
A. time periods defined by the life forms
C eon
___
___
A period
___
D era
___ epoch
B
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
– correlation
– superposition
Relative-Age Dating of Rocks
RELATIVE-AGE DATING OF ROCKS
Uniformitarianism the forces that continually
change the surface features of Earth today
have been occurring since Earth formed.
Only the rate, intensity, and scale have changed.
The resulting sediments and rocks record & preserve:
• the environment and fossils within the rocks
• preserve evidence of the life-forms that lived
Relative-Age Dating of Rocks
PRINCIPLES FOR DETERMINING RELATIVE AGE
Relative-age dating places the ages of
rocks and the events that formed
them in order, but without exact
dates. One event or rock layer is
compared to another.
Relative-Age Dating of Rocks
PRINCIPLES FOR DETERMINING RELATIVE AGE
Original horizontality –
Sedimentary rocks are
deposited in horizontal or
nearly horizontal layers.
Superposition - In an
undisturbed rock sequence,
the oldest rocks are at the
bottom and each
successive layer is younger
than the layer beneath.
Cross-cutting
relationships - 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
caused by an erosional surface becoming buried
by younger rocks.
http://www.classzone.com/books/earth_scienc
e/terc/content/visualizations/es2902/es2902pa
ge01.cfm?chapter_no=visualization
Unconformity Visualization
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
A different type of unconformity exists when
sedimentary rocks overlie nonsedimentary rocks.
The contact point
between rock is
called a
nonconformity.
Angular unconformity
is created when
horizontal
sedimentary rocks
are uplifted & tilted,
then 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. principle which states that
A
___ original horizontality
C superposition
___
B unconformity
___
D correlation
___
sedimentary rocks are
deposited in horizontal layers
B. a gap in the rock record
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.
http://science.discovery.com/videos/100-greatestdiscoveries-shorts-radiometric-dating.html
Radiometric Dating Videos
http://www.pbs.org/wgbh/evolution/library/03/3/quickt
ime/l_033_01_56.html
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.
Geologists use the half-life of an isotope to date
samples.
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 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 halflife 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.
Tree Rings
– The age of a tree - 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. (Indiana)
– 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.
– http://vsx.onstreammedia.com/vsx/AFG/search/PB
SPlayer?assetId=67655&ccstart=914479&pt=1
Indiana Fossils – Falls of the Ohio
Absolute-Age Dating of Rocks
SECTION ASSESSMENT
1. Match the following terms with their
definitions.
A. alternating light- and dark___B half-life
___D dendrochronology
___A varve
___ key bed
C
colored 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 - preservation requires
extraordinary circumstances such as freezing,
drying out, or oxygen-free environments.
http://www.classzone.com/books/earth_science/terc/c
ontent/visualizations/es2901/es2901page01.cfm?chap
ter_no=visualization
How Fossils are Formed
Remains of Organisms in the Rock Record
TYPES OF FOSSILS
Altered Hard Parts
– Altered hard parts are fossils (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
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
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
(events), such as climatic changes.
The study of fossils allows geologists to locate
energy resources (oil, gas, & coal companies hire
geologist to search for new wells or mines).
http://fossils.valdosta.edu/
Fossil Home Page for help on your project
Remains of Organisms in the Rock Record
SECTION ASSESSMENT
1. Match the following terms with their
definitions.
A. fossils in which all organic
C original
___
A preservation
___ altered hard part
B
___ permineralization
D cast
___
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.
Section 21.1 Study Guide
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
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
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
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.