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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
Original preservation
Eon
Absolute
Altered hard part
Era
Half-life
Radiometric Dating
Period
Varve
geologic time scale
Epoch
Mold
Uniformitarianism
Key bed
Cast
Unconformityperposition
Paleontology
Law of Crosscutting
Index fossil
Law of Superposition
Trace fossil
Dendrochronology
Evolution
Permineralization
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.
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.
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 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
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.
 A period is the shortest time span.
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.
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.
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.
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.
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
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.
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
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
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.
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 relationshipsAn intrusion or a fault is
younger than the rock it
cuts across.
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.
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)
OTHER MEANS OF DETERMINING RELATIVE AGE
The unconformity
is called a
disconformity
when horizontal
sedimentary
rocks overlie
horizontal
sedimentary
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.
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.
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
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.
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
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
Radioactive decay is the emission of radioactive
particles and the resulting change into other
elements over time.
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
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.
USE OF RADIOACTIVE ISOTOPES
– 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.
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.
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.
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)
SECTION ASSESSMENT
1. Match the following terms with their
definitions.
A. alternating light- and dark___B half-life
___
D dendrochronology
___
A varve
___
C key bed
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
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.
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
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.
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.
How Fossils are Formed
http://www.classzone.com/books/earth_science/terc/c
ontent/visualizations/es2901/es2901page01.cfm?chap
ter_no=visualization
TYPES OF FOSSILS
– 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.
TYPES OF FOSSILS
Form by mummification, amber, tar seeps,
freezing, & petrification.
– 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.
TRACE FOSSILS
Carbon films, molds, casts, coprolites, & gastroliths.
– 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.
– Carbon films -surface features on carbon film.
– Coprolites -solid wastes from the animal.
– Gastroliths -stones dinosaurs swallowed.
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.
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
SECTION ASSESSMENT
1. Match the following terms with their
A. fossils in which all organic
definitions.
C preservation
___
A altered hard part
___
B permineralization B.
__
D cast
___
material has been removed and
bones or shells have been
structurally changed
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
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.
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.