Objectives Describe the geologic time scale. The Geologic Time Scale Distinguish among the following geologic time scale divisions: eon, era, period,

Objectives• Describe the geologic time scale.

The Geologic Time Scale

• Distinguish among the following geologic time scale divisions: eon, era, period, and epoch.

– geologic time scale

– eon

– era

– period

– epoch

Vocabulary

• 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 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 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 second-longest span of time on 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.

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.

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.

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.

Section Assessment

1. Match the following terms with their definitions.

___ eon

___ period

___ era

___ 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

C

A

D

B

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.

FL: SC.D.1.4.3

______ Precambrian Time represents 90 percent of geologic time.

______ Human ancestors developed during the Paleozoic Era.

______ The boundaries between the periods of the Cenozoic are marked by mass extinctions.

______ Organisms in Precambrian Time had soft bodies with no shells or skeletons.

Section Assessment

3. Identify whether the following statements are true or false.


true

false

false

true

FL: SC.D.1.4.3

Objectives• Apply the principles for determining relative age to

interpret rock sequences.

• Describe an unconformity and how it is formed within the rock record.

– uniformitarianism

– original horizontality

– superposition

– cross-cutting relationships

– unconformity

– correlation

Vocabulary

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.

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.

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.


Geologic Principles


– The principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across.


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 usually caused by an erosional surface becoming buried by the deposition of younger rocks.

Other Means of Determining Relative Age• The unconformity is called a disconformity when

horizontal sedimentary rocks overlie horizontal sedimentary 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.

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.


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


___ original horizontality

___ superposition

___ unconformity

___ correlation


A. principle which states that 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

A

C

B

D

FL: SC.D.1.4.3

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.

FL: SC.D.1.4.3

Section Assessment



______ Relative-age dating allows geologists to determine the age of rock formations.

______ A limestone layer is older than a shale layer that is above it.

______ Rock layers are often found undisturbed if you dig deep enough.

______ The grains in a rock layer can be from a younger layer of rock.

false

true

false

false

FL: SC.D.1.4.3

– radioactive decay

– radiometric dating

– half-life

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

Absolute-Age Dating of Rocks

– dendrochronology

– varve

– key bed

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.


• Because it often takes a long time for the entire amount of an isotope to decay, geologists use the half-life of an isotope.

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.


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.

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.


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.


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.


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.

Section Assessment


___ half-life

___ dendrochronology

___ varve

___ key bed


A. alternating light- and dark-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

B

D

A

C

FL: SC.D.1.4.3

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.

FL: SC.D.1.4.3

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.

FL: SC.D.1.4.3

– fossil

– evolution

– original preservation

– altered hard part

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

Remains of Organisms in the Rock Record

– permineralization

– index fossil

– mold

– cast

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 because their preservation requires extraordinary circumstances such as freezing, drying out, or oxygen-free environments.

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.

Types of Fossils

Altered Hard Parts


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.

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.

Types of Fossils

Molds and Casts


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, such as climatic changes.

• The study of fossils allows geologists to locate energy resources.

Section Assessment


___ original preservation

___ altered hard part

___ permineralization

___ 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

C

A

B

D

FL: SC.D.1.4.3

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.

FL: SC.D.1.4.3

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.

FL: SC.D.1.4.3, SC.G.2.4.1

Chapter Resources Menu

Chapter Resources Menu

Study Guide

Section 21.1

Section 21.2

Section 21.3

Section 21.4

Chapter Assessment

Image Bank

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 21.1 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 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 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.


1. Which of the following is not an eon?

a. Phanerozoic c. Cenozoic

b. Proterozoic d. Archean

Multiple Choice

Chapter Assessment

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.

FL: SC.D.1.4.3

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.

Multiple Choice

Chapter Assessment

FL: SC.D.1.4.3

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

Chapter Assessment

After five half-lives of 5730 years (28 650 years total), around 3.2 percent of the C-14 would remain.

FL: SC.D.1.4.3

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

Chapter Assessment

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.

FL: SC.D.1.4.3

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

Chapter Assessment

The order of the divisions of geologic time classification beginning with the largest is eon, era, period, and epoch.

FL: SC.D.1.4.3

Short Answer

6. What does the principle of superposition state?

Chapter Assessment

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.

FL: SC.D.1.4.3

Short Answer

7. What does the principle of cross-cutting relationships state?

Chapter Assessment

The principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across.

FL: SC.D.1.4.3

True or False


______ Fallout from a major volcanic eruption can form

a key bed.

______ Homo sapiens evolved in the past .01 M.Y.B.P.

______ Varves are direct evidence of past life-forms.

______ Dendrochronology is limited to the age of the

oldest living tree.

______ The first fish appeared in the Paleozoic Era.

______ All organisms contain carbon.

Chapter Assessment

true

true

false

false

true

true

FL: SC.D.1.4.3

Image Bank

Chapter 21 Images

Image Bank

Chapter 21 Images

Image Bank

Chapter 21 Images

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Objectives Describe the geologic time scale. The Geologic Time Scale Distinguish among the following geologic time scale divisions: eon, era, period,

Documents

geologic time scaledistinguish

geologic timeprecambrian

percent of geologic

geologic period

geologic time scalethe

geologic timeplants

time units

rest of geologic history