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Geologic Time

Decoding the Age of our Planet& North Carolina

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 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.

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 TimePlants 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.

______ 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

End of Section 1

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


• 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.


Geologic Principles


– The principle of original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers.

– The principle of superpositionstates 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 relationshipsstates that an intrusion or a fault is younger than the rock it cuts across.

End of Section 2

– 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 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.


• 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.


• Half-life is the length of time it takes for one-half of the original amount of an isotope to decay.



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.



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

1. Match the following terms with their definitions.

___ 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

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.

End of Section 3

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