Life and Geologic Time. Geologic Time Trilobites are small, hard-shelled organisms that crawled on...

Life and Geologic TimeLife and Geologic Time

Geologic TimeGeologic Time

TrilobitesTrilobites are are small, hard-shelled small, hard-shelled organisms that organisms that crawled on the crawled on the seafloor. seafloor.

They are considered They are considered to be to be index fossilsindex fossils because they lived because they lived over vast regions of over vast regions of the world during the world during specific periods of specific periods of geologic time. geologic time.

The Geologic Time ScaleThe Geologic Time Scale

Paleontologists have been Paleontologists have been able to divide Earth’s history able to divide Earth’s history into time units based on the into time units based on the life-forms life-forms that lived only that lived only during certain periods. during certain periods.

This division of Earth’s history This division of Earth’s history makes up the makes up the geologic time geologic time scale.scale.

Major Subdivisions of Geologic Major Subdivisions of Geologic TimeTime

FourFour major subdivisions of major subdivisions of geologic time are used— geologic time are used— eons, eras, periods, and eons, eras, periods, and epochs.epochs.

The longest subdivisions—The longest subdivisions—eonseons—are based upon the —are based upon the abundance of certain fossils. abundance of certain fossils.


Next to eons, the longest Next to eons, the longest subdivisions are the subdivisions are the eras,eras, which are marked by major, which are marked by major, striking, and worldwide striking, and worldwide changes in the types of fossils changes in the types of fossils present.present.


Eras are Eras are subdividedsubdivided into periods into periodsPeriodsPeriods are units of geologic time are units of geologic time

characterized by the types of life existing characterized by the types of life existing worldwide at the time. worldwide at the time.

Periods can be divided into smaller units Periods can be divided into smaller units of time called of time called epochsepochs..

Epochs are also characterized by Epochs are also characterized by differences in differences in life-forms,life-forms, but some of but some of these differences can vary from these differences can vary from continent to continent.continent to continent.

Dividing Geologic TimeDividing Geologic Time Sometimes it is Sometimes it is

possible to distinguish possible to distinguish layers of rocklayers of rock that that formed during a formed during a single year or season.single year or season.

In other cases, thick In other cases, thick stacks of rock that stacks of rock that have have no fossilsno fossils provide little provide little information that could information that could help in subdividing help in subdividing geologic time.geologic time.

Precambrian TimePrecambrian Time

Precambrian timePrecambrian time is the longest is the longest part of Earth’s history and includes part of Earth’s history and includes the Hadean, Archean, and the Hadean, Archean, and Proterozoic Eons.Proterozoic Eons.


Precambrian time lasted from about Precambrian time lasted from about 4.5 billion4.5 billion years ago to about 544 years ago to about 544 million years ago. million years ago.


Although the Precambrian was the Although the Precambrian was the longestlongest interval of geologic time, interval of geologic time, relatively little is known about the relatively little is known about the organisms that lived during this organisms that lived during this time.time.

One reason is that many One reason is that many Precambrian rocks have been so Precambrian rocks have been so deeply buried that they have been deeply buried that they have been changed by changed by heat and pressure.heat and pressure.


In addition, most Precambrian In addition, most Precambrian organisms didn’t have organisms didn’t have hard hard partsparts that otherwise would that otherwise would have increased their chances have increased their chances to be preserved as fossils.to be preserved as fossils.

The Paleozoic EraThe Paleozoic Era

An abundance of An abundance of organismsorganisms with hard parts, such as shells, with hard parts, such as shells, marks the beginning of the marks the beginning of the Paleozoic Era. Paleozoic Era.

The The Paleozoic Era,Paleozoic Era, or era of or era of ancient life, began about 544 ancient life, began about 544 million years ago and ended million years ago and ended about 248 million years ago. about 248 million years ago.

Paleozoic LifePaleozoic Life

Many of the life-forms scientists know Many of the life-forms scientists know about were about were marine,marine, meaning they lived meaning they lived in the ocean.in the ocean.

Trilobites were common, especially Trilobites were common, especially early in the early in the Paleozoic.Paleozoic.

Other organisms developed shells that Other organisms developed shells that were easily preserved as were easily preserved as fossils.fossils.

Vertebrates,Vertebrates, or animals with or animals with backbones, also evolved during this era.backbones, also evolved during this era.

Mountain BuildingMountain Building

Several mountain-building Several mountain-building episodes occurred during the episodes occurred during the Paleozoic Era.Paleozoic Era.

The The Appalachian Appalachian MountainsMountains, for example, , for example, formed during this time.formed during this time.


The first mountain-building episode The first mountain-building episode occurred as the occurred as the oceanocean separating separating North America from Europe and North America from Europe and Africa closed.Africa closed.

Several volcanic island chains that Several volcanic island chains that had formed in the ocean had formed in the ocean collidedcollided with the North American Platewith the North American Plate

The collision of the island chains The collision of the island chains generated generated high mountains.high mountains.


The next mountain-building episode The next mountain-building episode was a result of the African Plate was a result of the African Plate collidingcolliding with the North American with the North American Plate.Plate.

When Africa and North America When Africa and North America collided, rock layers were collided, rock layers were folded and folded and faulted.faulted.

SedimentsSediments were uplifted to form an were uplifted to form an immense mountain belt, part of which immense mountain belt, part of which still remains today.still remains today.

End of an EraEnd of an Era

At the end of At the end of the Paleozoic the Paleozoic Era, more than Era, more than 90 percent of 90 percent of all marine all marine species, and species, and 70 percent of 70 percent of all land species all land species died off.died off.

End of an EraEnd of an Era

Near the end of the Near the end of the Permian PeriodPermian Period, , the continental plates came together the continental plates came together and formed the supercontinent and formed the supercontinent Pangaea.Pangaea.

Mountain-buildingMountain-building processes processes caused caused seas to close and deserts to spread seas to close and deserts to spread over North America and Europe.over North America and Europe.

Many species, especially marine Many species, especially marine organisms, couldn’t adapt to these organisms, couldn’t adapt to these changes, and became changes, and became extinct.extinct.

The Mesozoic EraThe Mesozoic EraThe Breakup of PangaeaThe Breakup of Pangaea

The The Mesozoic Era,Mesozoic Era, or era of or era of middle life, was a time of middle life, was a time of many changes on Earth.many changes on Earth.

At the beginning of the At the beginning of the Mesozoic Era, all continents Mesozoic Era, all continents were joined as a single were joined as a single landmass called landmass called Pangaea. Pangaea.


Pangaea separated into Pangaea separated into twotwo large landmasses. large landmasses.

The northern mass was The northern mass was Laurasia,Laurasia, and and Gondwanaland Gondwanaland was the was the southern landmass.southern landmass.


Reptile’s Reptile’s skin skin helps it retain bodily helps it retain bodily fluids. fluids.

This characteristic, along with their This characteristic, along with their shelled eggs, enabled reptiles to shelled eggs, enabled reptiles to adapt readily to the adapt readily to the drier drier climate climate of the Mesozoic Era.of the Mesozoic Era.

Reptiles became the most Reptiles became the most conspicuous animals on land by conspicuous animals on land by the the Triassic period.Triassic period.

The Cenozoic EraThe Cenozoic Era

The The Cenozoic Era,Cenozoic Era, or era of recent life, or era of recent life, began about 65 million years ago and began about 65 million years ago and continues today. continues today.

The Cenozoic Era is subdivided into The Cenozoic Era is subdivided into twotwo periods.periods.

The first of these is the The first of these is the TertiaryTertiary period. period. The present-day period is the The present-day period is the

QuaternaryQuaternary Period. It began about 1.8 Period. It began about 1.8 million years ago.million years ago.

Times of Mountain BuildingTimes of Mountain Building

Many mountain Many mountain ranges formed ranges formed during the during the Cenozoic Era.Cenozoic Era.

These include These include the the AlpsAlps in in Europe and the Europe and the AndesAndes in in South America.South America.

Times of Mountain BuildingTimes of Mountain Building

The The HimalayaHimalaya formed as India formed as India moved northward moved northward and collided with and collided with Asia. Asia.

The collision The collision crumpled and crumpled and thickenedthickened Earth’s Earth’s crust, raising the crust, raising the highest mountains highest mountains presently on Earth.presently on Earth.

Question 1Question 1

Which of these geologic time Which of these geologic time subdivisions is longest?subdivisions is longest?

A. eraA. eraB. eonB. eonC. epochC. epochD. periodD. period

AnswerAnswer

The answer is B. The answer is B. Eons are the Eons are the longest longest subdivisions of subdivisions of geologic time geologic time and are based and are based upon the upon the abundance of abundance of certain fossils.certain fossils.


Which was the longest period Which was the longest period of geologic time?of geologic time?

A. Paleozoic EraA. Paleozoic EraB. Pennsylvanian PeriodB. Pennsylvanian PeriodC. Phanerozoic EonC. Phanerozoic EonD. Precambrian TimeD. Precambrian Time

AnswerAnswer

The answer is D. The answer is D. Precambrian Precambrian Time was the Time was the longest period longest period of geologic of geologic time, lasting time, lasting about 4 billion about 4 billion years.years.


The era of recent life is the The era of recent life is the __________.__________.

A. Cenozoic EraA. Cenozoic Era

B. Mesozoic EraB. Mesozoic Era

C. Paleozoic EraC. Paleozoic Era

D. Precambrian TimeD. Precambrian Time

AnswerAnswer

The answer is A. The Cenozoic The answer is A. The Cenozoic Era began about 65 million Era began about 65 million years ago.years ago.

Life and Geologic Time. Geologic Time Trilobites are small, hard-shelled organisms that crawled on...

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