Geologic Geologic Time ScaleTime Scale

Chapter 9, Chapter 9,

Section 3Section 3

Measuring TimeMeasuring Time The Earth has The Earth has

existed for 4.6 existed for 4.6 billion yearsbillion years

The geologic time The geologic time scale divides all scale divides all those years into those years into sectionssections

Each section shows Each section shows a specific change in a specific change in lifelife or in the Earth’s or in the Earth’s surfacesurface

Giving Time a NameGiving Time a Name The largest sections are called “eons”The largest sections are called “eons” ““Eons” are divided into “eras” (the 2Eons” are divided into “eras” (the 2ndnd

largest section)largest section) ““Eras” are divided Eras” are divided

into “periods”into “periods” Then “periods” are Then “periods” are

divided into “epochs” divided into “epochs” Usually the oldest Usually the oldest

sections are shown sections are shown on the bottom and on the bottom and the most recent the most recent sections are shown on the topsections are shown on the top

How is Time Divided?How is Time Divided? Major changes in Earth’s history mark the Major changes in Earth’s history mark the

boundaries between the sectionsboundaries between the sections Most sections have been divided because a Most sections have been divided because a

major organism developed or went extinct. major organism developed or went extinct.

ExtinctionExtinction Extinction = the death Extinction = the death

of every member of a of every member of a speciesspecies

It can happen because It can happen because the environment the environment changed, and some changed, and some organisms could not organisms could not survive in the new survive in the new environment.environment.

Mass ExtinctionMass Extinction Mass extinction = the Mass extinction = the

extinction of many species at extinction of many species at the same timethe same time

Can you think of an example Can you think of an example of a mass extinction?of a mass extinction?

It can be caused by gradual It can be caused by gradual changes in climate or ocean changes in climate or ocean currents (Uniformitarianism)currents (Uniformitarianism)

Or by catastrophic events, Or by catastrophic events, like an asteroid hitting the like an asteroid hitting the earth and blocking out the earth and blocking out the sun (Catastrophism)sun (Catastrophism)

Let’s Talk About the ErasLet’s Talk About the Eras

Precambrian TimePrecambrian Time

Precambrian TimePrecambrian Time Time from the formation of the Earth (4.6 Time from the formation of the Earth (4.6

billion years ago) to 542 million years agobillion years ago) to 542 million years ago The Earth was very different thenThe Earth was very different then At first, there was At first, there was

no oxygen or living things no oxygen or living things Then the first organisms Then the first organisms

appeared in the oceans appeared in the oceans 3.6 billion years ago 3.6 billion years ago

They were called ProkaryotesThey were called Prokaryotes

ProkaryotesProkaryotes Only one cell and no nucleusOnly one cell and no nucleus One type, Cyanobacteria, One type, Cyanobacteria,

were the first organisms on were the first organisms on EarthEarth

Through photosynthesis, Through photosynthesis, they released oxygen into they released oxygen into the airthe air

As oxygen began to build As oxygen began to build up, the ozone was formedup, the ozone was formed

The ozone protects the Earth The ozone protects the Earth against the sun’s harmful against the sun’s harmful rays (deadly to land rays (deadly to land organisms)organisms)

Now organisms could live on Now organisms could live on land too!land too!

EukaryotesEukaryotes After another 1 After another 1

billion years, more billion years, more complex complex organisms organisms developeddeveloped

They were called They were called EukaryotesEukaryotes

Many cells with a Many cells with a nucleusnucleus

Evolved into Evolved into organisms on organisms on Earth todayEarth today

Paleozoic EraPaleozoic Era

Began 542 million years ago and Began 542 million years ago and ended 251 million years agoended 251 million years ago

Many new life forms appeared Many new life forms appeared during the 1during the 1stst period, the Cambrian period, the Cambrian Period = “Cambrian Explosion”Period = “Cambrian Explosion”

For the first time on Earth, For the first time on Earth, organisms had hard parts (shells, organisms had hard parts (shells, exoskeletons)exoskeletons)

Some organisms that were alive Some organisms that were alive during this period still exist today during this period still exist today (ferns and salamanders) but they (ferns and salamanders) but they were giants!were giants!

Permian ExtinctionPermian Extinction The largest mass extinction The largest mass extinction

happened 251 million years happened 251 million years agoago

Marked the end of the Marked the end of the Paleozoic Era and the start of Paleozoic Era and the start of the Mesozoic Erathe Mesozoic Era

Scientists are not sure what Scientists are not sure what caused this mass extinction caused this mass extinction (maybe climate change & (maybe climate change & volcanoes)volcanoes)

90% of ocean life and 78% of 90% of ocean life and 78% of land life diedland life died

Reptiles and amphibians Reptiles and amphibians survived!survived!

Mesozoic EraMesozoic Era

Mesozoic EraMesozoic Era Began 251 million years agoBegan 251 million years ago Surviving reptiles and amphibians evolved Surviving reptiles and amphibians evolved

into many other species (like dinosaurs) = into many other species (like dinosaurs) = “Age of Reptiles”“Age of Reptiles”

Cretaceous-Tertiary ExtinctionCretaceous-Tertiary Extinction Another mass extinction Another mass extinction

happened 65 million years happened 65 million years ago ago

Marked the end of the Marked the end of the Mesozoic Era and the Mesozoic Era and the beginning of the Cenozoic beginning of the Cenozoic EraEra

All of the dinosaurs and All of the dinosaurs and half of the other animals & half of the other animals & plants went extinctplants went extinct

Scientists think an asteroid Scientists think an asteroid hit Earth, the dust clouds hit Earth, the dust clouds blocked out the sun, plants blocked out the sun, plants died, then herbivores, then died, then herbivores, then carnivores.carnivores.

Cenozoic EraCenozoic Era

Cenozoic EraCenozoic Era Began 65 million years ago and continues Began 65 million years ago and continues

todaytoday

Scientists know the most about this Era Scientists know the most about this Era because the fossils are in the top rock layers because the fossils are in the top rock layers and are easier to findand are easier to find

Land & climate has Land & climate has changed a lot changed a lot

What big change What big change will happen that will happen that will end this era will end this era & start the next? & start the next?