Geologic Time Scale

Objectives

• Explain why geologic time scale is used to show Earth’s history.

• Describe the different units of the geologic time scale.

Imagine squeezing Earth’s 4.6 billion yearhistory into a 24-hour day.Earth forms at midnight.

7:00 am- Earliest one-celled organisms appear.

7:00 am-9:00 pm- Simple, soft-bodied organisms such as jellyfish and worms appear.

Little after 9:00 pm- More complex organisms evolve in the oceans.

Little after 10:00 pm-Reptiles and insects first appear.

Just before 11:00 pm- Dinosaurs arrive11:30 pm- Dinosaurs go extinct.11:59:59- Humans appear one second

before midnight.

Geologic Time ScaleA record of the life forms and geologic events

in Earth’s history.First developed the geologic time scale by

studying rock layers and index fossils worldwide.

Scientist placed Earth’s rocks in order by relative age.

Later, radioactive dating helped determine the absolute age of the divisions in geologic time scale.

Divisions of Geologic TimeAs they studied the fossil record, they found

major changes in life forms at certain times.They used these changes to mark where one

unit of geologic time ends and the next begins.

Divisions of the geologic time scale depend on events in the history of life on Earth.

Geologic time begins with a long span of time called Precambrian Time. Covers 88% of earth’s history. It ended 544 million years ago.

Time between Precambrian and now is divided into 3 eras—Paleozoic, Mesozoic and Cenozoic.

Eras are subdivided into periods. Names of periods come from place around the world where geologist 1st described the rocks and fossils of that period.

• Why is a time scale used to represent Earth’s history instead of a calendar?

• Earth’s history is so long• When did Geologic time begin?• 4.6 billion years ago• When does Geologic time end?• It doesn’t. We are now in the Quarternary

Period of the Cenozoic Era.• How long is a geologic period?• A period doesn’t have an exact length.

Early Earth• Earth formed 4.6 billion years ago.• Scientist think that Earth began as a ball of

dust, rock and ice.• Gravity pulled this mass together.• As Earth grew larger, gravity increased.

Pulled in nearby dust, ice and rock.• As object hit the Earth at high speeds, their

kinetic energy changed into thermal energy.

• Energy from collisions caused Earth’s temp to rise until planet was very hot.

• Scientist believe Earth may have become so hot it melted.

• Denser materials sank toward the center—formed Earth’s dense iron core.

• At same time, Earth continuously lost heat to cold of space.

• Less dense molten material hardened to form Earth’s outer layers.

• As Earth travelled around the sun, its gravity also captured hydrogen and helium.

• This was the Earth’s 1st atmosphere.• It was lost when the sun released a strong

burst of particles.• These particles blew away Earth’s first

atmosphere.

Life Develops• Cannot pinpoint when and where life began.• Fossils of single-celled organisms in rocks

that formed 3.5 billion years ago.• Similar to present day bacteria.• About 2.5 billion years ago, organisms began

using energy from sun to make their own food. Photosynthesis

• Released oxygen into air-slowly increased.

• Processes in atmosphere changed oxygen to ozone.

• Ozone blocked deadly UV rays of sun.• Shielded from sun’s UV rays, organisms

could live on land.

Paleozoic Era• Paleo- means ancient or early.• Lasted from 544-244 million years ago.• Event that marked the beginning of the era

was the many different kind of organisms that evolved.

Cambrian Period• Cambrian Explosion-so many new life forms

appeared within a short time.• 1st time organisms had hard parts-shells and

outer skeletons.• All animals lived in the sea.• Many were invertebrates-jellyfish, sponges,

worms drifted through the water.• Brachipods (clams) and trilobites were

common in the Cambrian seas.

Ordovician Period• Seas still cover the earth• Ice cap covers what is now North Africa.• Ancestors of squid and octupus appear.• Jawless fish evolve—first vertebrates.

Silurian Period• Coral reefs develop.• Prior to this period, only one-celled

organisms lived on land. Land plants appear.

• Fish with jaws appear.• Insects and spiders appear.

Devonian Period• Often called Age of Fishes.• Every main group of fishes were present in

the ocean.• Most fish had jaws, skeletons and scales on

their bodies.• Sharks appeared late in the period.• Animals begin to invade land.• 1st vertebrates on land were lungfish. 1st

amphibians evolved from the lungfish.

Carboniferous Period• Broken into Mississippian and

Pennsylvanian.• Life expanded over continents.

Appalachian Mountains began to form.• Small reptiles developed• Winged insects evolved-huge dragonflies

and cockroaches.• Giant ferns & cone bearing plants and trees

formed. “Coal forest”

Mass Extinction• When many types of living things became extinct

at the same time.• Affected plants and animals on land and in the

seas.• Trilobites suddenly went extinct.• Hypothesize climate change from continental

drift may have caused extinction.• Formation of Pangaea caused deserts to expand

in tropics. Sheets of ice covered land closer to South Pole. Organisms could not survive.

Mesozoic Era• Meso- means middle.• Began 245-66 million years ago.• Known as Age of Reptiles.

Triassic Period• Fish, insects, reptiles and cone bearing trees

survived Permian extinction.• 1st dinosaurs appear.• 1st mammals appear-size of mouse.• 1st turtles and crocs appear.• Conifers, palm-like trees (cycads), ginko trees

dominate forest.• Pangaea holds together. Hot, dry conditions

dominate center of Pangaea.

Jurassic Period• Dinosaurs become dominant land animal.• Some were plant eaters, some were meat

eaters.• Different types lived at different times.• 1st birds appear. Archaeopteryx means

“ancient wing thing.”• 1st flying reptiles-pterosaurs appear.• Pangaea breaks apart-N. America and S.

America separate.

Cretaceous Period• Continents moved toward present location.• Widespread volcanic activity.• Flying reptiles and birds competed.• Birds hollow bones and feathers made them better

adapted.• 1st flowering plants. Flowering plants produce

seeds that are inside fruit. Fruit helps seed survive.

• Dinosaurs dominate-T-rex.• 1st snakes appear.

Another Mass Extinction• Wiped out over half of all plant and animal

groups.• No dinosaurs survived.• Two theories:

1. Asteroid hit earth. Impact threw huge amounts of dust and water into atmosphere blocking sunlight. No sun—

plants died and plant eating animals starved.

2. Climate changes were caused by increased volcanic activity. Volcanic output would block sun as well and same process would follow.

Cenozoic Era• Ceno- means recent.• -Zoic means life.• Began 66 millions years ago until now.• Called Age of Mammals• Mammals evolved to live on land, in water

and in the air.

Tertiary Period• Climate was warm and mild.• Rocky Mts and Himalayas form.• Continental glacier covers Antarctica.• 1st grasses appear-provide food source

for grazing animals. (ancestors of cattle, deer, sheep.)

• Flowering plants thrive.• Ancestors of humans evolve.

Quarternary Period• Climate cooled-causing a series of ice ages.• 20,000 years ago climate began to warm.• Modern humans-Homo sapiens evolved as early

as 100,000 years ago. 12,000-15,000 humans had migrated around the world.

• Mammals, flowering plants and insects dominate land.

• Giant mammals of N. America and Eurasia become extinct.

Key Terms

• Geologic Time Scale• Era• Period

Objectives

• Explain why geologic time scale is used to show Earth’s history.

• Describe the different units of the geologic time scale.