NOTE CHECK

• What is the difference between relative and absolute dating?

• What is superposition?

• What era did humans first appear?

Fossils & The Geologic Time Scale

Exploring Geologic Time

Geologic time notes - page 23

Exploring Geologic Time(geologic time notes page 29)

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Geologic Time Scale

• The geologic time scale – a “calendar” of Earth history

•Subdivides geologic history into units•Originally created using relative dates

• Structure of the geologic time scale•Eon – the greatest expanse of time•Era – an eon is divided into eras

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Geologic Time

Chapter 8

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The 4.6 billion years of Earth’s history can be divided into major time segments called

eons. Earth = 4.6 billion

years PreCambrian Eon

4600 to 542 mya This boundary continually

changes as older soft bodied fossils are recognized

Phanerozoic Eon 543 mya to present

• mya means million of years ago

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An Eon is divided into eras. Then each era can be subdivided into periods. Periods can be subdivided into epochs.

EON to ERA to PERIOD TO EPOCHBIGGEST ………………………..SMALLEST

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How do scientists pick when to start a new time segment?

• Each time segment is characterized by changes in landforms (tectonics changes), climate, and in life forms.

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Precambrian Eon is whenEarth Formed

85% of Earth’s history

Intense volcanic activity; oceans form; meteorite bombardment

Atmosphere develops

(blue-green algae and bacteria) andLife begins with one cell organisms evolves to multi-cell organisms like worms

Few fossils exist because most did not have hard parts

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The Phanerozoic Eon began when life developed hard bodies.

The Phanerozoic Eon is divided into time segments called eras.

Paleozoic 542 Mya to 251 Mya

Mesozoic 251 Mya to 65 MyaCenozoic 65 Mya to present

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The Paleozoic Era-Age of Ancient Plants and

Animals The formation of

Pangaea

Six periods are marked by significant differences in lifeforms

Appearance of fish, insects, amphibians, and some reptiles

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The Mesozoic Era Age of Dinosaurs

Known as age of reptiles; dinosaurs are prominent life forms

Last part of this period (Cretaceous) witnesses large-scale extinction of marine and flying reptiles as well as dinosaurs; extinction may be due to massive asteroid collision with Earth and massive volcanic eruptions

Pangaea breaks up

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The Cenozoic Era Age of Mammals and

Flowering Plants Age of mammals and

flowering plants

Climates cool generating widespread glaciation

Humans evolve

High level of tectonic activity as plates separate and evolve

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What is a fossil?

• Fossils give clues about organisms that lived long ago.

• They help to show that evolution has occurred.

• They also provide evidence about how Earth’s surface has changed over time.

• Fossils help scientists understand what past environments may have been like.

• A fossil is the preserved remains of a once-living organism.

What do fossils tell us?

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HOW IS A FOSSIL FORMED?

1. SedimentAn animal is buried by sediment, such as volcanic ash or silt, shortly after it dies. Its bones are protected from rotting by the layer of sediment.

4. ErosionErosion from rain, rivers, and wind wears away the remaining rock layers. Eventually, erosion or people digging for fossils will expose the preserved remains.

2. LayersMore sediment layers accumulate above the animal’s remains, and minerals, such as silica (a compound of silicon and oxygen), slowly replace the calcium phosphate in the bones.

3. MovementMovement of tectonic plates, or giant rock slabs that make up Earth’s surface, lifts up the sediments and pushes the fossil closer to the surface.

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FIVE MAIN TYPES OF FOSSILS

PetrifiedFossils

Molds andCasts

CarbonFilms

TraceFossils

PreservedRemains

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• The word “petrified” means “turning into stone.”

• Petrified fossils form when minerals replace all or part of an organism.

• Water is full of dissolved minerals. It seeps through the layers of sediment to reach the dead organism. When the water evaporates, only the hardened minerals are left behind.

PETRIFIED FOSSILS

PETRIFIED FOSSILThe Field Museum in Chicago

displays a fossil of a Tyrannosaurus rex.

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MOLDS AND CASTS• A mold forms when hard parts of an

organism are buried in sediment, such as sand, silt, or clay.

• The hard parts completely dissolve over time, leaving behind a hollow area with the organism’s shape.

MOLD FOSSILThis mold, or imprint, is of an

extinct mollusk called an ammonite.

• A cast forms as the result of a mold.

• Water with dissolved minerals and sediment fills the mold’s empty spaces.

• Minerals and sediment that are left in the mold make a cast.

• A cast is the opposite of its mold.

CAST FOSSILThis ammonite cast was discovered in the United

Kingdom.

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• All living things contain an element called carbon.

• When an organism dies and is buried in sediment, the materials that make up the organism break down.

• Eventually, only carbon remains.

• The thin layer of carbon left behind can show an organism’s delicate parts, like leaves on a plant.

CARBON FILMS

FERN FOSSILThis carbon-film fossil of a

fern is more than300 million years old.

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Relative Dating Relative dating: looks at where the fossil is

located to determine its age relative to other fossils. This only works if the area has been undisturbed.

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Absolute Dating• Uses radioactive elements near the fossils

to determine the actual age of the fossils.•By determining the age of the radioactive element, scientists can calculate the age of the fossil buried nearby.

The absolute age of fossils is estimated by dating associated igneous rock and lava flows.

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TRACE FOSSILS

• Trace fossils show the activities of organisms.

• An animal makes a footprint when it steps in sand or mud.

• Over time the footprint is buried in layers of sediment. Then, the sediment becomes solid rock.

FANCY FOOTWORKThis dinosaur footprint was

found in Namibia, Africa.

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Some organisms get preserved in or close to their original states. Here are some ways that can happen.

PRESERVED REMAINS

Amber An organism, such as an insect, is trapped in a tree’s sticky resin and dies. More resin covers it, sealing the insect inside. It hardens into amber.

Tar An organism, such as a mammoth, is trapped in a tar pit and dies. The tar soaks into its bones and stops the bones from decaying.

Ice An organism, such as a woolly mammoth, dies in a very cold region. Its body is frozen in ice, which preserves the organism—even its hair!

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Index Fossils

• Organisms are constantly changing or evolving. Some die out.

• Some fossils remain unchanged for only a specific, short period of time.

• These organisms produce the index fossils.

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Four features of an index fossil.

• Existed for short period of time

• Abundant• Widespread

geographically• Easily recognizable

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What is the index fossil of the Paleozoic Era?

Trilobite

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Superposition

The idea that younger rock lies above older rock.

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ASSIGNMENT

•Read back through the notes you have just taken. Design 4 multiple choice questions about the content.

•

•Design one question which asks for a description or explanation

with details.

Video: Becoming a Fossilhttp://www.pbs.org/wgbh/evolution/library/04/3/l_043_01.html

PBS

Article: Major Fossil Findhttp://www2.scholastic.com/browse/article.jsp?id=3751945

Scholastic News Online

Online Exhibit: Fossil Hallshttp://www.amnh.org/exhibitions/permanent/fossilhalls

American Museum of Natural History

Interactive Game: Fossil Hunthttp://channel.nationalgeographic.com/episode/dino-death-trap-2998#tab-fossil-hunt

National Geographic

For more on fossils visit:

Scholastic Classroom Magazines. www.scholastic.com Photo Credits: PAGE 1: UTAH MUSEUM OF NATURAL HISTORY (DINOSAUR); PHOTOTAKE INC./ALAMY (ANT). PAGE 3: 5W INFORGRAPHIC (GRAPHIC). PAGE 4: Jason Lindsey/Alamy (DINOSAUR); Peter Bowater/Alamy (MOLD); David Lyons/Alamy (FERN); Hoberman Collection UK/Alamy (FOOTPRINT); John Cancalosi/Alamy (MANTIS). PAGE 5: Gary Crabbe/Alamy (DINOSAUR). PAGE 6: Peter Bowater/Alamy (MOLD); Detail Heritage/Alamy (CAST). PAGE 7: David Lyons/Alamy (FERN). PAGE 8: Hoberman Collection UK/Alamy (FOOTPRINT). PAGE 9: John Cancalosi/Alamy (MANTIS); R1/Alamy (TAR PITS); Gianni Dagli Orti/Corbis (MAMMOTH).