A Little Bit of HistoryA Little Bit of History……• The history of geologic time begins with the human interest in mining.

• Interests in rock units began to flourish with commercial mining in the 1500s and 1600s

• People began to identify strata (rock layers) by the types of fossils that were found within them.

• Two scientists were credited with contributing the most research to this area -- Nicolaus Steno and James Hutton

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Nicolaus StenoNicolaus Steno

Adapted two geologic principlesAdapted two geologic principles

1.1. Law of SuperpositionLaw of Superposition : sedimentary rocks : sedimentary rocks are found with the oldest on the bottom and are found with the oldest on the bottom and youngest on top. youngest on top. E.x. Paint layersE.x. Paint layers

2.2. Principle of Original HorizontalityPrinciple of Original Horizontality:: If left If left undisturbed rock layers will stay in horizontal undisturbed rock layers will stay in horizontal orderorder

James HuttonJames Hutton

18th-century Scottish18th-century Scottish physician and farmerphysician and farmer

Studied his farm land for geologic changesStudied his farm land for geologic changes *The Present is the Key to the Past*The Present is the Key to the Past Principle of UniformitarianismPrinciple of Uniformitarianism: : geologic geologic

processes from the past are the same as the processes from the past are the same as the current geologic processes. current geologic processes.

E.x. What happened then happens nowE.x. What happened then happens now

Relative Age: gives the age of a rock layer based on the ages of layers around it (younger or older)

What sort of events can change the order of rock layers?

Law of Crosscutting Relationships Fault or body of rock is younger than any other body of rock that it cuts through.

Igneous intrusions that may be uplifted to the surface would also be younger than the rock they push through

Unconformities- breaks in geologic time where rock layers erode away or when sediment isn’t deposited

PaleontologyPaleontology The study of fossils. The study of fossils. Fossils: remains of plantsFossils: remains of plants and animals that lived in a previous and animals that lived in a previous

geologic timegeologic time

Organisms have changed throughout Organisms have changed throughout the geologic past. the geologic past.

Fossils provide the clues behind Fossils provide the clues behind environmental changes of the past.environmental changes of the past.

What kinds of rocks are fossils found in?What kinds of rocks are fossils found in?

FossilizationFossilization Organisms with hard Organisms with hard

parts are typically parts are typically preserved. preserved.

Those without hard Those without hard parts are fossilized by parts are fossilized by quick burial. quick burial.

How are fossils formed?How are fossils formed? MummificationMummification AmberAmber Tar seepsTar seeps FreezingFreezing PetrificationPetrification

MummificationMummification

Drying of organisms; found in deserts.

AmberAmber

Hardened tree sap, insects get stuck in sap and sap hardens

La Brea Tar PitsLa Brea Tar Pits

•Thick petroleum•Animals get stuck in tar and it preserves them

FreezinFreezingg

Animals preserved because bacteria cannot survive in cold climate to decay the bodies

PetrificatioPetrificationn

When organic material is replaced with minerals

Types of fossilsTypes of fossils

Trace Trace FossilsFossils

-Evidence that -Evidence that

an animal an animal existed. existed.

Ex. footprintsEx. footprints

Types of Fossils continuedTypes of Fossils continued…… MoldsMolds: animal : animal

remains dissolve remains dissolve away, but the away, but the shape of the shape of the animal remains.animal remains.

CastCast: when a mold : when a mold fills in with fills in with sediment and sediment and becomes becomes hardened.hardened.

ImprintsImprints- - carbonized carbonized imprintsimprints

CoproliteCoprolitess

fossilized animal droppingsfossilized animal droppings(learn eating habits)(learn eating habits)

GastrolithsGastroliths- stones found in dinosaur - stones found in dinosaur stomachs to help them digest foodstomachs to help them digest food(are generally round and polished)(are generally round and polished)

Index fossilIndex fossil:: Unique fossil that Unique fossil that lived during a specific timelived during a specific time

Trilobites lived 750 million years ago

Specific index fossils for every time periodSpecific index fossils for every time period

The Geologic Time Scale is a type of calendar geologists use to put events in their proper order

Calculating Earth's AgeCalculating Earth's Age

Earth’s age was Earth’s age was estimated by measuring estimated by measuring the radioactive decay of the radioactive decay of uraniumuranium to to leadlead. .

With improved With improved techniques and evidence techniques and evidence from from tree rings tree rings and and glaciersglaciers, the age of , the age of Earth is estimated to be Earth is estimated to be aboutabout

4.5 – 4.6 billion years 4.5 – 4.6 billion years oldold..

Geologic ColumnGeologic Column• By applying relative ages to sedimentary By applying relative ages to sedimentary

rocks we began to look at fossils to learn rocks we began to look at fossils to learn to date them.to date them.

• Not one place contains a complete record Not one place contains a complete record of geologic time.of geologic time.

• Geologic ColumnGeologic Column-- ordered ordered arrangementarrangement of rock layers based on of rock layers based on relative ages of the rockrelative ages of the rock

• Once geologists have dated a lot of the Once geologists have dated a lot of the rock layers, they can fill in the gaps.rock layers, they can fill in the gaps.

Divisions of Geologic TimeDivisions of Geologic Time

MYA- millions of years agoMYA- millions of years ago BYA- billions of years agoBYA- billions of years ago

Geologic time is typically divided by Geologic time is typically divided by clear clear changeschanges. .

Ex. Ex. extinctionsextinctions and and dominant dominant fossilsfossils

Divisions of Geologic Time Divisions of Geologic Time cont…cont…

• EonsEons – – largestlargest unit of geologic time (4 total eons). unit of geologic time (4 total eons).• We call the first three eons the Precambrian time We call the first three eons the Precambrian time

which has little fossil evidence.which has little fossil evidence.

Microbe-rich Microbe-rich stromatolite reefsstromatolite reefs, like this one in Shark Bay, , like this one in Shark Bay,

Australia, were among the first Australia, were among the first sourcessources of oxygen on Earth. of oxygen on Earth. They flourished throughout the planet's shallow waters 3.5 They flourished throughout the planet's shallow waters 3.5 billion years ago, but are extremely rare today.billion years ago, but are extremely rare today.

Continued…Continued…• EraEra- Unit of time that includes two or more - Unit of time that includes two or more periodsperiods..• There are 3 eras that the Phanerozoic Eon can be There are 3 eras that the Phanerozoic Eon can be

divided into:divided into:1.1. PaleozoicPaleozoic *Zoic = “animal life”*Zoic = “animal life”2.2. MesozoicMesozoic3.3. Cenozoic.Cenozoic.• Different fossils are represented in each of these eras: Different fossils are represented in each of these eras:

marine life, birds and reptiles and mammals.marine life, birds and reptiles and mammals.

Paleozoic: explosion of diverse life

Beginning Beginning – OceansOceans

Middle to the EndMiddle to the End– Forests and jungles Forests and jungles – Land animals evolvedLand animals evolved

Devonian Period: Age of Fishes

Mesozoic: Age of Reptiles

Jurassic Period:

Dinosaurs ruled the Earth

Cenozoic: Age ofMammals

• Eras are divided into shorter time Eras are divided into shorter time units called units called PeriodsPeriods..

• Each period is characterized by Each period is characterized by specific specific fossilsfossils and is named for and is named for the the locationlocation they were they were discovered.discovered.

• EpochsEpochs-when the fossil record is -when the fossil record is so so completecomplete we can keep we can keep dividing periods into epochs.dividing periods into epochs.

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