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Earth’s History

• Planet Earth is approximately 4.6 billion years old–Rocks of the crust provide clues

to Earth’s past• By analyzing these clues we can infer events from the past

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Earth’s History• Principle of Uniformitarianism

–Major assumption in geology–Events in the past occurred the same way

that they are occurring today. Examples Include:• Weathering/erosion• Deposition• Volcanism• Plate tectonics

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Geologic DatingGeologic Dating

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There are two types of Geologic Dating

• AbsoluteAbsolute–Determines how many years old

something is

• RelativeRelative–Used to determine if one thing is

younger or older than another

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Tools used to determine

• Using radiometric dating

• Law of superposition• Use of index fossils• Correlation of rock

layers

Absolute Age Relative Age

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Relative Dating

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Law of Superposition• In a series of sedimentary rocks the

bottom layer is the oldest and the top layer is the youngest– Lower layers must be in place before

younger rocks can be deposited on top of them

– Exception: when something occurs to overturn layers

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Grand Canyon

GRAND CANYON- LAW OF SUPERPOSITION

YOUNGEST ON TOP

OLDEST ON BOTTOM

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Law of Superposition• Rock layers are older than folds found in them

– Layers were there before they were folded

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Folds/Tilted

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Law of Superposition

• Rock layers are older than faults found in them

• This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

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Law of Superposition• Fossils are generally the same

age as the rock layers in which they are found–Animal remains are deposited

along with the sediments that will turn into sedimentary rocks

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Law of Superposition• Intrusions are younger than the rock that

they cut through or flow out of

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Fossils• Generally found in

sedimentary rock layers

Why don’t fossils exist in most igneous or metamorphic rock?

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Fossils • A study of the fossil record shows:

–A great variety of plants, animals and simpler life forms have lived on Earth in the past

–That life forms have evolved through time

–Most life forms of the geologic past have become extinct

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Index FossilsIndex Fossils• Index Fossils--used to date rock

layers–Fossils from creatures that existed for

a geologically short period of time

–Fossils from creatures that had a wide geographic distribution

Which fossil would make the best index fossil?Which fossil would make the best index fossil?

Found in only 1 layer (short lived)Found in all samples (widespread)