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Earth’s History

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Earth’s History

• Planet Earth is approximately 4.5 X 109 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 Time

• Plate motions and mountain building events can be placed on the geologic time scale

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Geologists have divided Earth’s history into time units based on the the fossil record

Geologic Time

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Geologic Time• 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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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 SuperpositionLaw of Superposition

• Sediments are laid down underwater in horizontal layers and form sedimentary rocks

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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• Igneous intrusions are younger than the

rock that they cut through or flow out of

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Igneous Intrusion - Cross Cutting

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UnconformityUnconformity• When a new rock layer is formed

atop an eroded surface

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Upper Silurian Carbonates

Tilted Ordovician Shales and Sandstones

Taconic Unconformity

unconformity

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Practice: what happened here?

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CorrelationCorrelation

• Matching similar rock layers in different locations to see if they formed at the same time

Which fossil might be found in Devonian rock layers?

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Volcanic Ash Falls• Can also be used to correlate rock layers

over a large area

• Ash is a good indicator because:– The ash from one explosion has distinct

characteristics

– ash can be deposited around the globe

– The event occurs at one, geologically brief, time

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Fossils• Naturally occurring preserved remains

or impressions of living things

• Generally only hard parts get preserved – Bones, teeth, shells

• Hard parts are replaced by naturally occurring minerals

Eurypterus

NY State Fossil

Silurian index

fossil

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Fossils

• Can be a mold (impression)–Ex: footprint

• Or a cast–Ex: filled in footprint

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Fossils (Cont)

• Other types– Ice– Tar– Carbonaceous film– Amber– Imprints– Petrified

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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• The living organism

that made the fossil lived during the time the rock layer was forming – i.e. when the sediment

was being deposited

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Fossils• Fossils can provide

information about ancient environments– Marine fossils indicate

a marine environment, wooly mammoths indicate a cold environment, etc…

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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 (strata)–Fossils from creatures that existed for

a geologically short period of time• Ex. less than 2.0 x 107

–Fossils from creatures that had a wide geographic distribution

Location A Location B Location C

Rock layer 1 W W W Z

Rock layer 2 W Z

Y Z

Rock layer 3 W X X X Z

Which letter would make a good index fossil?

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Absolute Dating

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Radioactive Decay• When an unstable radioactive

element changes into a stable element

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Unstable Radioactive Isotope Stable Decay Product

Carbon 14 (C14)

Uranium 238 (U238)

Nitrogen 14 (N14)

Lead 206 (Pb206)

Parent Daughter

Example

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Radiometric DatingHalf lifeHalf life: The time required for 1/2 of a parent

material to break down to daughter material

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• Another look

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Radiometric Dating• The age of objects can be

determined by measuring the relative amounts of radioisotope and decay products

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Radiometric Dating• The half lives of radioisotopes

vary depending upon the isotope–Examples:

• C14 = 5,770 years

• U238 = 4,500,000,000 years

• Rb87 = 47,000,000,000 years

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• Carbon 14 is used to date biological remains–Carbon is incorporated into the

cells of living organisms and begins to decay when the organism dies

• Uranium 238 is used to date rocks–Larger half life

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Half Life Practice

Problems

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• If half the K -40 of a rock has decayed into Ar-40, how old is the rock?

• How many years would it take for 75 % of a sample of K-40 to disintegrate?

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How old are:

• A sample that has ½ c-14 and ½ n-14?

• A sample that has ¼ K-40 and ¾ Ar-40?

• A sample that has ½ U-238 ½ Pb-206?

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How old are:

• A sample that has 1/8 Rb-87 and 7/8 Sr-87?

• A sample that has ¼ C-14 and ¾ N-14?

• A sample that has ¾ C-14 and ¼ N-14?

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What fraction remains of:

• A 50 billion year old sample has ____ Rb-87 and ____ Sr-87.

• A 2.6 billion year old sample has____ K-40 and ____Ar-40.