Dynamic EarthDynamic Earth

Class 3Class 3

17 January 200617 January 2006

Any Questions?Any Questions?

Radiometric DatingRadiometric Dating Key principle: Key principle: Half LifeHalf Life = time required = time required

for 1/2 of the nuclei in a sample to for 1/2 of the nuclei in a sample to decaydecay

Earth formed about 4.6 billion years Earth formed about 4.6 billion years agoago

Oldest rocksOldest rocks

Youngest rocksYoungest rocks

Principle of SuperpositionPrinciple of Superposition

Principle of Original HorizontalityPrinciple of Original Horizontality

Principle of Cross-cuttingPrinciple of Cross-cutting RelationshipsRelationships

CORRELATIONCORRELATION

Process used to tie separated strata Process used to tie separated strata togethertogether

Based on matching physical features such Based on matching physical features such asas Physical continuityPhysical continuity - trace of rock unit - trace of rock unit Similar rock typesSimilar rock types - marker beds, coal - marker beds, coal

seams, rare minerals, odd colorseams, rare minerals, odd color

Within sedimentary layers there are Within sedimentary layers there are often the remains of small animals often the remains of small animals ((fossilsfossils))

Fossils are quite useful for correlating Fossils are quite useful for correlating between two sections that are not between two sections that are not laterally continuouslaterally continuous

CORRELATIONCORRELATION

PaleontoloPaleontologygy

The study of life in the past based on The study of life in the past based on fossilized plants and animals.fossilized plants and animals.

FossilFossil: Evidence of past life : Evidence of past life

Fossils preserved in sedimentary rocks are Fossils preserved in sedimentary rocks are used to determine: used to determine: 1) Relative age 1) Relative age 2) Environment of deposition2) Environment of deposition

Ammonite FossilsAmmonite Fossils Petrified WoodPetrified Wood

Chip Clark Tom Bean

Using Fossils to Correlate Rocks

Fossils have evolved through time, so Fossils have evolved through time, so when we find a fossil of the same type in when we find a fossil of the same type in two different areas, we are pretty sure two different areas, we are pretty sure that the rocks are about the same agethat the rocks are about the same age

CORRELATIONCORRELATION

TThis technique is not very useful in his technique is not very useful in HawaiiHawaii -- -- Why?Why?

Fossils helpful in sedimentary rocks, Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocksbut usually no fossils in volcanic rocks

CORRELATIONCORRELATION

Radiometric Radiometric dates dates

provide provide absolute absolute

ages to the ages to the local local

stratigraphstratigraphyy

Radiometric dating of lava flows provides Radiometric dating of lava flows provides absolute dating of glacial events in Hawaiiabsolute dating of glacial events in Hawaii

Bracketing agesBracketing ages

MagnetostratigraphyMagnetostratigraphy An alternate method for absolute age An alternate method for absolute age

dating that works well with volcanic dating that works well with volcanic rocksrocks

MagnetostratigraphyMagnetostratigraphy Technique that works best in volcanic Technique that works best in volcanic

rocksrocks Time scale based on polarity reversal of Time scale based on polarity reversal of

Earth's magnetic fieldEarth's magnetic field Major problem is that Earth's magnetic Major problem is that Earth's magnetic

field has been constant for the past field has been constant for the past 700,000 yrs (no reversals), so this 700,000 yrs (no reversals), so this doesn't work for very young rocksdoesn't work for very young rocks

Earth’s Magnetic

Field

Magnetization of Magnetization of MagnetiteMagnetite

Lavas Lavas record record

magnetimagnetic c

reversalreversalss

Magnetic Magnetic reversals reversals

over the past over the past 20 million 20 million

yearsyears

Magnetic time scale Magnetic time scale 0-700,000 -- Normal0-700,000 -- Normal 700,000 - 2.5 my -- Reversed700,000 - 2.5 my -- Reversed > 2.5 my -- Normal> 2.5 my -- Normal Ko`olau lavas mostly reversed in Ko`olau lavas mostly reversed in

polarity, so they must be older than polarity, so they must be older than 700,000 yrs, but younger than 2.5 my700,000 yrs, but younger than 2.5 my

Lavas on Kaua`i and in Wai`anae Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they Range show normal polarity, so they must be older than 2.5 mymust be older than 2.5 my

The Geologic time scaleThe Geologic time scale Divisions in the worldwide Divisions in the worldwide

stratigraphic column based on stratigraphic column based on variations in preserved fossilsvariations in preserved fossils

Built using a combination of Built using a combination of stratigraphic relationships, cross-stratigraphic relationships, cross-cutting relationships, and absolute cutting relationships, and absolute (isotopic) ages(isotopic) ages

The Geologic The Geologic Column Column

and and Time ScaleTime Scale

Hominoid fossils Hominoid fossils can be dated can be dated

using both using both radiometric and radiometric and paleomagnetitic paleomagnetitic

techniques.techniques.

Generalized Generalized Stratigraphic Stratigraphic

Section of Section of Rocks Rocks

Exposed in Exposed in the Grand the Grand CanyonCanyon

Some of the Geologic Units Exposed in Some of the Geologic Units Exposed in the Grand Canyonthe Grand Canyon

Michael Collier

The first ~4 billion years of Earth’s history was nearly devoid of life – this time is known as the Precambrian.

After the Precambrian, life began to develop very rapidly.

The Paleozoic lasted for 300 my.

Dinosaurs dominated the Mesozoic, which lasted for almost 200 my.

After the dinosaurs died out, mammals began to dominate in the Cenozoic.

These are the main subdivisions that you should know.

The geologic The geologic timescale timescale

and absolute agesand absolute ages

Isotopic dating of intebedded Isotopic dating of intebedded volcanic rocks allows volcanic rocks allows assignment of an absolute age assignment of an absolute age for fossil transitionsfor fossil transitions

Bracketing agesBracketing ages

The geologic The geologic timescale timescale

and absolute agesand absolute ages

Isotopic dating of intebedded Isotopic dating of intebedded volcanic rocks allows volcanic rocks allows assignment of an absolute age assignment of an absolute age for fossil transitionsfor fossil transitions

What What are these are these subdivisions based on?subdivisions based on?

Boundaries are marked by mass extinctionsBoundaries are marked by mass extinctions Cretaceous-Tertiary boundary (death of the Cretaceous-Tertiary boundary (death of the

dinosaurs): 65 million yearsdinosaurs): 65 million years Permian-Triassic boundary (the biggest Permian-Triassic boundary (the biggest

mass extinction of them all): 250 million mass extinction of them all): 250 million yearsyears

Start of the Cambrian (first hard animal Start of the Cambrian (first hard animal parts--nice fossils): 550 million yearsparts--nice fossils): 550 million years

The Geologic The Geologic Column Column

and and Time ScaleTime Scale

How do we know How do we know when it started?when it started?

What is the age What is the age of the Earth?of the Earth?

Many methods have been Many methods have been used to used to

determine the age of the determine the age of the EarthEarth

1) Bible:1) Bible: In 1664, Archbishop Usher of In 1664, Archbishop Usher of Dublin used chronology of the Book of Dublin used chronology of the Book of Genesis to calculate that the world began Genesis to calculate that the world began on Oct. 26, 4004 B.C.on Oct. 26, 4004 B.C.

2) Salt in the Ocean:2) Salt in the Ocean: ( (ca.ca. 1899) Assuming 1899) Assuming the oceans began as fresh water, the rate the oceans began as fresh water, the rate at which rivers are transporting salts to the at which rivers are transporting salts to the oceans would lead to present salinity in oceans would lead to present salinity in ~100 m.y.~100 m.y.

Many methods have been Many methods have been used to used to

determine the age of the determine the age of the EarthEarth

3) Sediment Thickness:3) Sediment Thickness: Assuming the rate of Assuming the rate of deposition is the same today as in the past, deposition is the same today as in the past, the thickest sedimentary sequences (the thickest sedimentary sequences (e.g.e.g., , Grand Canyon) would have been deposited in Grand Canyon) would have been deposited in ~ 100 m.y.~ 100 m.y.

4) Kelvin’s Calculation:4) Kelvin’s Calculation: (1870): Lord Kelvin (1870): Lord Kelvin calculated that the present geothermal calculated that the present geothermal gradient of ~30°C/km would result in an gradient of ~30°C/km would result in an initially molten earth cooled for 30 – 100 m.y.initially molten earth cooled for 30 – 100 m.y.

Flawed Flawed assumptionsassumptions

• • Bible is not a science text or history bookBible is not a science text or history book

• • Salt is precipitated in sedimentary Salt is precipitated in sedimentary formationsformations

• • Both erosion and non-deposition are Both erosion and non-deposition are major parts of the sedimentary recordmajor parts of the sedimentary record

• • Radioactivity provides another heat Radioactivity provides another heat sourcesource

The heat inside the The heat inside the EarthEarth

The discovery of radioactivity at the turn The discovery of radioactivity at the turn of the century by Bequerel, Curie, and of the century by Bequerel, Curie, and Rutherford not only provided the source of Rutherford not only provided the source of the heat to override Kelvin’s calculations the heat to override Kelvin’s calculations but provided the basis for all later but provided the basis for all later quantitative estimates of the ages of quantitative estimates of the ages of rocks.rocks.

Oldest rocks on Oldest rocks on EarthEarthSlave Province, Northern CanadaSlave Province, Northern Canada

Zircons in a metamorphosed granite dated Zircons in a metamorphosed granite dated at 3.96 Ga by the U-Pb methodat 3.96 Ga by the U-Pb method

Yilgarn block, Western AustraliaYilgarn block, Western Australia Detrital zircons in a sandstone dated at 4.10 Detrital zircons in a sandstone dated at 4.10

Ga by U-Pb method.Ga by U-Pb method.

Several other regions dated at 3.8 Ga by Several other regions dated at 3.8 Ga by various methods including Minnesota, various methods including Minnesota, Wyoming, Greenland, South Africa, and Wyoming, Greenland, South Africa, and Antarctica.Antarctica.

Age of the Age of the EarthEarth

Although the oldest rocks found on Earth Although the oldest rocks found on Earth are 3.96 Ga (or even 4.1), we believe that are 3.96 Ga (or even 4.1), we believe that the age of the Earth is approximately 4.6 the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.0 Ga Ga. All rocks of the age 4.6 to 4.0 Ga have been destroyed (the rock cycle) or have been destroyed (the rock cycle) or are presently covered by younger rocks.are presently covered by younger rocks.

Three types of rocksThree types of rocks

Igneous Igneous

SedimentarySedimentary

MetamorphicMetamorphic

Igneous rocks make up most of the earth

sedimentary rocks make up most of the

surface.

Major Rock GroupsMajor Rock GroupsIGNEOUS SEDIMENTARY METAMORPHIC

High Relief of AndesHigh Relief of Andes

Low Relief of Western AustraliaLow Relief of Western Australia

TheTheRockRockCyclCycl

ee

Age of the Age of the EarthEarth

Although the oldest rocks found on Earth Although the oldest rocks found on Earth are 3.96 Ga (or even 4.1), we believe that are 3.96 Ga (or even 4.1), we believe that the age of the Earth is approximately 4.6 the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.0 Ga Ga. All rocks of the age 4.6 to 4.0 Ga have been destroyed (the rock cycle) or have been destroyed (the rock cycle) or are presently covered by younger rocks.are presently covered by younger rocks.

Age of the Age of the EarthEarthThis is based on the age of rocks brought This is based on the age of rocks brought

back from the Moon (4.4 Ga), and back from the Moon (4.4 Ga), and meteorites (4.6 Ga), that are thought to meteorites (4.6 Ga), that are thought to be good representatives of the early solar be good representatives of the early solar system as well as more complicated system as well as more complicated geochemical modeling. This data geochemical modeling. This data suggests that the present chemical suggests that the present chemical composition of the crust must have composition of the crust must have evolved for more than 4.5 Ga.evolved for more than 4.5 Ga.

The big The big assumptionassumption

The half-lives of radioactive The half-lives of radioactive isotopes are the same as they isotopes are the same as they

were billions of years ago.were billions of years ago.

Test of the Test of the assumptionassumption

Meteorites and Moon rocks (that are Meteorites and Moon rocks (that are

thought to have had a very simple thought to have had a very simple

history since they formed), have been history since they formed), have been

dated by up to dated by up to 10 independent isotopic 10 independent isotopic

systemssystems all of which haveall of which have given the given the

same answersame answer. However, scientists . However, scientists

continue to critically evaluate this data.continue to critically evaluate this data.

Age of the EarthAge of the Earth

We have not found a way to determine the We have not found a way to determine the exact age of the Earth directly from Earth rocksexact age of the Earth directly from Earth rocks

Earth's oldest rocks have been recycled and Earth's oldest rocks have been recycled and destroyed (rock cycle)destroyed (rock cycle)

We have been able to determine the probable We have been able to determine the probable age of the Solar System and to calculate an age of the Solar System and to calculate an age for the Earthage for the Earth

Assume the Earth and the rest of the solid Assume the Earth and the rest of the solid bodies in the Solar System formed at the same bodies in the Solar System formed at the same time and are, therefore, of the same age time and are, therefore, of the same age

Age of the EarthAge of the Earth

Oldest rocks have been dated by a number of Oldest rocks have been dated by a number of radiometric dating methods and the consistency radiometric dating methods and the consistency of the results give scientists confidence that the of the results give scientists confidence that the ages are correct to within a few percentages are correct to within a few percent

Interestingly, these rocks are not from any sort Interestingly, these rocks are not from any sort of "primordial crust" but are lava flows and of "primordial crust" but are lava flows and sediments deposited in shallow watersediments deposited in shallow water Indicates that Earth history began well before these Indicates that Earth history began well before these

rocks were deposited rocks were deposited

Age of the EarthAge of the Earth Moon rocks returned to Earth on the Apollo Moon rocks returned to Earth on the Apollo

missions are generally 4.4-4.5 billion years oldmissions are generally 4.4-4.5 billion years old No rock cycle on the MoonNo rock cycle on the Moon

Minimum age of our nearest planetary neighborMinimum age of our nearest planetary neighbor More than 70 meteorites of many different More than 70 meteorites of many different

types date 4.53 to 4.59 billion years oldtypes date 4.53 to 4.59 billion years old Primitive objects provide best ages the formation of Primitive objects provide best ages the formation of

the Solar Systemthe Solar System So, the best age for the Earth comes not from So, the best age for the Earth comes not from

dating individual rocks but by considering the dating individual rocks but by considering the Earth and meteorites as part of the same Earth and meteorites as part of the same evolving system evolving system

Any Questions?Any Questions?

TThurhursdaysday Homework #1 is due – check web Homework #1 is due – check web

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