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TimeHow we achieved a modern sense

of time.

Learning Goals• Relative Geologic Time

– Superposition (oldest on bottom)– Crosscutting and inclusion

• Intrusions younger than host• Cobbles are older than host

• Radiometric dating (absolute time)– Half lives and exponential decay

• Geologic Time – Hadean, Archean, Proterozoic,

Phanerozoic

Yearly Calendars are Ancient

• Stonehenge is 2000+ BC and indicates that ancient cultures counted days and knew precisely the repeat cycle of the seasons.

Clocks

• Sundials are relative (solar) clocks.• Mechanical clocks developed late

13th century (pre Renaissance).• Clocks spread through Europe in 14-

16th Centuries.• Renaissance Italy was ‘obsessed’

with measurement, for painting, sculpture, as well a practical matters (commerce and war).

Sundial: Solar ClockClocks

• Mechanical Clocks spread through Europe in the 14-16 Centuries.

• This elaborate one is in Prague.

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Deep Time (Clocks in the rocks)

• Bishop Ussher 17th Cent. (biblical): 4004BC• Buffon 18th Cent. (Cooling of spheres):

~50000 Y• Hutton late 18th Cent. (Geological cycles):

Infinite• Darwin late 19th Cent. (Biological changes):

Billions• Kelvin late 19th C (Sun’s energy): 40 Million

Max (Kelvin was wrong!)• Modern (Radiometric): 4.55 Billion

Relative Age of Rocks• Original Horizontality

– Sediments were originally flat-lying

• Superposition– The oldest ones are on the bottom

• Cross-cutting– The disturbed (host) rocks are older

than disturbing rocks

Superposition: Oldest on bottomCorrelation of Layers

• Physical Continuity– Horizontal tracing

• Similarity of rock types and sequences

• Correlation of Fossils– Faunal succession

Correlation of Layers

Host rocks (red) are older than the intruding rocks (black).

Crosscutting:

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CrosscuttingPrinciple of

Cross-cutting Relationships

Grand Canyon

Angular Unconformity, GCNP

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Angular Unconformity, GCNP Angular Unconformity, GCNP

Angular Unconformity, GCNP The Age of the Earth• Bishop Ussher 17th Cent. (biblical): 4004BC• Buffon 18th Cent. (Cooling of spheres):

~50000 Y• Hutton late 18th Cent. (Geological cycles):

Infinite• Darwin late 19th Cent. (Biological changes):

Billions• Kelvin late 19th C (Sun’s energy): 40 Million

Max (He was wrong!)• Modern (Radiometric): 4.55 Billion

Ice Ages

Mammals and Flowering Plants

Dinosaurs

Fish

Trilobites

Time scale is NOT linear

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Relative Age of Rocks• By the mid 19th century a relative time

scale had been worked out for the sedimentary rocks of Europe (Phanerozoic).

• They lacked an absolute time scale.• Kelvin and classical physicists advocated

40 million max. • Darwin and evolutionary biologists

advocated billions of years.• Discovery of radioactivity at about 1900

confirmed billions.

Event Sequence

1.Original horizontality2.Superposition (oldest

on the bottom)3.Crosscutting

(Intruding igneous rocks are younger than their hosts)

Event Sequence

Radiometric Dating:Establishing an absolute time scale

• Minerals contain naturally radioactive elements– K, U, Th, Rb, Sm

• These elements decay to stable daughter elements

• When minerals crystallize from melt, they contain parent only.

• If we measure the concentration of daughter element in a mineral and we know the decay rate, we can calculate when the mineral crystallized.

Radiometric DatingExample: 40K - 40Ar

• A K-feldspar (KAlSi3O8) crystallizes in a granite and initially contains no Ar.

• Natural K is 0.012% 40K• Atmospheric Ar has 3 stable isotopes

– 36K, 38K, 40K– No 36K, or 38K in feldspar.

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Radiometric DatingExample: 40K - 40Ar

• A K-feldspar (KAlSi3O8) crystallizes in a granite and initially contains no Ar.

• Natural K is 0.012% 40K• 40K decays to 40Ar with a half-life of

1.31 x 109 years (1.3 billion years).• If we measure the 40Ar content of the

feldspar, we can get a crystallization date of the mineral.

• Isotope measurements are made with a mass spectrometer.

Radiometric Dating• Igneous and metamorphic rocks can

be dated directly by radiometric methods.

• Sediments cannot be dated directly.• Igneous rock fragments in sediments

can be dated. (Sed must be younger)• Igneous rock intruding sediments can

be dated. (sed must be older)• 14C can be used to date organic

matter less than ~50000 yrs old.

Inclusion:Host rocks are younger than the included rocks (cobbles).

Intrusion:Host rocks (red) are older than

the intruding rocks (black).

Radiometric Dating• Igneous and metamorphic rocks can

be dated directly by radiometric methods.

• Sediments cannot be dated directly.• Igneous rock fragments in sediments

can be dated. (Sed must be younger)• Igneous rock intruding sediments can

be dated. (sed must be older)• 14C can be used to date organic

matter less than ~50000 yrs old.

14C system is different from other radiometric dating

systems.• 14C in the atmosphere comes from 14N• Plants take 14C from atmosphere• 14C has half-life of 5730 years• There is no 14C in rocks.• 14C can be used to date plant and animal

matter that is younger than about 50,000 years.

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Exponential DecayTypes of Radioactive Decay

• Particle composed of: Mass# Atomic # Example• alpha 2 neutrons+ 4 2 U, Th,

2 protons• beta- electron 0 -1 40K• beta+ positron 0 +1 40K• gamma photon 0 0 all

nuclear reactions

• neutron neutron 1 0 235U

Naturally Radioactive Isotopes Naturally Radioactive IsotopesParent DaughterHalf life Decay

• 40K 40Ar 1.3 Gy +

• 87Rb 87Sr 49 Gy -

• 238U 206Pb 4.5 Gy 8, • 235U 207Pb 0.7 Gy 7, • 232Th 208Pb 14 Gy 6, • 14C 14N 5700 y -

Geologic Time Scale• Eon Era Period Began (My ago)

• Phanerozoic Cenozoic Quaternary 0.01

• Pleistocene 1.6

• Tertiary Pliocene 5.3

• Miocene 23.7

• Oligocene 36.6

• Eocene 57.8

• Paleocene 66.4

• Mesozoic Cretaceous 144

• Jurassic 208

• Triassic 245

• Paleozoic Permian 286

• Pennsylvanian 320

• Mississippian 360

Geologic Time Scale• Phanerozoic Cenozoic

• Mesozoic

• Paleozoic Permian 286

• Pennsylvanian 320

• Mississippian 360

• Devonian 408

• Silurian 438

• Ordovician 505

• Cambrian 550

• Proterozoic 2500

• Archean 4000

• Hadean 4550

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Some Major Events• Latest warming 7000y • Ice ages ~1.8 MY to 7000 years ago • Dinosaur extinction 66 MY• Dinosaurs ~245 MY• Vertebrates ~400 MY• Multi-cell life forms ~550 ‘Cambrian Explosion’• ‘Snowball earth’ 600 MY• Free O2 ~ 2.5 GY (CH4 and NH3 decline)• Single cell life forms ~3.7 GY• Oceans: at least by 4.3 GY• Accretion: 4.55 GY

Tree of Life

Ice Ages

Mammals and Flowering Plants

Dinosaurs

Fish

Trilobites

Geologic Time Terms• Hadean• Archean• Proterozoic• Phanerozoic• Paleozoic• Mesozoic• Cenozoic(Tertiary)• Cambrian• Unconformity• Angular unconformity

• Half-life• Alpha particle• Beta particle• Gamma ray• Neutron

A conglomerate contains some granite cobbles. K-Ar date on

the granite gives 180 MY.

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A conglomerate contains some granite cobbles. K-Ar date on

the granite gives 180 MY. It is only rarely possible to use

radiometric methods to directly date sedimentary rocks.

But igneous and metamorphic rocks can be dated directly.

A conglomerate contains some granite cobbles. K-Ar date on

the granite gives 180 MY. • A. The conglomerate is older than 180

MY• B. The conglomerate is younger than

180 MY• C. The conglomerate is 6000 y old• D. No age inference can be made.

Host rocks (red) are older than the intruding rocks (black).

A shale is intruded by a basalt dike that has an age of 1100 MY.

ClickerA shale is intruded by a basalt dike

that has an age of 1100 MY.

• A. The shale is older than 1100 MY• B. The shale is younger than 1100 MY• C. The shale is less than 1 million

years old.• D. No relative age inference can be

made.

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Exponential Decay Geologic Time Scale• Eon Era Period Began (My ago)

• Phanerozoic Cenozoic Quaternary 0.01

• Pleistocene 1.6

• Tertiary Pliocene 5.3

• Miocene 23.7

• Oligocene 36.6

• Eocene 57.8

• Paleocene 66.4

• Mesozoic Cretaceous 144

• Jurassic 208

• Triassic 245

• Paleozoic Permian 286

• Pennsylvanian 320

• Mississippian 360

The era of dinosaurs is subdivided into Triassic,

Jurasssic, and Cretaceous. Together these are known

as the:

• A. Archean• B. Proterozoic• C. Paleozoic• D. Mesozoic• E. Cenozoic

Why can’t 14C be used to date limestones?

• A. No carbon in limestone• B. No 14C in limestone• C. 14C half-life too long• D. 14C half-life too short• E. Daughter 14N not retained by limestone

Half-lives: If the amount of radioactive isotope is ¼

the amount originally present, how many half-

lives have gone by?

• A. 1• B. 2• C. 3• D. 4

Half-lives: If the amount of radioactive isotope is ¼

the amount originally present, how many half-

lives have gone by?

• A. 1• B. 2• C. 3• D. 4

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Event Sequence

1. The basaltic dike is older than the graniteA. TrueB. False

Event Sequence

2. The basaltic sill is older than the granite.A. TrueB. False

Event Sequence

3. Sediment layer ‘a’ is older than the graniteA. TrueB. False

Event Sequence

4. Sediment layer ‘q’ is older than the graniteA. TrueB. False