Geological Time

© NASA

Outline of Talk

Part 1. The Age of the EarthPractical: Saltiness of Ocean

Part 2. The History of the EarthPractical: Dawn of a New Age

Part 3. Radiometric dating

Age of the Earth (1): Eternity

Aristotle (384-322 BC)

en.wikipedia.org/wiki/Image:Aristoteles_Louvre.jpg

• In classical philosophy the Earth was eternal, so Age of the Earth irrelevant

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Age of the Earth (2): The Bible

4004 BC

• In 1654 Bishop Ussher calculated that the Earth was created in 4004 BC

• He got this figure using evidence from the Bible and other Middle Eastern literature

• The date became so popular that it was printed with the Book of Genesis

Bishop Ussher (1581-1656)

en.wikipedia.org/wiki/Image:Ussher.jpg

en.wikipedia.org/wiki/Image:Genesis.jpg

Age of the Earth (3): Experiments

Comte de Buffon (1707-1788)

• In 1760, Buffon measured the cooling time of red-hot iron balls of different sizes

• He scaled up to the size of the Earth (75,000 years to cool)

http://en.wikipedia.org/wiki/Image:Buffon_1707-1788.jpg

One ofBuffon’s iron balls

Age of the Earth (4): The Sun

http://en.wikipedia.org/wiki/Image:Hermann_von_Helmholtz.jpg

http://upload.wikimedia.org/wikipedia/en/9/93/Cropped_Earth_with_Sunburst.PNG

Hermann von Helmholtz (1821-1894)

• In 1858, calculated the time it would take for the sun to condense to present diameter from gas nebula (around 20 million years)

Age of the Earth (5): More Physics

http://en.wikipedia.org/wiki/Image:Lord_Kelvin_photograph.jpg

Lord Kelvin (1824-1907)

• In 1862, Lord Kelvin assumed that Earth originally had a temperature of 7000°F

• Knew geothermal gradient (1°F/50 ft)

• Calculated cooling age (20 million years)

Geothermal gradient

Age of the Earth (6): Geology

Charles Lyell(1797-1875)

• Sediment accumulates at the same rate today as in the past so Earth must be really ancient to account for geological record (hundreds of millions of years)

http://en.wikipedia.org/wiki/Image:Charles_Lyell.jpg http://en.wikipedia.org/wiki/Image:Colorado_River_edit.jpg

Age of the Earth (7): Evolution

Lord Kelvin’s “views on the recent age of the world have been for some time one of my sorest troubles” (Darwin to Wallace)

• In 1869, Thomson argued that there was not enough time for Darwin’s evolution by natural selection

http://en.wikipedia.org/wiki/Image:Lord_Kelvin_photograph.jpg

Charles Darwin

Lord Kelvin

http://en.wikipedia.org/wiki/Image:Darwin aged 54.jpg

Age of the Earth (8): Sea Salt• In 1899, John Joly calculated the Earth’s age using the saltiness of the ocean (80-150 million years)

• How much salt was in the Ocean?• How much did rivers add each year?

Obituary Notices of F.R.S., 1, 260 (1933)

John Joly (1857-1933)

http://www.bigfoto.com/miscellaneous/photos-16/index.htm

salt crystals

Age of the Earth (9): Assumptions

• All these estimates were based on assumptions that couldn’t be proven

Practical Exercise 1

Calculating the Age of the Earth using the Saltiness of the Ocean

http://www.bigfoto.com/miscellaneous/photos-16/index.htm

Geological History (1): Strata

Nicolas Steno (1638-1686)

http://en.wikipedia.org/wiki/Nicolas_Steno

Oldest strata

Youngest strata

© Howard Falcon-Lang

Geological History (2): Neptunism

Abraham Werner (1749-1817)

http://en.wikipedia.org/wiki/Image:Abraham_Gottlob_Werner.jpg

• Werner argued all rocks had been deposited in a worldwide ocean (think Noah’s Flood)• Geologists could figure out the order in which rocks formed• Divided geological record into four main divisions

© Howard Falcon-Lang

Granite

Geological History (3): Gaps

• Hutton’s unconformity showed that there were big gaps in the geological record

James Hutton (1726-1797)

Copyright © Marli Miller, University of Oregon http://upload.wikimedia.org/wikipedia/commons/c/c6/James_Hutton.jpg

Siccar Point

unconformity

Geological History (4): Maps

William Smith(1769-1839)

http://en.wikipedia.org/wiki/Image:William_Smith.g.jpg

The Map that changed the World

http://en.wikipedia.org/wiki/Image:Geological_map_of_Great_Britain.jpg

Some of Smith’s fossils

http://en.wikipedia.org/wiki/Image:Smith_fossils2.jpg

Geological History (5): Fossils

Cuvier

• Cuvier showed that some animals had gone extinct• Lyell used the proportion of living fossils to divide up geological time• Older rocks contained more extinct types than younger rocksCharles Lyell

(1797-1875)Georges Cuvier (1769-1832)

en.wikipedia.org/wiki/Image:Georges_Cuvier.jpg en.wikipedia.org/wiki/Image:Charles_Lyell.jpg

Geological History (6): Stratigraphy

A tug-of-war as rocks got sorted into geological periods in the new science of stratigraphy

commons.wikimedia.org/wiki/Image:Adam_Sedgwick.jpg

en.wikipedia.org/wiki/Image:Roderick_Murchison.jpg

en.wikipedia.org/wiki/Image:Geological_map_of_Great_Britain.jpg

MurchisonSedgewick

Geological History (7): Periods

Carboniferousen.wikipedia.org/wiki/Geologic_time_scale

Cambrian

Ordovician

Silurian

Devonian

Permian

Triassic

Jurassic

Cretaceous Tertiary

Quaternary

Geological History (8): The Column

Geological Time: Eons, Eras, Periods and Epochs

http://en.wikipedia.org/wiki/Geologic_time_scale

Geological History (9): Example

Impacthttp://en.wikipedia.org/wiki/Image:KT_boundary_054.jpg

http://en.wikipedia.org/wiki/Image:Impact_event.jpg

en.wikipedia.org/wikiImage:Palais_de_la_Decouverte_Tyrannosaurus_rex_p1050042.jpg

en.wikipedia.org/wiki/Image:Tyrannosaurus_BW.jpg

Extinction

Cretaceous

Paleogene

Practical Exercise 2The Anthropocene: The Dawn of a New Age?

http://en.wikipedia.org/wiki/Image:Midtown_Manhattan_Oct_2007.jpg

New York skyline

Radiometric dating (1): Discoveryhttp://en.wikipedia.org/wiki/Image:Henri_Becquerel.jpg

Henri Becquerel (1852-1908)

• In 1896, Discovery of radioactivity paved the way for the precise dating of events in the geological record

Radiometric dating (2): Decay

en.wikipedia.org/wiki/Image:Alpha_Decay.svg

• Radioactive ‘parent isotopes’ spontaneously emit protons and neutrons and decay into ‘daughter isotopes’• E.g., Uranium-238 decays into Lead-206

Radiometric dating (3): Half life

• The rate of decay from parent to daughter isotope depends on its half life. The half life is the amount of time needed for half the parent isotope to decay to daughter isotope

Half life: 0

Half life: 1

Half life: 2

Linear

Exponential

Radiometric dating (4): Clocks

• Different radioactive isotopes have different half lives• Isotopes with long half lives are useful for dating old rocks. It is

important to use the right tool for the right job

Decay series Half life

40K to 40Ar 1250 Ma

147Sm to 143Nd 1060 Ma

235U to 207Pb 704 Ma

238U to 206Pb 4468 Ma

14C to 14N 5370 years

Geological timescales

Archaeology

Radiometric dating (5): Pioneers

Arthur Holmes (1890-1965)

Ernest Rutherford (1871-1937)

en.wikipedia.org/wiki/Image:Ernest_Rutherford2.jpg

en.wikipedia.org/wiki/Image:A Holmes.jpg

• Rutherford figured out a technique to date the age of rocks in 1904• Holmes developed this kind of ‘radiometric dating’ still further.• In 1913 Holmes dated some rocks from Ceylon to 1600 million years

Radiometric dating (6): Oldest Rock

• Oldest rocks on Earth are the Acasta Gniess of northern Canada • 4030 million years old

en.wikipedia.org/wiki/Image:Zircon_microscope.jpg

© NASA

Acasta Gneiss

Zircon mineral

Radiometric dating (7): Oldest Grain

www.geology.wisc.edu/zircon/Earliest%20Piece/Images/5.jpg

4404 Mazircon grain

www.geology.wisc.edu/zircon/Earliest%20Piece/Images/2a-team.jpg

© NASA

• Ancient mineral grain found at Jack Hills, Australia • Mineral grain eroded from first crust and then deposited in a new rock

• Dates the Earth’s first crust to around 4404 million years

Radiometric dating (8): Meteorites

• Radiometric age of meteorites date the formation of the Solar System and Earth (4550 million years old)

en.wikipedia.org/wiki/Image:Canyon-diablo-meteorite.jpg

http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg

Crab Nebula

Canyon Diablo meteorite

Radiometric Dating (9): History

dinosaurs

humans

first life

origin of Earth

http://en.wikipedia.org/wiki/Geologic_time_scale

en.wikipedia.org/wiki/Image:Eopraptor_sketch5.png© World Health Org.

© NASA

first complex cells

Geological Time

© NASA