geo time
TRANSCRIPT
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
© NASA
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