Transcript timelect

Geological Time
© NASA
Outline of Talk
Part 1. The Age of the Earth
Practical: Saltiness of Ocean
Part 2. The History of the Earth
Practical: Dawn of a New Age
Part 3. Radiometric dating
Age of the Earth (1): Eternity
en.wikipedia.org/wiki/Image:Aristoteles_Louvre.jpg
© NASA
• In classical philosophy the Earth was
eternal, so Age of the Earth irrelevant
Aristotle
(384-322 BC)
Age of the Earth (2): The Bible
• 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
4004 BC
en.wikipedia.org/wiki/Image:Ussher.jpg
Bishop Ussher
(1581-1656)
en.wikipedia.org/wiki/Image:Genesis.jpg
Age of the Earth (3): Experiments
One of
Buffon’s iron balls
• 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
Comte de Buffon
(1707-1788)
Age of the Earth (4): The Sun
Hermann von
Helmholtz
(1821-1894)
http://en.wikipedia.org/wiki/Image:
Hermann_von_Helmholtz.jpg
• In 1858, calculated the time it would
take for the sun to condense to
present diameter from gas nebula
(around 20 million years)
http://upload.wikimedia.org/wikipedia/en/9/93/Cropped_Earth_with
PNG
Age of the Earth (5): More Physics
• 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
http://en.wikipedia.org/wiki/Image:Lord_
Kelvin_photograph.jpg
Lord Kelvin
(1824-1907)
Age of the Earth (6): Geology
http://en.wikipedia.org/wiki/Image:Charles_Lyell.jpg
Charles Lyell
(1797-1875)
http://en.wikipedia.org/wiki/Image:Colorado_River_edit.jpg
• 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)
Age of the Earth (7): Evolution
Lord Kelvin
http://en.wikipedia.org/wiki/Image:
Lord_Kelvin_photograph.jpg
Charles
Darwin
• In 1869, Thomson
argued that there was
not enough time
for Darwin’s evolution
by natural selection
http://en.wikipedia.org/wiki/Image:
Darwin aged 54.jpg
Lord Kelvin’s “views on the recent age of the world have been
for some time one of my sorest troubles” (Darwin to Wallace)
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?
salt
crystals
Obituary Notices of F.R.S., 1, 260 (1933)
John Joly (1857-1933)
http://www.bigfoto.com/miscellaneous/photos-16/index.htm
Age of the Earth (9): Assumptions
• All these estimates
were based on
assumptions that
couldn’t be proven
Geological History (1): Strata
© Howard Falcon-Lang
Youngest strata
http://en.wikipedia.org/wiki/Nicolas_Steno
Nicolas Steno
(1638-1686)
Oldest strata
Geological History (2): Neptunism
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
Abraham Werner
(1749-1817)
Granite
© Howard Falcon-Lang
Geological History (3): Gaps
Copyright © Marli Miller, University of Oregon
http://upload.wikimedia.org/wikipedia/
commons/c/c6/James_Hutton.jpg
Siccar Point
• Hutton’s unconformity showed that there
were big gaps in the geological record
James Hutton
(1726-1797)
Geological History (4): Maps
http://en.wikipedia.org/wiki/Image:
Geological_map_of_Great_Britain.jpg
http://en.wikipedia.org/wiki/Image:Smith_fossils2.jpg
http://en.wikipedia.org/wiki/Image:
William_Smith.g.jpg
Some of
Smith’s fossils
William Smith
(1769-1839)
The Map that
changed the World
Geological History (5): Fossils
en.wikipedia.org/wiki/Image:Georges_Cuvier.jpg
en.wikipedia.org/wiki/Image:Charles_Lyell.jpg
Cuvier
Georges Cuvier
(1769-1832)
Charles Lyell
(1797-1875)
• 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 rocks
Geological History (6): Stratigraphy
Sedgewick
Murchison
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
A tug-of-war as rocks got sorted into geological periods in the
new science of stratigraphy
Geological History (7): Periods
en.wikipedia.org/wiki/Geologic_time_scale
Carboniferous
Triassic
Cambrian
Ordovician
Silurian
Permian
Devonian
Jurassic
Quaternary
Cretaceous
Tertiary
Geological History (8): The Column
http://en.wikipedia.org/wiki/Geologic_time_scale
Geological Time: Eons, Eras, Periods and Epochs
Geological History (9): Example
en.wikipedia.org/wikiImage:Palais_de_la_Decouvert
e_Tyrannosaurus_rex_p1050042.jpg
http://en.wikipedia.org/wiki/Image:KT_boundary_054.jpg
Impact
http://en.wikipedia.org/wiki/Image:Impact
_event.jpg
Paleogene
Extinction
Cretaceous
en.wikipedia.org/wiki/Image:Tyrannosaurus_BW.jpg
Practical Exercise 2
The Anthropocene: The Dawn of a New Age?
http://en.wikipedia.org/wiki/Image:Midtown_Manhattan_Oct_2007.jpg
New York skyline
Radiometric dating (1): Discovery
http://en.wikipedia.org/wiki/Image:
Henri_Becquerel.jpg
• In 1896, Discovery of radioactivity paved
the way for the precise dating of events
in the geological record
Henri Becquerel
(1852-1908)
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
Half life: 0
Half life: 1
Exponential
Linear
Half life: 2
• 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
Radiometric dating (4): Clocks
Decay series
40K
to 40Ar
147Sm
to 143Nd
Half life
1250 Ma
1060 Ma
235U
to 207Pb
704 Ma
238U
to 206Pb
4468 Ma
14C
to 14N
5370 years
Geological
timescales
Archaeology
• 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
Radiometric dating (5): Pioneers
• 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
en.wikipedia.org/wiki/Image:
en.wikipedia.org/wiki/Image:
A Holmes.jpg
Ernest_Rutherford2.jpg
some rocks from Ceylon
Ernest Rutherford Arthur Holmes to 1600 million years
(1890-1965)
(1871-1937)
Radiometric dating (6): Oldest Rock
Zircon mineral
en.wikipedia.org/wiki/Image:Zircon_microscope.jpg
© NASA
• Oldest rocks on Earth
are the Acasta Gniess of
northern Canada
• 4030 million years old
Acasta Gneiss
Radiometric dating (7): Oldest Grain
www.geology.wisc.edu/zircon/Earliest%20Piece/Images/2a-team.jpg
• 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
© NASA
4404 Ma
zircon grain
www.geology.wisc.edu/zircon/Earliest%20Piece/Images/5.jpg
Radiometric dating (8): Meteorites
http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg
Canyon Diablo meteorite
en.wikipedia.org/wiki/Image:Canyon-diablo-meteorite.jpg
Crab Nebula
• Radiometric age of meteorites
date the formation of the
Solar System and Earth
(4550 million years old)
Radiometric Dating (9): History
http://en.wikipedia.org/wiki/Geologic_time_scale
© World Health Org.
first life
en.wikipedia.org/wiki/Image:Eopraptor_sketch5.png
dinosaurs
© NASA
origin of Earth
first complex cells
humans
Geological Time
© NASA