Deep Time

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Deep Time
How old is Earth?

• Key Concepts
• Different Age Determinations
• Ages based on human history
• Pre-20th century physical estimates.
• Radioactive Isotope Dating
• Best current estimate 4.6 0.1 Ga

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Historical Age Determinations

• Based on the concept of creation
• Paradigm
• Human History Physical History of Earth
• Traditional Jewish calendar starts from 3760 BC

www-astronomy.mps.ohio-state.edu/
pogge/Ast161/Unit5/deeptime.html
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Historical Age Determinations

• Eusebius of Caesarea (AD 260-340)
• 3184 BC, based on biblical chronology
• James Ussher (1581-1656)
• Evening of Sunday, October 23, 4004 BC (Julian)

www-astronomy.mps.ohio-state.edu/
pogge/Ast161/Unit5/deeptime.html
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Pre-20th century physical estimates

• George-Louis Leclerc, Comte de Buffon (1707-1788)
• Cooling of the Earth 75,000 years
• Lord (William) Kelvin (1824-1907)
• Refined Buffon's calculation in 1897 to obtain
  an age of 25 Ma
• Why is this number wrong?

Radioactivity!
www-astronomy.mps.ohio-state.edu/
pogge/Ast161/Unit5/deeptime.html
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Radioactivity

• Radioactive isotopes are unstable over time they
  decay to produce stable isotopes
• Radioactive decay occurs in the nucleus of an
  atom
• Different types of radiation are emitted by this
  process (?, ? and ? rays)

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Nuclear Decay
Daughter
Parent
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Exponential decay of the parent
P P0e-?t

• ?? is called the decay constant
• Half life (t1/2) is more commonly used

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Half life

• The time required (in a closed system) for the
  concentration of a parent isotope to be reduced
  by half.
• t1/2 ln(2)/? 0.693/?
• Each radioactive isotope has a unique half life

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Example U-Pb system
t1/2 0.70 Ga
t1/2 4.5 Ga
http//www-astronomy.mps.ohio-state.edu/pogge/Ast
161/Unit5/deeptime.html
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Radiometric age dating the Geological Hour Glass
Parent D t1/2 Uranium-238 Lead-206 4.5 Ga
Uranium-235 Lead-207 704 Ma Potassium-40
Argon-40 1.25 Ga Carbon-14 Nitrogen-14
5568 years
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How big is Earth?

• In 200 BC, Eratosthenes estimated Earths
  circumference by geometry.
• He used the length of a building shadow in
  Alexandria at noon on the summer solstice.
• He knew that, simultaneously, sunlight was
  hitting the bottom of a water well in Aswan.

Eratosthenes 276-194 BC
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How big is Earth?
Alexandria

• His estimate (250,000 stadia) is only slightly
  larger than todays known global circumference
  (40,075 km)

Aswan
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Journey to the Centre of the Earth

• The earth is divided into four main layers the
  inner core, outer core, mantle, and crust.

Radius 6,378 km
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Why is Earth differentiated?
When Earth formed, immense amount heat released
by meteorite bombardment melted the planet.
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How hot does it get?
Early Earth during the Hadean period showing the
so-called magma ocean.
http//rst.gsfc.nasa.gov/Sect19/Sect19_2a.html
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Global heat flow

• Highest heat flow at mid-ocean ridges
• Thermal energy production from Earth 3.8x1013 W
• Energy use of U.S.A. 3.0x1011 W

http//geophysics.ou.edu/geomechanics/notes/heatfl
ow/global_heat_flow.htm
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Geothermal gradient

• On continents, the near-surface geothermal
  gradient is 16oC / km
• The geothermal gradient decreases with depth

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Extrapolating temperature into Earths interior
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Sources of Earths internal heat

• Primordial (from planet formation)
• Decay of radioactive elements

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Primordial Heat

• Gravitational contraction
• Core formation (heats mantle by 1000o C)
• Heavy meteorite bombardment during the Hadean
  Period (4.5 - 3.8 Ga)

http//www.palaeos.com/Hadean/Hadean.htm
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Formation of the Moon
Leading theory moon formed by collisional
ejection of mantle material at 4.45 Ga, due to
collision of Mars-sized body