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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
Geo-speak
for “billion
years”
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
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
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
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)
Nuclear Decay
Parent
Daughter
Exponential decay of the parent
P = P0e-t
  is called
the decay
constant
 Half life (t1/2)
is more
commonly
used
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
Example: U-Pb system
t1/2 = 4.5 Ga t1/2 = 0.70 Ga
http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit5/deeptime.html
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
Radiocarbon
Potassium-Argon
Uranium-Lead
method
method
method
(useful
(useful
(useful
for geologically
for
for very
intermediate-age
oldyoung
igneous rocks)
(<
igneous
50,000
rocks)
years), previously living material)
How big is Earth?
 In 200 BC, Eratosthenes
Eratosthenes
276-194 BC
estimated Earth’s
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.
How big is Earth?
Alexandria
 His estimate
(250,000 stadia) is
only slightly larger
than today’s known
global
circumference
(40,075 km)
Aswan
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
Why is Earth differentiated?
When Earth
formed, immense
amount heat
released by
meteorite
bombardment
melted the planet.
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
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/heatflow/global_heat_flow.htm
Geothermal gradient
 On continents, the
near-surface
geothermal gradient
is ~ 16oC / km
 The geothermal
gradient decreases
with depth
Extrapolating temperature
into Earth’s interior
Sources of Earth’s internal heat
 Primordial (from planet formation)
 Decay of radioactive elements
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
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