How Old is the Earth?

1
How Old is the Earth?
1654 Archbishop James Ussher of Ireland studied
the timing of biblical events described in the
Old Testament, and concluded that the the world
was created on - October 23, 4004 B.C.E. at 900
AM. Thus, the age of the earth, based on
Ussher's results, would be 4004 2003 6,007
years old. Many calculations of this type have
been done by other biblical scholars, and in
general, the Earth was found to be about 6,000
years old based on these types of calculations.
2
Current Estimate of the Age of the Earth
Scientific methods of estimating the age of the
earth are based on analysis of evidence found in
rocks and on radiometric dating . Based on
these methods, scientists have concluded that the
earth is much more than 6,000 years
old.   Recent estimates of the age of the earth
4.6 billion years old (4.6 billion
4,600,000,000)
3
Age of the Earth
4
Geologic Time
Geologic time is measured in millions of
years. How do we know about such long periods
of time? Geochronology The science of
determining the ages of rocks.
5
Relative Geologic Ages
1669 - Nicholas Steno Studied layers of rock
called strata. Steno described how the position
of strata could yield relative ages of rock units
(i.e., older versus younger). Steno applied the
following simple rule that has come to be called
the principle of superposition In any sequence
of "undisturbed" strata of sedimentary rocks, the
oldest layer is on the bottom, and successively
higher rocks are successively younger.
Sedimentary rocks Rocks formed from weathered
products of pre-existing rocks (sediments) that
have been transported, deposited, and lithified
(converted into rock).
6
Geologic Time Scale
During the 16, 17 and 1800's geologists
developed a geologic time scale based on relative
ages.
7
Absolute Age of the Earth
During the 16, 17 and 1800's geologists developed
a geologic time scale based on relative ages.
But What about absolute ages? What is the
absolute age of the earth? Define Absolute age
of the earth amount of time since the earth
solidified. At some point in the earth's early
history, the entire planet was a mass of molten
rock material. When did the earth cool and
solidify into its present state?
8
Methods for Estimating the Age of the Earth

• Fossils and evolution
• Rates of deposition of sediments
• Cooling of the Earth from a molten state
• Radiometric Dating

9
Fossils and Evolution
Fossils remains of ancient life. For example,
molds of ancient sea shells.    Mid-1800's
Charles Lyell - Used evolution to estimate
absolute ages of rocks.  
10
Fossils and Evolution
Mid-1800's Charles Lyell - Used evolution to
estimate absolute ages of rocks. To estimate
ages of rocks Need to find a natural process
that leaves a tangible record in the rocks.
Then if you can estimate the amount of time
associated with that process, the age of the rock
can be estimated. Evolution is such a process.
  
11
Geology and Geophysics
Geology is the science that deals with the
history of the earth as recorded in rocks.
12
Uniformitarianism
The Principle of Uniformitarianism The
physical, chemical and biological laws that
operate today have also operated in the geologic
past. Lyell compared the amount of evolution
exhibited by marine fossils found in rocks of
different relative ages with estimates of rate of
evolution. Using estimates of rate of evolution
and observations of fossil forms -- estimate age
of rocks. How long a period of time would be
required for the evolution observed in the rock
strata to occur?
13
Fossils and Evolution

• How long a period of time would be required for
  the evolution observed in the rock strata to
  occur?
• Lyell calculated Certain rocks were about 80
  million years old. But, ages of older rocks were
  more difficult to estimate.  
• Problems
• Missing parts of fossil record.
• Rates of evolution difficult to estimate for
  older life forms.
• General conclusion based on this type of approach
  to estimating age of earth
• Earth is at least 10's of millions of years old.

14
Methods for Estimating the Age of the Earth

• Fossils and evolution
• Rates of deposition of sediments
• Cooling of the Earth from a molten state
• Radiometric Dating

15
Fossils and Evolution
How long a period of time would be required for
the evolution observed in the rock strata to
occur? General conclusion based on this type
of approach to estimating age of earth Earth
is at least 10's of millions of years old.
16
Uniformitarianism
The Principle of Uniformitarianism The physical,
chemical and biological laws that operate today
have also operated in the geologic past.
17
Age of the Earth
18
Methods for Estimating the Age of the Earth

• Fossils and evolution
• Rates of deposition of sediments
• Cooling of the Earth from a molten state
• Radiometric Dating

19
Rates of Deposition of Sediments
Estimate the amount of time required to deposit a
given thickness of strata. A Age of sequence
of sedimentary rocks. T Thickness of strata.
R Rate of sediment accumulation. A T/R
Example (100 ft)/(0.0002 ft/yr) 500,000 yr
20
Uniformitarianism
The Principle of Uniformitarianism The physical,
chemical and biological laws that operate today
have also operated in the geologic past.
21
Rates of Deposition of Sediments
TT total thickness of sedimentary strata that
has been deposited during earth history. Age of
Earth TT/R
22
Rates of Deposition of Sediments
Because of poor estimates of rates of deposition,
this method gave a very wide range of
results. But, this method generally suggests
that the earth is at least 10's of millions of
years old, and (based on both fossil evidence
and rates of deposition of sediments) Many
geologists and biologists in the 1800's concluded
that the earth was several hundreds of millions
of years old.
23
Methods for Estimating the Age of the Earth

• Fossils and evolution
• Rates of deposition of sediments
• Cooling of the Earth from a molten state
• Radiometric Dating

Based on fossils and evolution, and on rates of
deposition of sediments By the late 1800's
many geologists and biologists thought that the
earth was several hundreds of millions of years
old.
24
Cooling of the Earth from a Molten State
Late 1800's A physicist named Lord Kelvin, used
physics and mathematics to challenge geologists
and biologists who, by then estimated the age of
the earth to be at least 100's of millions of
years old. Kelvin estimated Earth was about 24
to 40 million years old. Many geologists and
biologists of the time argued that the earth was
much older.
25
Cooling of the Earth from a Molten State

• Kelvin's Analysis
• Assume that the Earth cooled from a molten state.
  
• Assume that cooling followed ordinary laws of
  heat conduction and radiation (uniformitarianism).
  
• Apply laws of heat conduction and radiation, and
  calculate the number of years it would take for
  the earth to cool from a hot molten mass to it's
  present state.
• Kelvin's Result Earth is 24 to 40 million years
  old.

26
Uniformitarianism
The Principle of Uniformitarianism The physical,
chemical and biological laws that operate today
have also operated in the geologic past.
27
Age of the Earth
28
Late 1800s

• Lord Kelvin  
   
• 10's of millions of years old
• Cooling of earth from molten state
• Physics and mathematics
• "Quantitative" arguments

• Geologists and Biologists  
• 100's of millions of years old
• Fossils, evolution, and rates of
  deposition of sediments
• Intuition
• "Qualitative" arguments

29
The real purpose of the scientific method is to
make sure that nature hasnt misled you into
thinking that you know something that you dont
actually know.
Robert M. Pirsig, Zen and the Art of Motorcycle
Maintenance An Inquiry into Values
30
Radioactivity

• Discovery of radioactivity in the 1890's showed
  that the geologists and biologists were right
• Radioactivity as an internal heat source.
• Radiometric dating.
• So, earth is much older than Kelvin thought

31
Methods for Estimating the Age of the Earth

• Fossils and evolution
• Rates of deposition of sediments
• Cooling of the Earth from a molten state
• Radiometric Dating

32
Age of the Earth
33
Radioactivity and Radiometric Dating