Transcript Age of the Earth I - PowerPoint Lecture Notes

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 9:00 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.
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)
Age of the Earth
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.
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).
Geologic Time Scale
During the 16, 17 and 1800's geologists
developed a geologic time scale based on
relative ages.
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?
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
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.
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.
Geology and Geophysics
Geology is the science that deals
with the history of the earth as
recorded in rocks.
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?
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.
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
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.
Uniformitarianism
The Principle of Uniformitarianism
The physical, chemical and biological
laws that operate today have also
operated in the geologic past.
Age of the Earth
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
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
Uniformitarianism
The Principle of Uniformitarianism
The physical, chemical and biological
laws that operate today have also
operated in the geologic past.
Rates of Deposition of Sediments
TT = total thickness of sedimentary
strata that has been deposited during
earth history.
Age of Earth = TT/R
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.
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.
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.
Cooling of the Earth from a Molten State
Kelvin's Analysis:
1. Assume that the Earth cooled from a molten
state.
2. Assume that cooling followed ordinary laws of
heat conduction and radiation (uniformitarianism).
3. 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.
Uniformitarianism
The Principle of Uniformitarianism
The physical, chemical and biological
laws that operate today have also
operated in the geologic past.
Age of the Earth
Late 1800’s
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
“The real purpose of the
scientific method is to make
sure that nature hasn’t misled
you into thinking that you know
something that you don’t
actually know.”
Robert M. Pirsig, Zen and the Art of
Motorcycle Maintenance: An Inquiry
into Values
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…
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
Age of the Earth
Radioactivity
and
Radiometric
Dating