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Rocks as time machines: principles
of geologic time
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 + 2012 =
6,016 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:
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.
4.6 billion years old
(4.6 billion = 4,600,000,000)
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.
Geologic Time Scale
During the 16, 17 and 1800's geologists
developed a geologic time scale based on
relative ages. Sedimentary rocks: Rocks formed from weathered products
of pre-existing rocks (sediments) that have been transported,
deposited, and lithified (converted into rock). 1
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Absolute Age of the Earth
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, it would seem
the entire planet was a mass of molten rock material.
When did the earth cool and solidify into its present
state?
MORE ON ALL OF THIS A BIT LATER
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. Uniformitarianism
The Principle of Uniformitarianism = The physical,
chemical and biological laws that operate today have also
operated in the geologic past.
THE IDEA WAS HUTTON’S BUT LYELL COINED THE TERM
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?
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
Fossils and Evolution
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.
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.
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Cooling of the Earth from a Molten State
Cooling of the Earth from a Molten State
Kelvin's Analysis:
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.
Determining geological ages
•  Relative dating – placing rocks and events in their
proper sequence of formation
•  Numerical (absolute) dating – specifying the actual
number of years that have passed since an event occurred
(known as absolute age dating)
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.
Relative Geological Time
Nicolas Steno’s Principles (1669):
Principle of Uniformitarianism:
“The present is the key to the past”.
Studying processes today gives insight into past events.
Principle of Original Horizontality:
Beds of sedimentary rock are deposited in a horizontal
orientation.
Principle of Superposition:
In a sequence of undisturbed sedimentary rocks, the
layers get younger upwards.
Principle of Lateral Continuity:
Original sedimentary layers extend laterally until they
thin at its edges.
Relative Geological Time
Principles (cont.):
Principle of Inclusions:
Inclusions are older than the rock in which they
are contained.
Principle of Cross-Cutting Relations:
A disrupted pattern is older than the cause of the
disruption.
Principle of Fossil Succession:
Fossils contained in strata are related to the age of
the rocks.
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Unconformities
Surface that represents a break in the
geologic record. The rock unit(s)
immediately above the break is/are
much younger than that/those below.
Usually represent buried erosional
surfaces – parcel of geology is missing!
Unconformities
Three types:
1) Disconformity: unconformity is parallel to layering,
but there is a gap in the geologic record.
Typically erosional and hard to spot.
2) Angular Unconformity: Younger strata overly older
rocks that were tilted/folded.
3) Nonconformity: A contact in which an erosional
surface on a plutonic or metamorphic rock has been
covered by younger sediments or volcanic rocks (i.e.,
unconformity separates different rock types).
Angular Unconformity
Nonconformity
Disconformity
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Unconformities present in the Grand Canyon
Correlation
Relationships between rocks of the
same age in different locations.
Means of correlation:
(i) Physical continuity;
(ii) Similarity of rock types, sequences;
(iii) Similarity of fossils, especially
where assemblages are present.
Physical Continuity
Principle of Lateral Continuity
in a Eroded Landscape
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