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TELLING TIME GEOLOGICALLY
UNCONFORMITIES
Not all the rocks that ever formed are preserved.
Many rocks are subjected to weathering and erosion.
Gaps in the geologic record exist.
These gaps are termed UNCONFORMITIES.
They occur when erosion has removed rocks or
none were deposited.
Some are small gaps in time.
Some are extensive amounts of time.
They exist in practically every sequence of sed. rocks.
TELLING TIME GEOLOGICALLY
UNCONFORMITIES
NONCONFORMITY
Cambrian Sawatch
Sandstone overlying the
Precambrian
Pikes Peak Granite
1.6 billion years missing
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UNCONFORMITIES
ANGULAR UNCONFORMITY
Siccar Point, Scotland
Birthplace of Unconformities
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UNCONFORMITIES
DISCONFORMITY
Wingate Sandstone,
overlying
Chinle Formation
Utah
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CORRELATION
In geology, we try to relate all the rocks on Earth into
a relative age scheme.
Consider sequences of sedimentary rocks from all over
the Earth and fit them together in the proper
order.
Process is called CORRELATION.
CORRELATION is the determination of equivalence
of age between geographically distant rock units
using paleontologic (fossils) or lithologic (rock)
similarities.
TELLING TIME GEOLOGICALLY
CORRELATION
The farther apart the units, the harder it is to correlate
the units.
With distance depositional environments change,
resulting in different facies.
TELLING TIME GEOLOGICALLY
CORRELATION
Fossils help in correlation.
KEY BEDS are also used.
KEY BEDS record a geological event of short duration
that affected a large area.
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
Relative age dating provides valuable information.
Puts rocks in proper sequence.
But…..
It is important to know in years, how long ago an
event happened or when a rock formed.
NUMERICAL or ABSOLUTE DATING can do this to
a point.
Generally depends on some type of “natural clock”.
Depends on a process that occurs at a known, constant
rate.
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
ISOTOPE DATING
Depends on the decay of radioactive isotopes.
Isotopes are varieties of elements that differ by the
number of neutrons in the nucleus.
Radioactive isotopes have nuclei that spontaneously
decay by emitting or capturing a variety of
subatomic particles.
The decaying isotope is known as the parent isotope.
By decay, the parent isotope forms a daughter isotope.
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
ISOTOPE DATING
Loss or gain of neutrons converts a parent isotope into
a daughter isotope of the same element.
Loss or gain of protons changes the parent isotope into
a daughter isotope of a completely different
element.
Through this process, unstable radioactive isotopes
decay to form stable, non-radioactive daughter
isotopes.
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
ALPHA () DECAY
Alpha () particles are composed of two protons and
two neutrons (He nucleus)
By expulsion of  particles, the atomic mass decreases
by 4 and the atomic number decreases by 2.
Produces a daughter isotope that is a completely new
element.
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
ALPHA () DECAY
238U decays
92
by alpha () decay to form 234Th90
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
BETA () DECAY
Beta () particles are essentially electrons.
These electrons are released from the nucleus of the
parent isotope.
Neutrons are composed of a proton and an electron.
Neutron decays, releasing an electron, while at the
same time produces a proton.
Beta () decay increases the atomic number by 1.
No change in the atomic mass.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
BETA () DECAY
40K decays
19
by beta () decay to form 40Ca20
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DETERMINING NUMERICAL OR ABSOLUTE AGE
ELECTRON OR BETA () CAPTURE
Electron or Beta () capture involves capture of an
electron from the surrounding orbiting cloud
by the nucleus.
These electrons join with a proton and form a neutron.
Electron or Beta () capture decreases the atomic
number by 1.
No change in the atomic mass.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
ELECTRON OR BETA () CAPTURE
40K decays
19
by beta () capture to form 40Ar18
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DETERMINING NUMERICAL OR ABSOLUTE AGE
Radioactive isotopes are incorporated in minerals and
rocks in a variety of ways.
As minerals crystallize from magma, radioactive
isotopes are included in mineral crystal structure.
At the time of crystallization, only parent isotopes are
included in the mineral.
Radioactive parent isotopes then begin to decay
producing daughter isotopes.
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
ISOTOPE DATING uses this process to measure the
amount of time elapsed since the mineral’s
formation.
With time, the amount of parent isotope will decrease
and the amount of daughter isotope will increase.
The DECAY RATE is constant and acts like a “clock”.
Decay rates are not affected by temperature, pressure,
or chemical reaction with the parent isotope.
By measuring the ratio of parent to daughter isotopes in
the mineral and comparing it with the rate of
radioactive decay, we can determine the numerical
age of a rock.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
The time it takes for HALF of the atoms of the parent
isotope to decay into daughter isotopes is known
as the isotope’s HALF-LIFE (t½).
1:1 parent to daughter
1:3
1:7
TELLING TIME GEOLOGICALLY
DETERMINING NUMERICAL OR ABSOLUTE AGE
To calculate the numerical age of a rock, mineral, bone,
etc., we determine the number of half-lives or
fraction thereof and multiply the number of
half-lives gone by by the known half-life (in years).
Simply put:
In a rock we find 23 atoms of 235U and 161 atoms of 207Pb
Half-life (t½) is 713 million years.
Age of the rock is 2.139 billion years.