UNCONFORMITIES

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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. The
y 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.
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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.
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CORRELATION
The farther apart the units, the harder it is to
correlate the units. With distance depositional
environments change, resulting in different
facies.
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CORRELATION
Fossils help in correlation. KEY BEDS are also
used. KEY BEDS record a geological event of short
duration that affected a large area.
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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.
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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.
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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.
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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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
ALPHA (?) DECAY
238U92 decays by alpha (?) decay to form 234Th90
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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
40K19 decays 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
40K19 decays 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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
ISOTOPE DATING uses this process to measure
the amount of time elapsed since the minerals
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 isotopes HALF-LIFE (t½).
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11 parent to daughter
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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.