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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.

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Telling time geologically

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

TELLING TIME GEOLOGICALLY

UNCONFORMITIES

NONCONFORMITY

Cambrian Sawatch

Sandstone overlying the

Precambrian

Pikes Peak Granite

1.6 billion years missing


Telling time geologically

TELLING TIME GEOLOGICALLY

UNCONFORMITIES

ANGULAR UNCONFORMITY

Siccar Point, Scotland

Birthplace of Unconformities


Telling time geologically

TELLING TIME GEOLOGICALLY

UNCONFORMITIES

DISCONFORMITY

Wingate Sandstone,

overlying

Chinle Formation

Utah


Telling time geologically

TELLING TIME GEOLOGICALLY

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

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

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

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

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

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

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

TELLING TIME GEOLOGICALLY

DETERMINING NUMERICAL OR ABSOLUTE AGE

ALPHA () DECAY

238U92 decays by alpha ()decay to form 234Th90


Telling time geologically

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.


Telling time geologically

TELLING TIME GEOLOGICALLY

DETERMINING NUMERICAL OR ABSOLUTE AGE

BETA () DECAY

40K19 decays by beta() decay to form 40Ca20


Telling time geologically

TELLING TIME GEOLOGICALLY

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.


Telling time geologically

TELLING TIME GEOLOGICALLY

DETERMINING NUMERICAL OR ABSOLUTE AGE

ELECTRON OR BETA () CAPTURE

40K19 decays by beta() capture to form 40Ar18


Telling time geologically

TELLING TIME GEOLOGICALLY

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

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.


Telling time geologically

TELLING TIME GEOLOGICALLY

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).



Telling time geologically

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.