TELLING TIME GEOLOGICALLY UNCONFORMITIES Not all the rocks that ever formed are preserved. Many...

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 ornone were deposited.

Some are small gaps in time.Some are extensive amounts of time.They exist in practically every sequence of sed. rocks.


UNCONFORMITIES

NONCONFORMITY

Cambrian Sawatch Sandstone overlying the Precambrian Pikes Peak Granite1.6 billion years missing


UNCONFORMITIES

ANGULAR UNCONFORMITY

Siccar Point, ScotlandBirthplace of Unconformities


UNCONFORMITIES

DISCONFORMITY

Wingate Sandstone,overlying Chinle FormationUtah


CORRELATION

In geology, we try to relate all the rocks on Earth intoa relative age scheme.

Consider sequences of sedimentary rocks from all overthe Earth and fit them together in the properorder.

Process is called CORRELATION.

CORRELATION is the determination of equivalenceof age between geographically distant rock unitsusing paleontologic (fossils) or lithologic (rock)similarities.


CORRELATION

The farther apart the units, the harder it is to correlatethe units.

With distance depositional environments change, resulting in different facies.


CORRELATIONFossils help in correlation.

KEY BEDS are also used.KEY BEDS record a geological event of short duration

that affected a large area.


DETERMINING NUMERICAL OR ABSOLUTE AGERelative age dating provides valuable information.Puts rocks in proper sequence.But…..

It is important to know in years, how long ago anevent happened or when a rock formed.

NUMERICAL or ABSOLUTE DATING can do this toa point.

Generally depends on some type of “natural clock”.Depends on a process that occurs at a known, constant

rate.


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.



ISOTOPE DATING

Loss or gain of neutrons converts a parent isotope intoa daughter isotope of the same element.

Loss or gain of protons changes the parent isotope intoa daughter isotope of a completely differentelement.

Through this process, unstable radioactive isotopesdecay to form stable, non-radioactive daughterisotopes.



ALPHA () DECAY

Alpha () particles are composed of two protons andtwo neutrons (He nucleus)

By expulsion of particles, the atomic mass decreasesby 4 and the atomic number decreases by 2.

Produces a daughter isotope that is a completely newelement.



ALPHA () DECAY

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



BETA () DECAYBeta () 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.



BETA () DECAY

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



ELECTRON OR BETA () CAPTUREElectron 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.



ELECTRON OR BETA () CAPTURE

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



Radioactive isotopes are incorporated in minerals androcks 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 areincluded in the mineral.

Radioactive parent isotopes then begin to decay producing daughter isotopes.



ISOTOPE DATING uses this process to measure theamount 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 ofradioactive decay, we can determine the numericalage of a rock.



The time it takes for HALF of the atoms of the parentisotope to decay into daughter isotopes is knownas the isotope’s HALF-LIFE (t½).

1:1 parent to daughter

1:3

1:7



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 207PbHalf-life (t½) is 713 million years.

Age of the rock is 2.139 billion years.

TELLING TIME GEOLOGICALLY UNCONFORMITIES Not all the rocks that ever formed are preserved. Many...

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