time and geology quantitative geologic time early attempts tried to place events in chronology of...

Time and Geology

Quantitative Geologic Time

Early AttemptsTried to place events in chronology of actual age

James Ussher, Archbishop of Armagh and Primate of All Ireland

1650

The Annals of the World

-The Annals of the World

Ussher calibrated the Julian calendar with the dates and eventsfrom the Old Testament.

Determined Earth was created on Sunday, 23 October, 4004 BC

Time and Geology


Sir John Lightfoot 1642

Determined that the Earth was created at 9 am

Geologic observations did not support this timeframe.

Later disproved by the theory of evolution

Time and Geology


Other attempts used depositional rates of sediment

If the depositional rate was known one could estimate the time required to deposit a given thickness of sediments

The total geologic time elapsed could be estimated by dividing the annual sediment influx into the total thickness of sedimentary rock.

Time and Geology


Sir Edmund Halley 1715

Speculated that the ocean must be the same age as the Earth

Ocean started out fresh

Weathering of rocks on landproduced salt

If the amount of salt in the ocean wasknown it could be divided by the amount added each year

John Joly tried this and came up with an age of ~90 my in 1899

Time and Geology


Lord Kelvin (William Thomson) 1862

Thomson challenged the views on geologic and biological change of the early uniformitarians

Argued Earth could not be that old

Assuming that the Earth started out asa liquid at 7000°F and has beencooling since formation

Using the cooling rate he determined the Earth was 24-40 my old

Time and Geology


Geologists felt the Earth had to be older and arguedNo PROOF!

Discovery of radioactivity provided the answer.

X-ray of Kelvin’s hand

Radioisotopic Methods for Dating Rocks

A Quick Review

The solid Earth is composed of rocks and minerals

ROCKS - aggregates of minerals

A MINERAL is anaturally occurring,inorganic solid, consisting of either a single element or compound,with a definite chemical composition (or varies within

fixed limits),and a systematic internal arrangement of atoms.


MINERAL CHEMISTRY

Minerals are composed of elements in specific proportions.

Elements cannot be broken down into any smaller units andstill be recognizable.

Atoms of one or more elements combine in specific proportionsto produce compounds.

Specific proportions are expressed by the compound’schemical formula.

HALITE NaClQUARTZ SiO2

Radioisotopic Methods for Dating RocksMINERAL CHEMISTRY

Atoms are extremely small.

Atoms are composed of three particles: Protons and Neutrons in the nucleus of the atom.Electrons in energy levels around the nucleus.

Protons have a positive charge.Electrons have a negative charge.Neutrons are neutral.

Radioisotopic Methods

ATOMIC CHEMISTRY

ATOMIC MASS of an element is the total number of protonsand neutrons in the nucleus of the atom.

ATOMIC NUMBER is the total number of protons in thenucleus of the atom.

Always the same number of protons in every atom of the sameelement.

Number of neutrons in the nucleus can vary.Variation in the number of neutrons in the same element

produces ISOTOPES of that element.


ATOMIC CHEMISTRY

Isotopes of OxygenAlways 8 protons in the nucleus.Oxygen has atomic number of 8 on the Periodic Table.16O has 8 protons and 8 neutrons.17O has 8 protons and 9 neutrons.18O has 8 protons and 10 neutrons.


ATOMIC CHEMISTRY

Some isotopes have very unstable nuclei and those break down spontaneously.

This reaction is termed RADIOACTIVITY.

Generally a large amount of heat is produced in the reaction.


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.


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.


FACTORS AFFECTING ISOTOPIC DATING

Most useful in igneous rocks.As minerals crystallize, radioactive isotopes become

incorporated in the minerals.No daughter isotopes at that time.Crystallization sets the isotopic “clock”.

Doesn’t work in sedimentary rocks. How come?


FACTORS AFFECTING ISOTOPIC DATING

Works best when a rock or mineral represents a “closed” system.

Parent and daughter isotopes cannot move in or out ofa mineral or rock.

Igneous rocks best fit this criteria.


time and geology quantitative geologic time early attempts tried to place events in chronology of...

Documents

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