12 Age Dating of Earth

7/30/2019 12 Age Dating of Earth

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1. Relative age dating

It could be done by the following:

a) Principle of superposition---Initial horizontality: In normal geologic

section, the oldest bed is at the bottom and the younger one is at the

top, when there is no deformation.

b) Correlation by faunal contents, rock type or marker bed (datum

surface).

Each bed (rock unit) has distinct and unique fauna and flora, unlike

those above or below. By tracing these fossils across a country, it is

possible to recognize contemporaneous deposits and piece together

the scattered fragments of the record and place them in proper

sequence

Age-Dating of Earth


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Lithologic and faunal correlation of Lower Cretaceous rocks in northern Sinai, Egypt


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c) Principles of inclusions and cross-cutting relationship:

The cutting rock is younger than the one that has been cut

d) Paleomagnetic signature:

2. Absolute age dating Early attempts for absolute age dating have used the following:

a) Rate of sedimentation

Age =

Thickness of geologic column is averaging: 100 000 to 300 000 mile

Rate of sedimentation is averaging: 30 cm/ 100 yrs to 3 cm/ 8000 yrs, so

the Earths Age is averaging: 3 myrs to 1.6 byrs

rateionsedimentattheofaverageYearlycolumnhicstratigraptheofthicknessTotal


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Varved layers (clay and silt) deposit yearly in lakes. Also, organic-

rich layers deposits also yearly in the oceans. Thus, counting of

these layers gives the absolute age of these layers.

Using rate of sedimentation in age dating is not reliable:

1) because the rate of sedimentation is not constant from place to

another. There is deposition of sediments in places and while at

others the deposition ceases.

2) Erosion


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b) Salinity of the Ocean:

Age =

When we use the above formula, we should assume that

primitive oceans were not salty----Na Cl produced by

weathering of the earths crust-----rate of adding salt to the

oceans is in yearly average for all the geologic time.

Thus, it was estimated that (In 1899) age of the ocean equal

100 000 000 yrs

rocean watetoadded(g)saltofaverageYearlyoceanin(g)saltofamountTotal


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This method is reasonable, but it is not completely satisfactory

because some salt produced from the sedimentary rocks rather

than from the mother rocks (igneous and metamorphic), and

millions of tons of salt extract from the ocean to be used in the

industry.

c) Temperature:

Lord Kelvin (1897) estimated the age of the earth of about 24

to 40 Myrs, based on the assumption that the earth has been

gradually cooling down from an original molten conditions.


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This method based on the isotopes of the element.

Atom consists of : clouds of electrons ve charge Protons +ve charge neutrons no charge

No. of protons gives the Atomic Number of the element, where as No.

of protons + neutrons gives its Atomic Weight.

Example: Hydrogen has only one proton---Thus its mass= 1

Carbon has 6 protons+ 6 neutrons----its mass= 12

In nature Carbon has three isotopes 12C, 13C and 14C. The only

radioactive carbon isotope is Carbon 14. It is generated in upper

layers of the atmosphere when neutrons from sun radiations hit

nitrogen-14 atoms and transform them to Carbon 14.

d- Radioactive isotopes(Radioactive Chronometer)


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Carbon 14 formation, the diagram fromhttp://vcourseware.calstatela.edu/VirtualDating

Animals and plants in the

oceans and on land get their

carbon from carbon dioxidein the air. So, when these

creatures die is possible to

determine the amount of

Carbon 14 left in their

shells/bodies. C14 half life

equals 5730 years.
http://vcourseware.calstatela.edu/VirtualDatinghttp://vcourseware.calstatela.edu/VirtualDating


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Some heavy elements such as Uranium, Thorium-232, Rubidium-87,

Potassium-40 and Samarium-147are radioactive, emitting radiations.

Uraniumhas 92 protons + 146 neutrons. Thus its atomic mass =

238. In nature Uranium has two isotopes 235U and 238U.

Some of these isotopes are not stable emitting energy (radiations):

Gamma rays () (= x-rays)Alpha particles () Beta particles ()

So, these isotopes will become stable after a certain time,

transforming into another elements. This process is known as

Radioactivity. Rate of radioactivity is definite for each unstable

elements and commonly expressed as Half Life.


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Half Life= length of time that is required for

any unstable elements (isotopes) to be

diminished by half.------------How do we know it? Seefirst this diagram and table in the next slide.

Transforming of235U (unstable isotope) into 207Pb (stable element) with time (t)


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Parent IsotopeStable Daughter

ProductHalf-Life Values

Uranium-238 Lead-206 4.5 billion years

Uranium-235

Lead-207

704 million years

Thorium-232 Lead-208 14.0 billion years

Rubidium-87Strontium-87

48.8 billion years

Potassium-40 Argon-40 1.25 billion years

Samarium-147 Neodymium-143 106 billion years

Half life is now known for certain elements see the

following table:


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To know how to determine the half-life of radioactive

isotopes, follow the dating by Uranium/Lead Ratio:

The mathematical expression that relates radioactive decay to geologic

time is called the age equation and is:


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Example: By knowing the rate of disintegration of235U

into 207Pb in 1 yrs.

It is found that one g235

U1/7600 000 000 g207

Pb

Thus, in tyrs x 235U g/ 7600 000 000 = 207Pb

tyrs = x 7600 000 000

Ug

Pbg

When Lead/Uranium Ratio in Uraninite rock = 0.050

t(age of rock) = 0.050 x 7600 000 000 = 380 000 000 yrs

12 Age Dating of Earth

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