Age-Dating of Earth تقدير عمر الأرض. Relative age dating العمر النسبى 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.
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Age-Dating of Earth تقدير عمر الأرض
Lithologic and faunal correlation of Lower Cretaceous rocks in northern Sinai, Egypt
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.
b) Salinity of the Ocean:
When we use the above formula, we should assume that primitive oceans were not salty----Na Cl produced by weathering of the earth’s 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
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.
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.
d- Radioactive isotopes
Carbon 14 formation, the diagram from
Transforming of 235U (unstable isotope) into 207Pb (stable element) with time (t)
Half life is now known for certain elements see the following table:
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:
Example: By knowing the rate of disintegration of 235U into 207Pb in 1 yrs.
It is found that one g 235U1/7600 000 000 g 207Pb
Thus, in t yrs x 235U g/ 7600 000 000 = 207Pb
t yrs = x 7600 000 000
When Lead/Uranium Ratio in Uraninite rock = 0.050
t (age of rock) = 0.050 x 7600 000 000 = 380 000 000 yrs