Relative age dating

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Age-Dating of Earth ????? ??? ?????

• 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

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Lithologic and faunal correlation of Lower
Cretaceous rocks in northern Sinai, Egypt
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• Principles of inclusions and cross-cutting
  relationship
• The cutting rock is younger than the one that
  has been cut
• d) Paleomagnetic signature

• 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

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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
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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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d- Radioactive isotopes (Radioactive Chronometer)

• 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.

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• Animals and plants in the oceans and on land get
  their carbon from carbon dioxide in 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.

Carbon 14 formation, the diagram from
http//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.
• Uranium has 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? See first
  this diagram and table in the next slide.

Transforming of 235U (unstable isotope) into
207Pb (stable element) with time (t)
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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 of
235U into 207Pb in 1 yrs. It is found that one g
235U?1/7600 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