Transcript “Half Life”.

Age-Dating of Earth ‫تقدير عمر األرض‬
1. 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.
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
Lithologic and faunal correlation of Lower Cretaceous rocks in northern Sinai, Egypt
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 ‫العمر المطلق‬
Early attempts for absolute age dating have used the following:
a) Rate of sedimentation
Total thickness of the stratigraphic column
Age = Yearly average of the sedimentation rate
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 Earth’s Age is averaging: 3 myrs to 1.6 byrs
Varved layers (clay and silt) deposit yearly in lakes. Also, organicrich 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
b) Salinity of the Ocean:
Age =
Total amount of salt (g) in ocean
Yearly average of salt (g) added to ocean wate r
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.
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.
d- Radioactive isotopes
(Radioactive Chronometer)
• This method based on the isotopes of the element.
• Atom consists of :  clouds of electrons
 Protons
 neutrons
–ve charge
+ve charge
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.
• 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
• 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”.
• 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)
Half life is now known for certain elements see the
following table:
Parent Isotope
Stable Daughter
Product
Half-Life Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Thorium-232
Rubidium-87
Lead-207
Lead-208
Strontium-87
704 million years
14.0 billion years
48.8 billion years
Potassium-40
Argon-40
1.25 billion years
Samarium-147
Neodymium-143
106 billion years
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 235U1/7600 000 000 g 207Pb
Thus, in t yrs x 235U g/ 7600 000 000 = 207Pb
t yrs =
Pbg x 7600 000 000
Ug
When Lead/Uranium Ratio in Uraninite rock = 0.050
t (age of rock) = 0.050 x 7600 000 000 = 380 000 000 yrs