Presentation on theme: "1.Relative age dating العمر النسبى It could be done by the following: a) Principle of superposition---Initial horizontality: In normal geologic section,"— Presentation transcript:
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 Age-Dating of Earth تقدير عمر الأرض
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 Age = Thickness of geologic column is averaging: to 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
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
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 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.
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 12 C, 13 C and 14 C. 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)
Carbon 14 formation, the diagram from 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.
Some heavy elements such as Uranium, Thorium-232, Rubidium-87, Potassium-40 and Samarium-147are radioactive, emitting radiations. Uranium has 92 protons neutrons. Thus its atomic mass = 238. In nature Uranium has two isotopes 235 U and 238 U. 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 235 U (unstable isotope) into 207 Pb (stable element) with time (t)
Parent Isotope Stable Daughter Product Half-Life Values Uranium-238Lead billion years Uranium-235 Lead million years Thorium-232Lead billion years Rubidium-87 Strontium billion years Potassium-40Argon billion years Samarium-147Neodymium billion years 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 235 U into 207 Pb in 1 yrs. It is found that one g 235 U 1/ g 207 Pb Thus, in t yrs x 235 U g/ = 207 Pb t yrs = x When Lead/Uranium Ratio in Uraninite rock = t (age of rock) = x = yrs