ABSOLUTE AGE OF ROCKS

 ABSOLUTE AGE OF ROCKS:

Relative Time: "older or younger" according to their relative position in stratigraphic succession.

Absolute Time: In numerical figure; like millions of years.

∙Geochronology: Radioactive decay by Henry Becquerel.

Most common element used in geochronology are :

ThoriUm(Th)

Uranium(U)

Potassium(K)

Rubidium(Rb)

∙Thorium, Uranium decay to form stable lead isotopes with expulsion of alpha particles.

²³²Th(atomic no=90),²³⁸U(atomic no=92)

∙ Minerals that contain more than 1% Th and U; Uraninite, Monzonite, Zircon and allinite are used for radiometric dating.

∙ Zircon happens to be one of the important minerals for radiometric dating since it is quite resistant to subsequent change that a rock may undergo after it's formation.

∙ The K- Ar method is also very versatile as it is applicable for dating all types igneous, metamorphic, sedimentary rocks that contain minerals such as mica, glauconite and illite.

Loss of Argon(Ar): Low apparent age of the rock and during subsequent thermal event(metamorphism);clocks have been reset and the ages give the time of metamorphism.

Accumulation of Argon(Ar): High anomalous ages.

∙Rb - Sr method is also very widely used because of common occurrence of Rb as a trace element in alkali feldspar and mica.

∙Radiocarbon method is used for determining the ages of very young rock which were formed less than 60Ka before present.

∙Radioactive methods are applicable mainly for dating igneous rocks(igneous rocks don't contain fossils)

∙Radiometric ages of these igneous rocks are indirectly used for determining the stratigraphic age of sedimentary rocks.

Thanks for reading 

-Ichbinaasif