How do Geologists determine the age of rocks and what are half-lifes?

Question

I mean how do Geologists determine the age of Rocks or fossils?

What is a half life?

Answer 1

Basically two methods:

1. Relative dating - if a sedimentary rock is lying on top of another rock, then the younger rock will be on top and the older rock will be below - provided it hasn't been overturned. Various cross-cutting relationships such as dikes, diapirs, and unconformities can be used as well. This is only good to determine which rock is younger, it won't give you an age in years. This method is widely used in sedimentary rocks such as sandstone and shale, and can be supported by index fossils.

2. Absolute dating - Certain radioisotopes decay at known rates, and can be used to provide an age in years. In general this works best on igneous rocks such as granite and basalt. This is not carbon dating, although it works on the same principle.

Fossils always have relative dates, unless there is some way to date their host rocks radiometrically.

Half-life is the time which half of a radioactive isotope decays.  For example, the half life of uranium 238 is 4.5 billion years, roughly the same age as the earth.

If you need more details, just enter the two basic dating methods in the magic little search box on the top of this page.