Absolute geologic age refers to how long ago a geologic event occurred or a rock formed, in numeric terms, such as 65.5 million years ago.
Some rocks and minerals can have their absolute age directly measured by analyzing the ratios of certain radioactive and non-radioactive isotopes they contain.
These ages have been derived from relative dating and absolute dating (radiometric dating) of rock layers and fossils.
With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?
Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.
The units commonly used for geologic age are mega-annum (Ma) for millions of years, giga-annum (Ga) for billions of years, and kiloannum (ka) ka for thousands of years.
Because these units are used according to the rules of the metric system, the M in Ma and the G in Ga must be capitalized, and the k in ka must not be capitalized.
This means that a quartz sandstone deposited 500 million years ago will look very similar to a quartz sandstone deposited 50 years ago.
Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments.The laws of physics and chemistry that governed geologic processes in the past are the same as those that govern processes now and in the future.The geologic timescale is a chronology (calendar) of events on Earth based on obtaining ages of past events.There are three main assumptions that must be made to accept radiometric dating methods.These must be accepted on faith in uniformitarian and naturalistic frameworks.Relative time places events or formations in order based on their position within the rock record relative to one another using six principles of relative dating.