Calendar dating systems
In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated.Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record.These units, called igneous rock, or magma in their molten form, constitute major crustal additions.By contrast, crustal destruction occurs at the margins of two colliding continents, as, for example, where the subcontinent of India is moving north over Asia.To achieve this precision, geochronologists have had to develop the ability to isolate certain high-quality minerals that can be shown to have remained closed to migration of the radioactive parent atoms they contain and the daughter atoms formed by radioactive decay over billions of years of geologic time.In addition, they have had to develop special techniques with which to dissolve these highly refractory minerals without contaminating the small amount (about one-billionth of a gram) of contained lead and uranium on which the age must be calculated.
This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the rocks or soil.
When these regions are later exposed in uptilted portions of ancient continents, a history of terrestrial rock-forming events can be deduced.
Episodes of global volcanic activity, rifting of continents, folding, and metamorphism are defined by absolute ages.
For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built.
Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence.