Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface.
When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events.
Since parent uranium atoms change into daughter atoms with time at a known rate, their relative abundance leads directly to the absolute age of the host mineral.
Just as the use of the fossil record has allowed a precise definition of geologic processes in approximately the past 600 million years, absolute ages allow correlations back to Earth’s oldest known rocks formed more than 4 billion years ago.
The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere.By mid-century the fossiliferous strata of Europe had been grouped into systems arrayed in chronological order.The stratigraphic column, a composite of these systems, was pieced together from exposures in different regions by application of the principles…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.Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.