Dating while seperated
Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents.These units, called igneous rock, or magma in their molten form, constitute major crustal additions.The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils.
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.
For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built.
These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.
Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present.
In the ideal case, the geologist will discover a single rock unit with a unique collection of easily observed attributes called a marker horizon that can be found at widely spaced localities.
Any feature, including colour variations, textures, fossil content, mineralogy, or any unusual combinations of these can be used.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.In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated.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.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.