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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.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.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.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.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.
Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.
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.
Great uplift, accompanied by rapid erosion, is taking place and large sediment fans are being deposited in the Indian Ocean to the south.
Using this established record, geologists have been able to piece together events over the past 635 million years, or about one-eighth of Earth history, during which time useful fossils have been abundant.
The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute isotopes has been improved to the point that for rocks 3 billion years old geologically meaningful errors of less than ±1 million years can be obtained.It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced.