Dating archaeological materials
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.A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself.Dating, in geology, determining a chronology or calendar of events in the history of Earth, using to a large degree the evidence of organic evolution in the sedimentary rocks accumulated through geologic time in marine and continental environments.To date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques.Continents move, carried on huge slabs, or plates, of dense rock about 100 km (62 miles) thick over a low-friction, partially melted zone (the asthenosphere) below.In the oceans, new seafloor, created at the globe-circling oceanic ridges, moves away, cools, and sinks back into the mantle in what are known as subduction zones (i.e., long, narrow belts at which one plate descends beneath another).
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.The results suggest that the present-day global tectonic scheme was operative in the distant past as well.plate tectonics has had a profound impact on the scientific understanding of our dynamic planet.It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced.In addition, because sediment deposition is not continuous and much rock material has been removed by erosion, the fossil record from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.
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Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated.