Radiocarbon dating of volcanic rocks

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.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.

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.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).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.

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