Geology by Jan Zalasiewicz

Author:Jan Zalasiewicz [Zalasiewicz, Jan]
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2018-05-04T00:00:00+00:00

Some fossils are more useful than others. The bones of Tyrannosaurus rex, magnificent as they are, are not terribly useful for biostratigraphy, being large, rare, fragile, and confined to part of north America. The really useful fossils are small, common, widespread, and rapidly evolving, like the graptolites of the Silurian or the ammonites of Jurassic and Cretaceous times. The microfossils are particularly useful, even though specialized and somewhat scary techniques have to be used to extract them from the rock, such as the use of powerful acids. However, as thousands of microfossils may be extracted from a small chip of rock, they can readily be obtained from the pounded-up ‘cuttings’ obtained when boreholes are drilled, a process in which larger fossils are obliterated.

There are now sets of biozones for each geological time period back to the beginning of the Cambrian Period, 541 million years ago, when easily visible fossils first became common. Their use extends to the present day, where the artefacts made by humans can be used as ‘technofossils’ to date very recent strata—such as in the precise dating of coastal cyclone deposits in Asia via the date stamps on the food-packaging litter that they contained.

The detailed subdivision of geological time by fossils has brought about enormous refinement of the Geological Time Scale. But there are other means of subdividing time in sedimentary rocks, such as by tracing changes in their chemical properties. These chemical changes in turn often relate to changes in climate or in environmental conditions on Earth—some as regular cycles, and other events that were haphazard, sudden, and catastrophic. By means of such interconnections, the timescale is evolving into an integrated history of the Earth.

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