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Modesdating

Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence.

By mid-century the fossiliferous strata of Europe had been grouped into systems arrayed in chronological order.The stratigraphic column, a composite of these systems, was pieced together from exposures in different regions by application of the principles…Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.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.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.

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.

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.

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.