Relative age dating of rocks

The thin, dark part of each ring represents slow autumn and winter growth.Several other processes result in the accumulation of distinct yearly layers that can be used for dating.For example, an especially warm summer might result in a very thick layer of sediment deposited from the melting glacier.Thinner varves can indicate colder summers, because the glacier doesn’t melt as much and carry as much sediment into the lake.These thick layers alternate with thin, clay-rich layers deposited during the winter.The resulting layers, called varves, give scientists clues about past climate conditions.

In other words, you can use superposition to tell you that one rock layer is older than another.But determining the absolute age of a substance (its age in years) is a much greater challenge.To accomplish this, scientists use a variety of evidence, from tree rings to the amounts of radioactive materials in a rock.Another example of yearly layers is the deposition of sediments in lakes, especially the lakes that are located at the end of glaciers.Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment.Probably the most reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that 240 million years have passed since the appearance of the first animals with shells.

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