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About Time:
How Geologists Get Dates


When geologists find horizontal layers of rock, like in this photo of the Grand Canyon in Arizona, they can generally be sure that the upper formations are younger than the lower ones.

Photo by Tobias Alt



Click on the image to watch Part II of the three Sea Change videos, about dating samples in the lab


Before the 20th century, the early pioneers of geology had no way to determine the actual age of rocks and fossils. Researchers could make educated guesses about which deposits were younger and which were older, providing what they called a relative age. They knew that geological processes, like the accumulation of sediment or the deposition of lava, put new rock on top of old. But they couldn’t tell how fast new rock had been added. Moreover, they knew that wind and water sometimes erode rock away, leaving gaps in the geological record.  Sometimes researchers could find layers of rock with distinctive fossils that could be identified in many places. But such layers typically represented vast spans of time that could not be measured in years.

However, in the1940s, researchers began to perfect techniques for determining the actual age of rocks (subject to uncertainty that depends on the technique).  The earliest absolute dating techniques took advantage of the process by which some naturally occurring elements decay into lighter versions or other elements. The famous carbon-14 method works this way. There are three naturally occurring varieties, or isotopes, of the element carbon, each of which has a unique weight. One of them, carbon-14,  can be used to date organic materials such as wood. Carbon-14, which has 6 protons and 8 neutrons in its nucleus, decays at a known rate to nitrogen-14, which has 7 protons and 7 neutrons in its nucleus.  Living things, such as trees, make tissue out of carbon taken from carbon dioxide in the air. Scientists know what fraction of the air’s carbon dioxide is composed of carbon-14. After a tree dies no new carbon is added to its tissues. So any carbon-14 in the wood gradually decays to nitrogen-14, until none is left. By measuring the ratios of carbon isotopes scientists can determine the wood’s age (time since death).

Scientists subsequently developed other absolute dating methods that also rely on changing ratios of isotopes. The Pliomax team dates their shells using a method that involves measuring the abundance ratio of two isotopes of strontium, strontium-86 and strontium-87.

Learn more about the strontium dating method