Radiometric Dating and The Kiowa Core

Everyone knows that some rocks are older than others. We know from paleontology that certain animals lived before others, stegosaurus lived before T. rex. If we find rocks with stegosaurus fossils in them next to rocks with T. rex fossils, we can usually say that the rocks with T. rex in them are younger. One of the problems facing geologists, though, is exactly how much younger are those rocks with the T. rex in them? In order to answer this question, we would need to figure out exactly how old, in years, the rocks with the T. rex and stegosaurus are. We do this by looking at the elements that make up the rocks that contain the fossils.

Certain elements found in rocks, like uranium, are radioactive. This means that they slowly disintegrate over time, changing from one material to another. Piece by very small piece individual atoms of uranium decay. As an atom of uranium loses a piece of itself, it changes into another element like lead. Luckily, for geologists, uranium, and other elements that are radioactive, lose pieces of themselves at a constant rate. That means we can measure how fast they decay and how fast they form new elements. Uranium happens to decay into lead. If we know how quickly uranium decays into lead and we can measure how much lead is in a sample of rock, we can find out how long it took for that lead to form and thereby the age of the rock. This technique can be used for many different elements. Potassium, a very common element in rocks, decays over time into the gas argon. If we measure the amount of argon in a sample of rock and measure how much potassium is left, we know how long it took for that argon to form and again the age of the rock.

What we actually measure in the age of a rock is how long ago, in years, that a certain element formed in that rock. In volcanic rocks, which can flow on the surface as lava, we measure when the lava cooled. For metamorphic rocks, which get buried deep below the surface, heated, and squeezed, we measure when this process stops. This raises an important point: If a piece of rock is formed and then heated at a later date, the reheating of the rock will reform the original elements. When we measure the age, we will only be able to tell when the rock was last heated, not necessarily the true age of the rock. It is very important to remember this when trying to date rocks.

Now that we know how to date rocks we need to ask why we would want to know how old a piece of rock is. Just like you like to remember important dates: your birthday, the year you got your drivers license; geologists like to remember important dates in the history of the earth. It is not only important to know what lived here but when it lived here. Denver used to be a tropical rainforest, we have fossil leaves to tell us that; but when was Denver a rainforest? If we know when Denver was a rainforest, and when it was not, we can begin to tell the story of Denver, not just make a statement about what Denver used to be like. And stories, as we all know, are much more interesting then statements.

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