Geology is like forensic work; it requires time, patience, and the right tools. To the layperson, rock sedimentary layers sometimes seem to blur together. The geologist uses common sense to determine how each layer is related to the other layers, what it is made of, and how it got there. Then, using this data, the geologist begins the tedious process of reconstructing a history for the region.
Superposition is one of those geology terms with a fancy name that is really just based on common sense. It is a principle that points out how sediments are deposited in nature. You cannot deposit a new layer of sediment below an existing layer. So the new layer must be younger than the existing layer. Applied to several layers in a rock sequence, you can tell the relative age of layers in relation to each other.
If you want to know more specifically the age of a layer rather than just whether it is younger or older than other layers, you need more advanced tools. In our interview, Dr. Quade described two of them: radiometric dating and paleomagnetic dating.
Radiometric dating relies on the fact that some chemical elements (the parents) decay into different chemical elements (the daughters). Some of these decay rates are constant. If you know how much of the parent existed in a sample at some start point and can measure the amount of daughter element that has been created, you can apply the decay rate and calculate the age of the sample.
Paleomagnetic dating, “in which you just look at the orientation of magnetic grains within the deposits”, says Dr. Quade, is another important tool. “You look at the pattern of reversals – we’ll call it reversals and normals – because in the past the Earth’s magnetic field has reversed in orientation, where literally there’s certain periods in geologic time if you were to go back and take a compass with you, you’d notice that your compass would point the opposite direction. North is south and south is north. We call them magnetic flip flops in the geologic record and they happen, major flip flops, on the order of every 500,000 to 800,000 years. The duration of these flip flops is uneven and so you can use this pattern. When you end up measuring a lot of geologic sections what it ends up looking like is a bit like a magnetic stripe like in a grocery store, where you have periods where for a long time it is reversed, for short times it is reversed and it is this pattern that you match to […] you just take that and match it to a master chronology.”
When you discover a fossil of a hominid, you want to know when the individual died to give an idea of when it lived. Superposition is a way to find out where the fossil fits in relative to the layers of sediments that surround it. Fossils from different species may also be useful for relative dating, especially since the fossils of some species are more numerous than others. Dr. Quade noted that fossil pigs are especially useful. “They are really good time markers because they evolve very quickly, they change quickly in the record, and so you can use the changes, particularly in the teeth morphology, to date deposits, broadly speaking.”
The image to the left is a molar from a fossil pig (Metritiocheourus compactus) that has been dated to be about 1.6 million years old. If you find another molar with similar characteristics in a section at another site, you may be able to conclude that the layer in each section match in age.
Geologists rely on relative dating using superposition, absolute dating using radiometric dating and paleomagnetic dating, and the availability of other tools of time such as fossil pigs to help reconstruct a chronology of events. Without these tools, the origins of humanity and the history of the planet Earth might forever be shrouded in mystery.