Last time, we learned about stratigraphy and the law of superposition. Now the students are going to use this information to come up with a reasonable interpretation of the age of fossils. Imagine that we already know the age of a specific layer of sediments in the Turkana Basin. According to the law of superposition, fossils found above this layer should be younger than that age and fossils found below should be older. But how do we know the exact age of that specific layer in the first place? Thankfully, volcanic activity has occurred periodically in East Africa for about the last 30 million years and, has allowed us to use some powerful dating tools in geology.
One of the most important methods of geologic dating in the Turkana Basin is radiometric dating, including K/Ar and Ar/Ar dating. Just like radiocarbon dating, these methods look at how much of the unstable isotopes have decayed over time and therefore, provide a precise age for a specific event. The event in this case, is a volcanic eruption. The rifting activity in East Africa created thin areas in the Earth’s crust. As a result, volcanic eruptions were relatively frequent, which shot out layers of ash that were transported and deposited in the Turkana Basin. Buried with the volcanic ash are minerals that contain radioactive Potassium and its decay product Argon gas, which are the targets of radiometric dating.