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How old is that rock? Relative dating of geologic material. This is NOT what is meant in geology when we use the term, “relative dating.” Uniformitarianism One of the most fundamental aspects of science is uniformitarianism. This is the idea that the processes that we can observe around us are likely the way the processes have happened in the past as well as the same processes that are happening in places that we cannot observe. For example... Uniformitarianism If I observe that a sandy beach in Virginia adds an average of 20 tons of sand per year per mile of beach... Uniformitarianism I can predict that a sandy beach in Africa probably adds close to 20 tons of sand per mile per year. Uniformitarianism I can also predict that 65 million years ago, sandy beaches probably grew at a similar rate. Uniformitarianism If the world spins at a rate causing a ~24 hour day today... Uniformitarianism A million years ago, the Earth probably had a day of ~24 hours. Uniformitarianism If light on Earth travels at ~3x108 m/s today... Uniformitarianism It probably moves at the same speed in the rest of the universe. Uniformitarianism If hydrogen has all of the properties of hydrogen every time we test it... Uniformitarianism Then all hydrogen in the universe probably acts/acted the same way. Original Horizontality Since we observe that sediments today are deposited horizontally, we infer that all sediments on the Earth probably started off horizontally. Superposition Since we observe that new sediments are deposited from deposition onto the top layers of the Earth, we infer that the lower the layer in the geological record, the older it is. Superposition Superposition doesn’t tell us how old things are, but it does tell us which layers are older than the others. Intrusions and Inclusions Intrusions and Inclusions As we have discussed, the Earth is not a static thing. Once crust is laid down, lots of things can change it. When magma pushes its way through the layers, it’s called intrusion. Intrusions and Inclusions As we have discussed, the Earth is not a static thing. Once crust is laid down, lots of things can change it. When magma pushes its way through the layers, it’s called intrusion. Intrusions are YOUNGER than the rock they are intruding upon. Intrusions and Inclusions Due to weathering and erosion, sometimes old rock can break off and mix into new rock. The rock inside the new rock is older and called an inclusion. Unconformities Erosion, weathering, deposition, and other movement of rocks on the surface of the Earth can make reading the geological record confusing. These big changes are different types of unconformities. Unconformities So in short, an unconformity is a mix up in the geologic record due to changes in the Earth.