Relative Dating Earth Science 2015

What is it? Relative dating is the process of identifying the relative ages of the layers of the Earth. The goal of relative dating is to identify which layers of Earth are older or younger compared to the layers around them. However, scientists do NOT use this process to provide an absolute date. It is only used to figure out the order in which the geologic events occurred.

How can we do this? The layers of rock in the crust are thousands to hundreds of thousands to million years old. How are scientists able to figure out which are older or younger? There are some rules and relationships we’ve figured out, but the most important one is this: “Processes we observe happening today happened the same way in the past.” We call this idea uniformitarianism. Without this idea, nothing else we discuss today would be valid.

The Laws: Law of Superposition Sedimentary rock forms when sediment collects on the surface, gets covered up by more sediment on top and then slowly gets compacted and cemented. So that means if it’s close to the surface, it probably only got there recently. The stuff way down near the bottom has been there for a long time. This is the Law of Superposition. In a cross-section (or sample of layers), layers of rock farther away from the surface are the oldest. Layers near the top are the youngest.

The Laws: Principle of Original Horizontality Layers of the Earth form in horizontal layers. That’s how gravity works. So if the layers of rock being analyzed aren’t parallel with the surface, then the event that made them not be horizontal happened after they were deposited. We can still figure out which are older by looking at how they’re angled and the farthest from the surface is the oldest.

The Laws: Principle of Cross-Cutting Relationships Start with the same idea: sedimentary rock gets deposited horizontally and the rock near the top is young, while rock near the bottom is old. What if some magma comes up through those layers and leaves an intrusion of igneous rock behind. That lava tube is younger, because the rock had to be there for it to intrude through it! Same principle is applied if the rock has a fault (crack) running through it. If the rock is cracked, it had to be there for it to crack!

Unconformities Rocks are constantly changing. When this happens, it leaves a gap in the relative dating record. We call these unconformities. If rock formation in an area stops, the top layers start getting weathered and eroded. If rock formation starts back up again afterwards, new horizontal layers will be deposited, but now there’s a lot of information missing!

Types of unconformities Disconformity: Everything is still horizontal, but there is an uneven layer in between two of the layers. The uneven layer is the disconformity. Angular unconformity: Layers become angled, then the exposed rock gets eroded away, and then horizontal layers are deposited on top. Nonconformity: An igneous or metamorphic intrusion reaches the surface, becomes eroded and a layer of newer sedimentary is deposited on top.