Principles of relative dating of geologic events
-Correlation by fossils- same fossil or fossil assemblage in widely separated rocks are same age. age of rock = number of half lives gone by X half life of isotope pair used How old is the Earth?The shorter the range of the fossil, the more accurate the correlation. -oldest rocks dated are from Greenland, Minnesota, etc.Numerical dating determines the actual ages of rocks through the study of radioactive decay.Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.Now imagine that you come upon a formation like this: What do you think of it? How can you make any conclusions about rock layers that make such a crazy arrangement?
It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness.
-Position in a sequence of rock- more than 1 layer is similar in rocks from different places.
-Key bed- distinctive or unusual bed present in different areas (coal bed, volcanic ash layer).
Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.
Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.