Cross cutting relationships used relative dating
Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.We could assume that this igneous intrusion must have happened after the formation of the strata.
Because the sedimentary rock had to have formed around the object for it to be encased within the layers, geologists can establish relative dates between the inclusions and the surrounding rock.Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.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?Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.Geologists use this type of method all the time to establish relative ages of rocks.
Now, what if instead of being horizontal, this rock layer was found in a tilted position?
Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.
Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes.
Try it risk-free Discover how geologists study the layers in sedimentary rock to establish relative age.
Learn how inclusions and unconformities can tell us stories about the geologic past.
If it had happened before the layers had formed, then we wouldn't see it punching through all the layers; we would only see it going through the layers that had existed at the time that it happened. The Principle of Cross-Cutting Relationships states that rock formations that cut across other rocks must be younger than the rocks that they cut across.