Relative dating quiz
These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.
We could assume that this igneous intrusion must have happened after the formation of the strata.Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. The forest layer is younger than the mud layer, right? When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history.The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock.Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.
We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.
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.
There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.
These interfaces between discontinuous layers of rock are called unconformities.
Geologists establish the age of rocks in two ways: numerical dating and relative dating.