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.
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.
Geologists establish the age of rocks in two ways: numerical dating and relative dating.
Numerical dating determines the actual ages of rocks through the study of radioactive decay.
These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.
The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.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.The same idea applies to fault lines that slide rock layers apart from each other; a fault that cuts across a set of strata must have occurred after the formation of that set.We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.How do we use the Law of Superposition to establish relative dates?