Relative age dating principles
We could assume that this igneous intrusion must have happened after the formation of the strata.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.
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.Now, what if instead of being horizontal, this rock layer was found in a tilted position?What could a geologist say about that section of rock?Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock strata.Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. sediments, which are deposited and compacted in one place over time.
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.
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Free 5-day trial Discover how geologists study the layers in sedimentary rock to establish relative age.
The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.
How do we use the Law of Superposition to establish relative dates?
Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock.