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.
In this lesson, we'll learn a few basic principles of stratigraphic succession and see whether we can find relative dates for those strange strata we found in the Grand Canyon.
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?
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Free 5-day trial Discover how geologists study the layers in sedimentary rock to establish relative age.
Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.
Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.
Throughout the history of life, different organisms have appeared, flourished and become extinct.
Many of these organisms have left their remains as fossils in sedimentary rocks.
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.