Principles of relative dating of geologic events
Let's look at these rock strata here: We have five layers total.
Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.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?Numerical dating determines the actual ages of rocks through the study of radioactive decay.Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.
Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.
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.
-Guide fossil (index fossil)- fossil with a short range (lived for a short period of time). and are ~3.9 BY old -meteorites and some moon rocks, however, dated at ~4.6 BY; believed to have formed at same time as Earth; age of Earth, therefore, believed to be ~4.6 BY.
-Assemblage range zones- use more than 1 fossil in rock layer; area of age overlap is age (figs. Relative geologic time scale- early geologists constructed based on correlation of rocks from all over world (mostly by fossils); only relative geologic time periods, no actual dates (fig. Absolute Dating Specific age given in number of years before present.
-Correlation by fossils- same fossil or fossil assemblage in widely separated rocks are same age. age of rock = number of half lives gone by X half life of isotope pair used How old is the Earth?