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Relative Age Dating
Transcript of Relative Age Dating
This is like you saying "I'm older than my sister, but younger than my brother."
* It does NOT tell us your exact age, but it does tell us how your age relates to others around you (your relative age)
Geologists can fill in gaps in the rock record by matching rock layers and fossils from separate locations. Geologists can correlate rocks by matching exposed rock layers in different locations.
We already know rocks can be correlated based on rock type and structure. They can also be correlated because of fossil evidence.
Relative Age Dating
Geologists are scientists who study Earth and rocks. They have developed a set of principles to compare the ages of rock layers.
Relative age is the age of rocks and geologic features compared with other rocks and features nearby.
5 Principles of Relative Age Dating
2. Original Horizontality
3. Lateral Continuity
5. Cross-cutting relationships
Principle that in undisturbed rock layers, the oldest rocks are on the bottom and the younger ones on top.
Most rock-forming materials are deposited in horizontal layers. Sometimes layers might be tilted or folded, but remember that all layers were originally deposited horizontally.
A piece of an older rock that becomes part of a new rock is called an inclusion. If one rock contains pieces of another rock, the rock containing the pieces is younger than pieces.
Principle that sediments are deposited in large, flat sheets. The layers continue in all directions until they thin out or meet a barrier.
For example, a river might erode the layers, but the order of the layers does not change.
If one geologic feature cuts across another feature, the feature it cuts across is older.
New rock layers can be deposited on top of old, eroded rock layers. An unconformity is a surface where rock has eroded away, and new rock layers over it.
They represent gaps in time because the eroded away rock is hard to trace back to a certain time.
There are 3 major types of unconformities.
Sedimentary layers are deposited on top of tilted or folded sedimentary layers that have been eroded.
Younger sedimentary layers are deposited on older igneous or metamorphic rock layers that have been eroded.
Younger, sedimentary layers are deposited on top of older, horizontal sedimentary layers that have been eroded.
If two or more rock formations contain fossils of about the same age, scientists can infer that the formations are also the same age.
Not all fossils are helpful. Index fossils are those that represent species that exited on Earth for a short length of time, were abundant, and inhabited many locations
Index Fossils Continued