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Copy of Relative Dating

Mr. Cruz's 8th Grade Earth History
by

Luis Cruz

on 25 October 2013

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Transcript of Copy of Relative Dating

Relative Dating: Age of an object/event in relation to other objects/events
In an undisturbed sequence of rock each layer is older than the one above
The younger layers are on top
The older layers are on the bottom
Father of Geology
Law of Superposition
Relative Dating vs. Absolute Dating and why it all matters.
Horizontality
States that sedimentary rocks are deposited in horizontal layers (Sheets)
Plus, if the layers are not horizontal, movements have occurred
Younger rocks can cause changes along their contacts with older rocks
James Hutton, father of Geology stated that the same processes that occur today have been constant over the geological past.

This geographic thought
is called uniformitarianism.
Younger
Older
But, wait. How do we know how old the fossil is in order to compare to the rock?
Faunal Succession
Fossils in layers of rock are in a definite sequence
Same fossils found in different rock outcrops are the same age
Absolute dating: using the radioactive decay of elements to find the precise age of a rock layer or fossil.
Geologist give a relative age to a layer of rock at one location.
Then give the same age to a matching layer of rock at other locations.

Using Fossils to date rocks
Index fossils are used to match rock layers.

To be an index fossil, it must be widely distributed and represent a type of organism that existed only briefly.

Matching Rock Layers
Ammonites
Evolved in shallow seas 500 mya

Extinct: 65mya
Group of hard shelled animals
Trilobites
Lived from 400 mya to 245 mya
Best-known and most studied fossils.
Exclusively marine animals.
Closest living relative is a horseshoe crab.
Fossil burrows and tracks have been found to show how they fed and hid.
Now that we know the age, we can match things up.
So, why does it all matter?
Full transcript