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A Lesson on DatingGeolog
ic
How old is old?
How do you date a rock?
• Absolute Dating– Analogous to your
age . . . i.e. an exact number
– Often found using the radioactive decay of certain elements
• Relative Dating– Dating with regard to
an order of events . . . analogous to knowing that you are the middle child
– Found by examining the local geology and discovering what happened before and after
Who would want to date a rock?
Three simple rules for dating rocks:
• Law of Original Horizontality– i.e. if you dump out a bunch of sand, it won’t stack
up vertically . . .
• Law of Cross-Cutting Relationships– In order to cut something, something has to be
there before you cut it
• Law of Superposition– If you are making a sandwich, you can’t put the
top on first . . .
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