geologic time and relative dating. geologic time geologic time—beer goggles for geologists – ex....
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Geologic Time and Relative Dating
Geologic Time
• Geologic time—beer goggles for geologists– Ex. geologically recent
events
• Earth is 4.6 billion years old– The passage of this
amount of time is difficult for humans to fathom
– Let’s think about it in a different way…
Earth Forms:noon
First single celledorganism: 4:00 pm
First Multi-celledOrganisms: 9:00 pm
First Dinosaurs:11:00 pm
How about the first Humans? 30 seconds before midnight
Geologic Time Continued…
• Geologists have two ways of measuring geologic time– Relative age dating: Refers only to the order of
events; does not assign a specific ages– Absolute age dating: Assigning an “exact” age to
something
Relative Dating
• Using one or more of the following principles to determine the order in which things (rocks) have changed over time:
Principle of Original Horizontality
• Sediment is deposited in nearly horizontal layers
Folded Rock Example
How in the world did this happen?
We start with horizontal layers…
Time passes, rocks are deformed
Time 1
Time 2
Erosion removes top of fold
Time 3
Principle of Superposition
• In an undisturbed sequence of rock, the youngest rocks are always at the top and the oldest on the bottom– Ex. Cake!
Oldest layers
Youngest layers
Principle of Crosscutting Relationships
• A rock must exist before something can happen to it– Picture ex: Order of
events?
• Here’s a geological example…
Layer A
Layer B
Layer C
Layer D
Layer E
Layer F
Layer G
Layer H
Layers A-H were deposited; dike c cuts Through layers A-H; dike B cuts dike C; Dike A cuts Dike B. Layer A is the oldest,Dike A is the youngest unit.
Principle of Inclusions
• An inclusion is a piece of one rock unit contained in another– Think about the game
“Operation”
Evolution and Fossils
• Evolution in a nutshell: – Critters change with
time—successive generations adapting to their environments
• Fossil = remains/traces of ancient critters preserved in rock
How Are Fossils Useful?
• Different animals and plants have lived at different times in the past– Ex: Dinosaurs, wicked-big insects
• Different fossils will appear and disappear in the same order that organisms evolved and died out—Law of Faunal Succession
• Rock correlation
Conformity and Unconformity
• Erosion may cause rock record to be incomplete– unconformity
• There are several types of unconformities:
Angular Unconformity: Horizontal Sedimentary rock layers over tilted rock layers
Disconformity: Erosion surface Separating horizontal sedimentaryRock layers
Nonconformity: Sedimentary rocks lie On top of igneous or metamorphic rocks