Transcript Geologic Time and Relative age Dating

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 Multi-celled
Organisms: 9:00 pm
How about the first Humans?
First single celled
organism: 4:00 pm
First Dinosaurs:
11:00 pm
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 1
Time passes, rocks are deformed
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!
Youngest layers
Oldest 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 H
Layer G
Layer F
Layer E
Layer D
Layer C
Layer B
Layer A
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 sedimentary
Rock layers
Nonconformity: Sedimentary rocks lie
On top of igneous or metamorphic rocks