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