Geologic Time

Earth’s History

Uniformitarianism

The present is the key to the past

Processes that happen today are the same processes that happened in ancient times

Modern RipplesFossilized Ripples Modern MudcracksFossilized Mudcracks

Dating – 2 types

• Relative Dating

• Absolute Dating

– Know order of events but not dates

–Know exact age

Relative Dating – Key Concepts

• Law of Superposition

• Principle of Original Horizontality

• Principle of Cross-Cutting Relationships

• Principle of Inclusions

Law of Superposition

Each bed is younger than the bed below it.

Principle of Original Horizontality

Sedimentary layers are deposited horizontally

Principle of Cross-Cutting Relationships

• Faults and intrusions are younger than the rocks they cut through

Inclusions

• Fragments of one rock unit that have been enclosed within another are older

Relative Dating

Youngest G A

BCFD

Oldest E

E is younger than D. Which Principle used?

Law of Superposition

G is younger than F. Which Principle is used?

Principle of Cross-Cutting Relationships

Principle of Inclusion

Using the Principle of Inclusions, which picture shows the granite intrusion to be the youngest event?

A B

B

Conformable Beds

• Beds that have not been disturbed

Uncomformities

– Angular

– Disconformity

– Nonconformity

• Substantial break or gap in the rock record

• 3 types

Angular Unconformity

• Older layers have been deformed, eroded, and younger layers deposited above

Disconformity

• Beds are parallel relative to each other but there is a break in sequence

• Implies a cessation of sedimentation or erosion, but no tilting

• Hard to recognize

Nonconformity

Sedimentary layers overlie igneous or metamorphic layers

Unconformities

1. Surface 1 is an example of ? Nonconformity2. Surface 2 is an example of ? Angular unconformity3. Surface 3, 4 and 5? Disconformity4. Granite is _________ than the Schist. Younger

Question: Answer:

Correlating Rocks

Matching rocks of the same age in different locations

How do we correlate?

Walking out the contact Matching fossils

Fossils

The remains or traces of prehistoric life

Types of Fossils

Body fossils– Bone, teeth, shells

Trace fossils– Record movement

Fossilization

Hard parts– Unaltered

Fossilization - Petrified

Turned into stone

Petrified Forest, Arizona

Fossilization - Replacement

Minerals replace organic matter

Fossilization – Molds & Casts

Mold – impression or cavity

Cast – deposited material that fills cavity

Mold Cast

Fossilization - Amber

Hardened Resin

Conditions favoring Preservation

Rapid burial Possession of hard parts

Dies Buried Eroded

Fossils and Correlation

William Smith

– Each rock formation contained fossils unlike the ones above & below

– Sedimentary strata in widely separated areas could be identified & correlated by their fossil content

Principle of Fossil Succession

Fossils occur in a definite, invariable sequence in the geologic record

– Apply with Law of Superposition and can document evolution of life through time

Index Fossils

Widespread geographically Limited to a short span of geologic time

Correlating Rocks & Fossils

Igneous rocks are dated radiometrically

Age in another area determined indirectly

Ages of fossils can be bracketed

Fossils in another area can be correlated containing same fossils

Doing this repeatedly the geologic time scale established

Geologic Time Scale