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 Mudcracks Fossilized Ripples Fossilized Mudcracks Modern Ripples

Dating – 2 types Relative Dating Absolute Dating Know order of events but not dates Absolute Dating 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 B C F D 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 A B Using the Principle of Inclusions, which picture shows the granite intrusion to be the youngest event? B

Conformable Beds Beds that have not been disturbed

Uncomformities Substantial break or gap in the rock record 3 types Angular Disconformity Nonconformity

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 Question: Answer: 1. Surface 1 is an example of ? Nonconformity 2. Surface 2 is an example of ? Angular unconformity 3. Surface 3, 4 and 5? Disconformity 4. Granite is _________ than the Schist. Younger

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 Trace fossils Bone, teeth, shells 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

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 Ages of fossils can be bracketed Fossils in another area can be correlated containing same fossils Age in another area determined indirectly Doing this repeatedly the geologic time scale established

Geologic Time Scale