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Geologic Time Earth’s History The present is the key to the past Uniformitarianism Processes that happen today are the same processes that happened in ancient times Fossilized Ripples Mudcracks Modern Ripples Modern Mudcracks Dating – 2 types • Relative 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 Oldest G A B C F D 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 1. 2. 3. 4. Question: Surface 1 is an example of ? Surface 2 is an example of ? Surface 3, 4 and 5? Granite is _________ than the Schist. Answer: Nonconformity Angular unconformity Disconformity 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 – Record movement Bone, teeth, shells Fossilization Hard parts – Unaltered Fossilization - Petrified Turned into stone Petrified Forest, Arizona Fossilization - Replacement Minerals replace organic matter Fossilization – Molds & Casts Mold Cast Mold – impression or cavity Cast – deposited material that fills cavity Fossilization - Amber Hardened Resin Conditions favoring Preservation Dies Rapid burial Possession of hard parts 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