1	Introduction to Geologic time
2	Geologic Time Absolute Age Geologic age of a fossil, rock, feature or event given in units of time, usually years Can be determined by radiometric dating Relative Age Geologic age of a fossil, rock, geologic feature, or event, defined relative to other fossils, rocks, features or events rather than in terms of years
3	Absolute Age We can pin a number on a rock or geologic event Use: Tree rings Isotopic dating radioactive isotopes that decay over time to a ‘daughter’ isotope, at a known rate
4	Absolute Age: Radiometric Dating Radioactive atoms are unstable and will emit radioactivity: a form of energy Half life: the time it takes for half of the nuclie in an isotope to decay 40K to 40Ar = half life ~ 1.3 billion years 238U to 206 Pb = half life ~ 4.5 billion years 14C to 14N = half life ~5,730 years
5	Relative Time How do we determine relative age? Based primarily on layered rock (sedimentary and volcanic) Use stratigraphy: a discipline of geology that looks at layered rocks and their relationship Principle of Uniformitarianism present is the key to the past Correlation
6	Relative Time Law of Superposition In an undeformed sequence of rocks, the oldest layer is at the bottom
7	Relative Time Principle of Original Horizontality Sediments are deposited in nearly horizontal layers. If they are tilted, folded or faulted, they have been tectonically shifted after deposition
8	Relative Time Principle of Cross-Cutting Relationships Faults or intrusions that disrupt other geologic units must be younger than those units A dike or batholith is younger than the surrounding rock A fault is younger than the rock it displaces Principle of Lateral Continuity Original layers of sediment extend horizontally until it tapers or thins
9	Relative Dating Additional time relationships: Inclusions The rock unit that is the source for inclusions must have existed to provide the material for inclusion – so it is older Contact metamorphism surrounding rock is “baked” by the intrusion, the rock was there before the granite
10	Unconformities No single place on earth has a continuous (conformable) rock record from the beginning of the earth
11	Types of Unconformities Disconformity An unconformity between beds that are parallel, & rock sequences are missing, due to erosion. Represents period of non-deposition or erosion. Disconformity often hard to distinguish from bedding planes
12	Types of Unconformities Angular Unconformity The younger underlying sedimenttary rocks are folded, tilted or faulted, and then eroded Followed by renewed sedimentation on top of the erosion surface Represents tectonic activity
13	Types of Unconformities Nonconformity erosion on plutonic or metamorphic rock is covered by sedimentary or volcanic rock Need period of uplift and erosion followed by deposition
14	Determining Relative Age by Correlation Correlation by: Match rock units of similar age from different regions and continents Correlation based on Physical Criteria Only good for local regions Physically tracing an outcrop (walking the outcrop) Where covered, finding distinctive minerals, or patterns of beds
15	Fossils Fossil Correlation Used to correlate over wider regions or between continents Index fossil - a species that had a short life span. Fossil assemblage- a group of fossils that lived at some period Geologic ages marked by dominance of particular fossil types – same sequence on all continents Faunal succession-stratigraphic ordering of fossils Fossils also used as environmental indicators Energy of depositional environment Shell thickness → location of ancient shorelines