learning objective how do geologists interpret earth’s history, and how does uniformitarianism...
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• LEARNING OBJECTIVE How do geologists interpret Earth’s history, and how does uniformitarianism help explain Earth’s
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UNIT 6 GEOLOGIC TIMEA. Geological time scaleRecord of life forms & geological events in Earth’s
history.Scientists developed the time scale by studying rock
layers & fossils world wide. 1. Relative age: is the age of compared to other
rocks or events.
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• 2. Age of the Earth.• 4.6 billion • 3. Catastrophism-earth’s history dominated by violent events.3. Uniformitarianism-geologic processes have
operated uniformly from origin of Earth to present time. The present is key to the past.
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• B. Age of The Earth• 1. Calculation from the Bible-(John Lightfoot,
Bishop Ussher) 4000-6000 B.C. • 2. Salt in ocean and thickness of sediments gives
ages of 100 million years, but rates are variable.• 3. Age of Sun-the Sun was believed to be burning
chemical reactions gave age of 10,000 years.
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• 1. Relative Age of rock or event as compared to other rocks or events.
• a. Principle of Original Horizontality-sediments are deposited in horizontal layers that parallel to surface on which they were deposited.
• Tilted or folded layers indicate that the crust has been deformed.
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• b. Principle of superposition-in series of undisturbed layers, oldest layer on bottom/each overlying layer is progressively younger/ youngest layer on top.
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• c. Evidence of Events • Igneous intrusions/extrusions, faults, joints, folds
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• d. Correlation techniques• Process of matching rocks /geologic events in
one location to rocks and events in another location.
• Continuity of rocks
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Fossil evidence in rocksIndex fossils lived over large areas for relatively shortperiods of time.
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• 2. Absolute age• a. Radioactive decay sequence acts as atomic
clock.• Radioactive isotopes decay (change) into
daughter isotopes at known rates. Parent isotopes emits subatomic particles.
• Half-life-amount of time needed for one-half of a radioactive parent to decay into doughter isotopes.
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neutrons protrons
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Five Radioactive Isotope PairsFive Radioactive Isotope Pairs
Half-LifeEffective Minerals and
Isotopes of ParentDating Range
Rocks That Can Parent Daughter
(Years)Be Dated
Uranium 238 Lead 206 4.5 billion 10 million to Zircon 4.6 billion UraniniteUranium 235 Lead 207 704 million Thorium 232 Lead 208 14 billion 48.8 billion
Rubidium 87 Strontium 87 4.6 billion 10 million to
Muscovite
Biotite
Potassium feldspar
Whole metamorphic
or igneous rock
Potassium 40 Argon 40 1.3 billion 100,000 to Glauconite 4.6 billion Muscovite Biotite Hornblende Whole volcanic rock
(Years)
4.6 billion
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Carbon 14 dating can be used to date fossils up to 75,000 years. For older fossils, some isotopes can be used to date sedimentary rock layers above and below the fossil.
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Carbon 14 CycleCarbon 14 Cycle
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3. Geologic Time Scale
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