• LEARNING OBJECTIVE How do geologists interpret Earth’s history, and how does uniformitarianism help explain Earth’s

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.

• 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.

• 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.

• 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.

• b. Principle of superposition-in series of undisturbed layers, oldest layer on bottom/each overlying layer is progressively younger/ youngest layer on top.

• c. Evidence of Events • Igneous intrusions/extrusions, faults, joints, folds

• d. Correlation techniques• Process of matching rocks /geologic events in

one location to rocks and events in another location.

• Continuity of rocks

Fossil evidence in rocksIndex fossils lived over large areas for relatively shortperiods of time.

• 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.

neutrons protrons

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

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.

Carbon 14 CycleCarbon 14 Cycle

3. Geologic Time Scale