Scientific Dating Methods

Unit 5

• Scientific dating methods of fossils and rock sequences are used to construct a chronology of Earth’s history expressed in the geologic time scale.

James Ussher (1581-1656)

• Archbishop of Armagh, Primate of all Ireland

• Constructed a chronology of human and Earth history in which he determined that Earth was only a few thousand years old.

• Catastrophism—Earth had been developed primarily by great catastrophes.

Relative Dating

• Placing rocks in their proper sequence of formation—which formed first, second, third, and so on

• Cannot tell us how long ago something took place, only that it followed one event and preceded another

Nicolaus Steno (1638-1686)

• Danish pioneer in geology

• Three defining principles of stratigraphy

Original Horizontality

• sedimentary layers are initially deposited in horizontal layers in a basin

• Forces break up rock into small pieces and these pieces wash down rivers to the ocean and settle on the seafloor

Rock Superposition

• Younger rock layers are on top of older rock layers

Lateral Continuity• Sediments are spread out

over continuous and extensive geographic areas

• They end by thinning out at the edge of the basin where they are deposited –or-

• They abruptly stop at a barrier –or-

• They grade into a different type of sediment

James Hutton

• Scottish physician and gentleman farmer

• Published Theory of the Earth

• Principle of Uniformitarianism—the earth was shaped by slow-moving forces still in operation today

Charles Lyell (1797-1875)

• British lawyer and geologist

• Popularized uniformitarianism

• Close and influential friend of Charles Darwin

Cross-Cutting Relationships

• Any event that cuts across an existing rock unit is younger than that unit

• Common events that can cut across existing rock units are unconformities, intrusions and faults.

Unconformities

• Angular Unconformity– Tilted or folded

sedimentary rocks are overlaid by younger, more flat-lying strata

– While layers were being set down, folding or tilting and erosion occurred

Unconformities

• Disconformity– Harder to identify– Strata on either side

are parallel

Unconformities

• Nonconformity– The break separates

older sedimentary metamorphic or intrusive igneous rocks from younger sedimentary strata