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 ( ) 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 ( ) 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 ( ) 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