Upload
diana-paul
View
219
Download
5
Embed Size (px)
Citation preview
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