Understanding Geologic Time. How old is it? How do we know? Absolute dating – process of assigning...
-
Upload
sabina-powell -
Category
Documents
-
view
222 -
download
2
Transcript of Understanding Geologic Time. How old is it? How do we know? Absolute dating – process of assigning...
How old is it? How do we know?
• Absolute dating – process of assigning a precise numerical age to an organism, object or event
• Relative dating – process of placing objects or events in sequence (first, second, third…)
Uniformitarianism
• James Hutton proposed that laws of nature operate today as they did in the past (take along time)
– Mountain building
– Erosion
– Sediment deposition and lithification to rocks
Principles of Relative Dating
• Original horizontality – sediments are deposited in layers, oldest on the bottom.
• Overlapping features – (cross cutting) if a rock or fault cuts across a rock layer it must be younger than what it cuts across.
• Unconformities – gaps in the rock record where erosion destroys “time” or deposition of new rock does not occur.
Fossils
• Remains or traces of organisms found in the rock record (usually SEDIMENTARY rocks)
– Correlation – matching fossils/rocks in one area with those in another (if B follows A in all places A is younger than B)
– Index fossil – evidence of organisms that lived over a large area but for a short period of time
Geologic Time Scale
• History of Earth as evidenced by fossils found in the rock record, boundaries determined by:
– Catastrophic geological events
– Major environmental changes
– Extinction and explosion of new life (change in fossils)
Absolute Dating
• Some minerals in IGNEOUS rocks are radioactive and decay in predictable ways (half life)
• Comparison of isotopes for the ‘parent’ and ‘daughter’ atoms provide a numerical “age”