The Grand Canyon - Geologic History
• Canyon = young, the rocks = old
• Canyon = 1 to 5 million years
• Rocks record over 2 bill. yrs.
Geologic Time - Deep Time
• Geologic Time Scale
• Calendar of Earth History
• Units of Time
– Eon, era, period
– Hierarchal
– First occurrence of life forms
• Frameworks of Time
• Relative Time
– Sequence of events
• Numerical Time
Geologic Time - Relative
• Relative Dating = sequence of events through time: establishing order
in with rocks are deposited and subsequently deformed and altered.......…
• Based on simple principles
Superposition
Cross-cutting
Fossil succession
Geologic Time - Relative
• Superposition
– Oldest layers on the bottom => younger layers near the top
• Cross Cutting
– Anything cutting through layered rocks = younger than everything it
cuts
Geologic Time - Relative
• Fossil succession - fossils appear in a recognizable order in the rock
record
• Index Fossils
– Short lived, common, widespread
– Fossils that lived during specific periods of time
Geologic Time - Numerical
• Assigning an actual date (in years) to the rock
• Radiometric Dating
• Decay of radioactive isotopes
• Decay occurs at constant rate - leaves a record
• Parent isotope - decays to - Daughter isotope
Geologic Time - Numerical
• Radioactive decay occurs at a constant, logarithmic rate.
• half life = the time it takes for 1/2 of the remaining parent atoms
to decay to daughters......
• Measure ratio of parent/daughter - known 1/2 life - time
• Done on igneous rocks
Geologic Time - Numerical
• How do we apply numerical time to layers of sedimentary rock?