Geologic Time (FINISH THIS)

principle of uniformitarianism

• processes that act today are the same as processes that acted in the past

• basis of first law of modern geology

• “present is key to the past”

relative time

• knowing location of an event within a sequence of events

• ex: “this rock is older than this rock, but younger than this rock”

• based on stratigraphic principles

absolute time/numerical time

• knowing age of event in years based upon radioactive dating techniques

• multiple different radioactive dating techniques used to measure absolute time

relative dating - basic facts

• based on stratigraphic principles aka Principles (or Laws) of Stratigraphy

• knowing location of event within a sequence of events

  • location of the event is the age of the event

• very accurate, but not precise

• works best in sedimentary rock

  • easier to interpret because of their layers

principles of stratigraphy 

1. principle of lateral continuity - layers are laterally continuous in all directions until they either thin out, are cut off, or end at barrier to deposition

2. principle of original horizontality - layers are originally deposited horizontally

3. principle of superposition - in an undisturbed sequence of rock, the oldest rock is on bottom & youngest rock is on top

4. principle cross-cutting relationships - deformation events that cut across rocks are younger than rocks they cut across

5. principle of inclusions - inclusions within rock are older than the rock that surrounds them

• principle of faunal succession - fossils succeed each other in a reliable order based upon evolution of life

principle of faunal (fossil) succession

• fossils succeed one another in strata in a predictable order

• fossil - evidence of life preserved in rock

• index fossil

  • a fossil species that is short - lived and geographically

• fossil assemblage

  • pattern of fossils found together in rock

absolute dating/numerical - dating basic fa

• based upon radioactive decay

• ages are expressed in years

• half - life

  • time it takes for half of the parent isotope to decay into daughter product

• measuring ratio of amount of parent isotope to daughter product

• not always as accurate, but calculations are very precise

• works best in igneous rock