Geologic Time

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 facts

• 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

Nonconformity

• sedimentary rock is deposited on igneous or metamorphic rock

• unconformity surface is erosional surface between sedimentary rock & the other two types of rock

disconformity

• most difficult to identify without additional information (typically fossils)

• missing time can occur because of erosion or nondeposition

angular unconformity

younger sedimentary layers are horizontal and lie above layers of older sedimentary rock that are tilted

Radioactive Decay

Parent is the starting isotope

Daughter is isotope after radioactive decay

Half-Life: time it takes for half of parent isotope to decay to daughter isotope

How to use half-lives to find age of rock

• need an unweathered sample of a mineral

• chemically analyze mineral for parent isotope & its daughter

• calculate percentage of each

• use graph to determine number of half-lives

• look up reference what half-life is for parent & daughter

• multiply number of half-lives by the value of the half-life

Correlation

• determination of time equivalent rocks

• stratigraphic correlation

  • primarily based on patterns within & between sequences

• lithostratigraphic correlation

  • primarily based on physical continuity & similarity of rock types

• biostratigraphic correlation

  • based on fossils

• chronostratigraphic correlation

  • primarily based on facies relationships

Geologic Time Scale

• geologists need way to compare age of rocks world wide

• based upon changes in life on Earth as evident in changes of fossil assemblages

Phanerozoic History of Earth

• Precambrian

  • name used for combining 3 oldest eons

• Paleozoic

  • mostly marine life with & without hard parts

  • development of vertebrates, land plants, insects, fish, amphibians, reptiles

  • Pangea being assembled

• Mesozoic

  • reptiles dominate the land

  • Pangea

• Cenozoic

  • mammals dominate on land

  • breaking of Pangea