Geo - Geologic Time

A please

principal of cross-cutting

faults, dikes, erosion, etc. must be younger than the material that is faulted

ex. volcano feeder dike cannot intrude rocks that aren’t there yet

principal of baked contacts

igneous intrusion cooks the host rock

ex. melted chocolate on cookie melts the cookie and replaces it

principal of inclusions

inclusions are always older than the enclosing materials

ex. animal falls into ice before it freezes, so animal is older than ice

principal of fossil succession

if the same fossils are present in different rock units, then they’re the same age

unconformity

time gap in rock record due to non-deposition or erosion

angular unconformity

horizontal sedimentary strata overlies tilted strata

siccar point scotland

nonconformity

igneous or metamorphic rocks are capped by sedimentary rocks

disconformity

unconformity within sedimentary layers due to interruption in sedimentation

formations

rock units unique enough to be mapped over a region

lithologic correlation

matching rock types

eons

hundreds to thousands of Ma (mega-annum/million years)

eras

subdivision of eons (65 to hundreds of Ma)

periods

subdivisions of eras (2 to 70 Ma)

Epochs

subdivisions of periods (0.011 to 22 Ma)