stratigraphy and geological time

principle of superposition

if rocks are undisturbed the youngest are on the top and the oldest are on the bottom

stratigraphy

sedimentary layers (strata) show a history of earths events

principle of original horizontality

if sed rock layers are tilted it is due to something that happened after deposition

principle of cross cutting relations

cross cutting feature (dyke, fault etc) is younger than surrounding rock

principle of inclusions

when a rock has smaller fragments in it the fragments are older than the surrounding rock

principle of fossil succession

younger fossils are generally on top of older fossils. layers with similar fossils are a similar age.

unconformities

large missing parts of the rock record between strata (erosional surfaces)

angular unconformity

an erosional surface between sedimentary layers with different orientations (ones below have been uplifted and tilted prior to erosion and ones above have not)

disconformity

erosional surface between sedimentary layers that are in the same orientation (hardest to see)

nonconformity

erosional surface between rock types that are not the same (sedimentary over metamorphic/igneous)

radioactive isotopes

decay at predictable rates parent→daughter and can be used for absolute dating

isotopes

same # of protons, dif # of neutrons. isotope number = atomic mass (protons + neutrons)

decay curve

represents % of parent/daughter we expect to find at a given time graphically.