Geologic time – 12 monthsYour birth occurred about 1/10 of a second before midnight!
Why do we care about time?Many important geologic problems can only beunderstood by studying the rate or frequency atwhich a process occurs.-How fast do mountains grow/erode? (L/t)-How frequently do earthquakes strike orvolcanoes erupt? (N/t)-Relative timing of different events can alsoprovide insight in to cause & effect.
Hypothesis 1:Continental rifting causesvolcanism.Hypothesis 2:Volcanism triggers continentalrifting.Consider two hypotheses proposed to explain anobserved association between volcanism andcontinental rifting:Would we expect the relative timing of rifting andvolcanism to differ for these two hypotheses?
Relative vs. Absolute AgesRelative Age:the occurrence of one event/rock relative to anotherWhich event happened first?Absolute Age:the age of an event/rock in years in the past (Ka, Ma,Ga)When did something happen?Put a numberon it!
1.Principle of SuperpositionIn a sequence of undisturbed layered rocks, theoldest rocks are on the bottom.2.Principle of Original HorizontalityLayered strata are deposited under the influenceof gravity as horizontal or nearly horizontal ornearlyparallel to the Earth’s surface.Principles of StratigraphyStratigraphy:The study of rock strata, especially the distribution, deposition,and age of sedimentary rocks.
Relative Ages and StratigraphyThe lower left sequence records a history of-deposition-tilting-erosion-more deposition.If left undisturbed by tectonicprocesses, younger rocks areat the top.Sediments are deposited inhorizontal layers and slowly changeinto sedimentary rocks:Marble Canyon, Grand Canyon
Sedimentary Formations•Aformationis a collection of sedimentary layers that exhibitsimilar physical properties and contain the same assemblage offossils over an extensive area—using fossils as recorders ofgeological time.•Useful for correlating across areas without exposures orbetween drill holes.