the key to the pastrelative time vs. absolute time

relative timeorder of events or objects from first (oldest) to last (youngest)she is older than he is; she was born first and he was born last

age of events or objects expressed numericallyshe is twenty-one and he is nineteen

absolute time

relative age dating conceptsoriginal horizontalityall sediment originally deposited in water formed

in horizontal layers

sediments will settleto bottom

and blanketthe sea floor

relative age dating concepts

superpositionwithin a sequence of undisturbed rocks, the oldest rocks are at the bottom and youngest at the top

….young upward…

oldest

youngest

relative age dating conceptscross-cutting relationshipsa disrupted pattern is older thanthe cause of the disruptione.g. an intrusion is youngerthan the rocks it intrudes

relative age dating conceptsinclusionsfragments of other rocks contained in a body of rockmust be older than the host rocke.g.

1) xenoliths in granite are olderthan granite and2) pieces of rock inconglomerate are older than conglomerate

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relative age dating conceptsunconformities• A break in the geologic record, a gap in time• Created when rock is eroded or when sediment is not deposited• If rock was consistently deposited, and never eroded, we would be able to see all rocks from all time periods (all 4.6 billion years)

relative age dating conceptsdifferent types of unconformitiesangular unconformity• found between horizontal layers of sedimentary rock and layers that have been tilted• contacts between formations may represent significant amounts of time

angular unconformity

angular unconformityfrom: http://www.uakron.edu/envstudies/parks/rmgcan2.html

angular unconformity

relative age dating conceptsdifferent types of unconformitiesdisconformity• contact separates sedimentary layers that are parallel• missing time is difficult to recognize (may need otherinformation)

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relative age dating conceptsdifferent types of unconformitiesnonconformity• horizontal sedimentary layers lie on top eroded igneous or metamorphic rock

• large gap in geologic record

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what events occur?

angular unconformity

what events occur?

nonconformity