2 Geologic Time Geologic time scale Relative dating principles Absolute dating w/ radioactivityCorrelation of rock layers
3 Relative Geologic Time Scale The relative geologic time scale has a sequence ofeonserasperiodsepochsbut no numbers indicating how long ago each of these times occurred
4 0-66.4 m.y.a 66.4-245 m.y.a 245-570 m.y.a Geologic time scale Eons Phanerozoic - last 570 million yearsCenozoic - recent lifeMesozoic - age of middle lifePaleozoic - ancient lifePrecambrian - from birth of Earth up to beforecomplex life forms developedErasm.y.a(mammals, humans)m.y.a(dinosaurs, 1st scrawny mammals)m.y.a(fish, trilobites, clams, corals, ferns )(algae, bacteria, some fossils without shells like jellyfish)
6 “Telling time geologically” Relative Dating: Key principles“Telling time geologically”Earth’s history concealed in rocksGoal of geology: unraveling Earth’s historyPrinciple time keeping devices:Relative dating - putting rocks/events in proper orderAbsolute dating - determining event’s actual time
7 Relative Dating: Key principles A bed of rock is older than that above, younger than that belowLaw of superpositionPrinciple of original horizontalityPrinciple of cross-cutting relationshipsInclusionsUnconformities
9 Relative Dating: Key principles Law of superpositionPrinciple of original horizontalityPrinciple of cross-cutting relationshipsInclusionsUnconformitiesA bed of rock is older than that above, younger than that belowSedimentary layers deposited flat
10 Relative Dating: Key principles Law of superpositionPrinciple of original horizontalityPrinciple of cross-cutting relationshipsInclusionsUnconformitiesA bed of rock is older than that above, younger than that belowSedimentary layers deposited flatFaults & dikes are younger than beds they cross
11 Relative Dating: Key principles InclusionsUnconformitiesFragments of one rock enclosed in another
13 Relative Dating: Key principles InclusionsUnconformitiesFragments of one rock enclosed in anotherBreak in time in deposition• An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.(Conformable layers: deposited w/out interruption)
14 Unconformities 3 types Angular unconformity Disconformity Relative Dating: Key principles3 typesAngular unconformityDisconformityNonconformity• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.• A disconformity is when two sedimentary rock layers are separated by an erosional surface.• A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.
21 Relative Dating Principles To view this animation, click “View” and then “Slide Show” on the top navigation bar.
22 Physical criteria Fossils Correlation of rock layers Matching up rocks from different placesthat are similar in agePhysical criteriaFossils
23 position of a bed in sequence of beds distinct/uncommon minerals Correlation of rock layersPhysical criteria (OK for short distances)position of a bed in sequence of bedsdistinct/uncommon minerals
24 “principle of fossil succession” Correlation of rock layersFossilsfossil organisms succeed each other insystematic fashionany time period can be determined byfossil content“principle of fossil succession”
25 “Index fossils” geographically wide-spread short span of time Correlation of rock layers“Index fossils”geographically wide-spreadshort span of timeT&L Figure 8.10
30 Chemistry review electrons nucleus Absolute dating w/ radioactivity Atommodelelectrons# protrons = atomic #,defines the elementnucleus# neutrons can vary:“isotopes”protronsneutrons
31 # of atoms Parent Daughter Now 1000 0 Next year 500 500 Absolute dating w/ radioactivityExample: an isotope with a half life of 1 year# of atomsParent DaughterNowNext year2 yrs from now3 yrs from now10 yrs from now
32 Radioactive DecayTo view this animation, click “View” and then “Slide Show” on the top navigation bar.