Geologic Time ScaleGeologic Time Scale

Historical GeologyHistorical Geology

Geologic Time ScaleGeologic Time Scale

Geologic Time ScaleGeologic Time Scale►Developed in 19Developed in 19thth

and 20and 20thth Century Century►Used relative Used relative

age datingage dating Principle of

Superposition Faunal succession Angular

unconformities X-cutting

relationships►Absolute ages

came later

Piecing Together Time ScalePiecing Together Time Scale► Examined outcrops Examined outcrops

around the worldaround the world► Each time interval = Each time interval =

set of rocks and set of rocks and fossilsfossils

► Time scale is Time scale is piecemealpiecemeal– Unconformities played

major role– Unit boundaries placed

at unconformities– Thought

unconformities had widespread correlation

– Unconformities can be local

Geologic Time ScaleGeologic Time Scale

Scale is divided by EonsScale is divided by Eons Eras, Periods and EpochsEras, Periods and Epochs Eon is the largest division Proterozoic and

PreCambrian Eons Before life became

abundant 7/8 of all time

Phanerozoic Eon visible life Represents 1/8 of time

Precambrian EonPrecambrian Eon

► Eons subdivided into ErasEons subdivided into Eras– Hadean Era (beneath the earth)

– 4.5-4.0 Ga– Archean Era (ancient)

4.0-2.4 Ga– Proterozoic Era (former life)

– 2.5 Ga-543 Ma

Phanerozoic EonPhanerozoic Eon

► Phanerozoic =Visible lifePhanerozoic =Visible life

► Boundaries defined by massive extinction events Boundaries defined by massive extinction events – Paleozoic Era (ancient life)

Age of Fishes– Mesozoic Era (middle life)

Age of Dinosaurs– Cenozoic Era (recent life)

– Age of Mammals

Geologic Time ScaleGeologic Time Scale

► Eras divided into PeriodsEras divided into Periods► Boundaries due to less drastic faunal and floral Boundaries due to less drastic faunal and floral

changeschanges► Different Periods named after localities or rock typesDifferent Periods named after localities or rock types

– Triassic– Three-part division of rock types in Germany

– Cretaceous– “Creta” - Latin for chalk

– Jurassic For Jura Mountains

Geologic Time ScaleGeologic Time Scale Periods subdivided into Periods subdivided into

EpochsEpochs► Based on % of recent Based on % of recent

fossilsfossils– Eocene

1.5% fossils = recent species

– Miocene 20-40% fossils

– Pliocene 50-100% fossils

– Pleistocene Later introduced

Naming Time ScaleNaming Time Scale

► SedgwickSedgwick– Mapped from

bottom of greywackes (Northern Wales)

– Identified Cambrian Latin for Wales

Naming Time ScaleNaming Time ScaleSedgwick & MurchisonSedgwick & Murchison

► MurchisonMurchison– Mapped from top

of greywackes– Identified Silurian

Ancient Welsh tribe

Sedgwick & MurchisonSedgwick & MurchisonControversyControversy

► Sedgwick used rocks; Murchison used fossilsSedgwick used rocks; Murchison used fossils► Discovered both systems overlappedDiscovered both systems overlapped► Dispute resolved after both diedDispute resolved after both died

– Broke out Ordovician Ancient Celtic tribe

Geologic Time ScaleGeologic Time ScaleMemorizeMemorize

Resources for Learning Times Resources for Learning Times ScaleScale

► http://www.ux1.eiu.edu/~rbjorstad/geoltime.htmlhttp://www.ux1.eiu.edu/~rbjorstad/geoltime.htmlmnemonic for learning time scalemnemonic for learning time scale► http://www.geosci.ipfw.edu/gildner/TimeScalePractice.htmlhttp://www.geosci.ipfw.edu/gildner/TimeScalePractice.htmlpractice naming time scale, insert names in blank fields and practice naming time scale, insert names in blank fields and

check answerscheck answers► http://pubs.usgs.gov/gip/geotime/contents.htmlhttp://pubs.usgs.gov/gip/geotime/contents.htmlUSGS online publication on geologic timeUSGS online publication on geologic time► http://www.athro.com/geo/hgfr1.htmlhttp://www.athro.com/geo/hgfr1.htmlcomprehending geologic time in days, meters, kilometers, and comprehending geologic time in days, meters, kilometers, and

yearsyears► http://www.ucmp.berkeley.edu/help/timeform.htmlhttp://www.ucmp.berkeley.edu/help/timeform.htmlUC Berkeley clickable geologic time scale—learn about the UC Berkeley clickable geologic time scale—learn about the

different periodsdifferent periods