Geologic Time Scale Historical Geology
Geologic Time Scale
Geologic Time Scale Used relative age dating Developed in 19th and 20th Century Used relative age dating Principle of Superposition Faunal succession Angular unconformities X-cutting relationships Absolute ages came later
Piecing Together Time Scale Examined outcrops around the world Each time interval = set of rocks and fossils Time scale is piecemeal Unconformities played major role Unit boundaries placed at unconformities Thought unconformities had widespread correlation Unconformities can be local
Geologic Time Scale Scale is divided by Eons Eras, 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 Eon Eons 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 Eon Phanerozoic =Visible life 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 Scale Jurassic Eras divided into Periods Boundaries due to less drastic faunal and floral changes Different 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 Scale Periods subdivided into Epochs Based on % of recent fossils Eocene 1.5% fossils = recent species Miocene 20-40% fossils Pliocene 50-100% fossils Pleistocene Later introduced
Naming Time Scale
Naming Time Scale Sedgwick & Murchison Mapped from bottom of greywackes (Northern Wales) Identified Cambrian Latin for Wales Murchison Mapped from top of greywackes Identified Silurian Ancient Welsh tribe
Sedgwick & Murchison Controversy Sedgwick used rocks; Murchison used fossils Discovered both systems overlapped Dispute resolved after both died Broke out Ordovician Ancient Celtic tribe
Geologic Time Scale Memorize
Resources for Learning Times Scale http://www.ux1.eiu.edu/~rbjorstad/geoltime.html mnemonic for learning time scale http://www.geosci.ipfw.edu/gildner/TimeScalePractice.html practice naming time scale, insert names in blank fields and check answers http://pubs.usgs.gov/gip/geotime/contents.html USGS online publication on geologic time http://www.athro.com/geo/hgfr1.html comprehending geologic time in days, meters, kilometers, and years http://www.ucmp.berkeley.edu/help/timeform.html UC Berkeley clickable geologic time scale—learn about the different periods