Outline 5: The Geologic Time Scale (Babcock text, p. 78) How it was created. A First Attempt, 1759 • Based on “appearance” of the rocks. • Latin terms: • Primary - crystalline rocks • Secondary - folded sedimentary rocks • Tertiary - undeformed sedimentary rocks • Quaternary - loose sediments Work in the 1800s • Based on relative ages of fossils rather than the appearance of rocks. • The Eras: (Greek terms) –Paleozoic: old life –Mesozoic: middle life –Cenozoic: recent life Work in the 1800s, cont’d • The periods of the eras were named after places or unique characteristics. • Cambrian - Latin for Wales • Ordovician - the Ordovices a Welsh tribe • Silurian - the Silures, a Welsh tribe • Devonian - Devonshire, England Work in the 1800s, cont’d • Carboniferous - carbon in coal –Mississippian: rocks in Mississippi River Valley –Pennsylvanian: coals in Pennsylvania • Permian - the Perm Basin in Russia Work in the 1800s, cont’d • Triassic - a 3-part division of rocks in Germany • Jurassic - the Jura Mountains of Switzerland • Cretaceous - Latin for chalks of Paris Basin Work in the 1800s, cont’d • The Cenozoic epochs have Greek roots. • Paleocene - ancient recent • Eocene - dawn of the recent • Oligocene - little recent • Miocene - less recent • Pliocene - more recent • Pleistocene - most recent • Holocene - completely recent Work in the 1800s, cont’d • What about really old rocks with no fossils? • Geologists didn’t know how to divide these rocks. • All were called Precambrian, an informal term we still use today. Work in the 1900s • Precambrian rocks were later divided into two eons: –Archean Eon: the beginning –Proterozoic Eon: earliest life • Younger rocks were grouped into the Phanerozoic Eon: visible life Work in the 1900s • Radiometric dating allowed the various eons, eras, periods, and epochs to be dated. • It turned out that the subdivisions are of very unequal duration. For instance, the Cretaceous Period at 79 MY is longer than the Cenozoic Era at 65 MY. Work in the 1900s, cont’d • Although the subdivisions are of unequal length, they still are very useful for talking about geologic time. • Geologists use the names of the Geologic Time Scale because it is a type of “shorthand” easier to remember than dates in years. A “Must Do” for You • Memorize the names of the Geologic Time Scale in your book. Don’t worry about dates in years. Do note that 88% of earth history is Precambrian. • Use the following pneumonic devices. Time Units • Eon –Era •Period –Epoch Eons • Phanerozoic • Proterozoic • Archean All Purpose Pill Phanerozoic Eras • Cenozoic • Mesozoic • Paleozoic Park My Car Phanerozoic Periods • Neogene • Paleogene • Cretaceous • Jurassic • Triassic • Permian • Carboniferous • Devonian • Silurian • Ordovician • Cambrian Can Ordinary Students Dupe Crabby Professors To Judge Class Projects Nicely? Cenozoic Epochs • Recent (Holocene) • Pleistocene • Pliocene • Miocene • Oligocene • Eocene • Paleocene People Eat Only Mario's Pan Pizza, Right? .