Geologic Time Scale(Earth is 4.6 billion years old)

Geologic Time ScaleTime scale that outlines the development of

Earth and of life on EarthTime scale divisions based on major changes

in the Earth’s surface, climate, or types of organisms

Time scale divisions usually based on dominant life forms

Geologic Time UnitsEon

Largest unit of time4 eons

Hadean, Archean, Proterozoic, & PhanerozoicPrecambrian time

Hadean + Archean + Proterozoic Covers about first 4 billion years of the Earth

Geologic Time Units(continued)Eons broken into Eras

Phanerozoic Paleozoic, Mesozoic, & Cenozoic

Eras broken into PeriodsPeriods broken into EpochsEpochs can be broken into Ages

Eons

Eras

Periods

Epochs

Ages

Smallertime units

Geologic ColumnsAn ordered arrangement of rock layers

that is based on the relative ages of the rocks in which the oldest rocks are at the bottom

Layers are distinguished by the type of rock and types of fossils in the rock

X

X

XX

X

Precambrian Time4.6 Ga (Billion years) – 542 Ma (million years)88% of Earth’s historyLittle known about this timeRocks are deformed and altered by tectonic

activity

Precambrian Rocks & LifeRocks

Shields – large exposed Precambrian rocksNearly ½ of valuable mineral deposits found in

shieldsLife

Very few fossilsStromatolites – blue-green algae deposits

Paleozoic Era542 Ma – 251 MaLand masses started out apart but ended up

together (Pangaea)Rocks have lots of fossilsPlant & animal species increase dramatically

at beginning of era7 Periods

Cambrian PeriodMarine life formsWarm shallow seas cover much of the

continentsNo evidence of land dwelling plants or

animals

Ordovician PeriodPrimitive fishLots of invertebratesFirst vertebrates (mostly fish) appearNo plant life on land

Silurian PeriodVertebrate and invertebrate marine life

dominateLand plants & animals evolve near end of

period

Devonian Period“Age of Fishes”AmphibiansLand plants and seed bearing plants evolve

Carboniferous PeriodIncludes Mississippian and Pennsylvanian

PeriodsClimate was warm and humidForests and swamps responsible for coal

depositsAmphibians, fish, large insects

Permian PeriodAppalachian Mountains createdShallow inland seas disappear

Mass extinctionMany invertebrates die outReptiles & amphibians survive

Mass ExtinctionsUsed to indicate end of an eraPermian mass extinction

90% of marine organisms70% of land organisms

Plenty of resources for other species to begin to dominate

Mesozoic Era251 Ma – 65.5 MaPangaea breaks apart to form continentsMountain ranges form

Sierra Nevadas, Andes“Age of Reptiles” Includes Triassic, Jurassic, & Cretaceous

Periods

Triassic PeriodDinosaurs – mostly 16 feet longReptiles in the oceansAmmonites – Mesozoic index fossilFirst mammals appear

Jurassic PeriodDinosaurs become dominant life form2 major groups of dinosaurs

Saurischians – lizard hipped Herbivores & Carnivores

Ornithischians – bird hipped Herbivores

Flying reptilesFirst birds

Cretaceous PeriodDinosaurs still dominateT RexHorned and duck-billed dinosaursFirst flowering plants (angiosperms)Magnolias, willows, oaks, maples, walnuts

Cretaceous-Tertiary Mass ExtinctionC-T ExtinctionEnd of Cretaceous PeriodNo dinosaur fossils found in rocks formed

after Cretaceous PeriodDue to environmental changes from

continental movement or volcanic activity

C-T Extinction (continued)Impact hypothesis

Giant meteorite crashed into the EarthProduces dust that blocks the sunPlanet cools, plant life dies off, animals die offIridium near suspected impact site (Iridium is

common in meteorites, but not Earth rocks

Cenozoic Era65.5 Ma to present dayContinents move to present day positionAlps and Himalayas formIce agesMammals become dominant

Cenozoic Era (continued)Divided into 2 periods

Tertiary Period Time before the last Ice Age

Quaternary Period From last ice age to present day

Divided into 7 epochs

Cenozoic EpochsPaleocene & Eocene

Primates evolveWhales, flying squirrels, batsStill lots of small reptilesWorldwide temp drops about 4oC at end of

Eocene

Cenozoic Epochs (Continued)Oligocene

Uplifting of HimalayasWorld climate becomes much cooler & drierGrasslands favored

MioceneAntarctic ice caps begin to formMediterranean Sea fills and empties several

timesLarge land mammalsEarliest human ancestors(?)

Cenozoic Epochs (Continued)Pliocene

Dramatic climate changes Ice caps grow, water levels fall Alaska and Russia connected by Bering land bridge North and South America become connected Species migrate into different continents

Cenozoic Epochs (Continued)Pleistocene

Ice agesFossils of earliest modern humans

Holocene11500 years ago as last ice age endsSea level rises 140 mGreat Lakes take their shapeHumans develop agriculture and begin to use

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