Geological Time Scale

The history of the earth has been divided into a geological time scale consisting of eons, eras, periods, and epochs. Major periods generally correspond to significant changes in the earth's climate or fossil record (e.g. appearance or extinction of various life forms). The following table is a summary of times (mya = millions of years ago) and some significant events. Time estimates are "start times" based on USGS (1/1999) information - different sources vary somewhat for dates.

	Hadean Time			4600 mya	Formation of the earth. Age estimate based on asteroid and moon rock analysis and assuming the earth was formed about the same time.
I.	Archean Eon
	Early			3800 mya	Appearance of prokaryotic cells (e.g. bacteria). Photosynthetic cyanobacteria (stromatolites) appear about 3600 mya. Atmosphere is mostly carbon dioxide, water vapor, sulphur dioxide, and nitrogen. Small "continents" form and split due to movements from higher heat of earth.
	Middle			3400 mya	Landmasses grow in size forming continental cratons.
	Late			2800 mya	Continental landmasses formed from early rock including gneiss, granite, greenstone, komatiites, and various sedimentary rocks. Mineral deposits formed including copper, zinc, nickel, and gold.
II.	Proterozoic Eon
	Early			2500 mya	Landmasses begin to form relatively stable proto-continents. Appearance of eukaryotic (containing nuclear membrane) cells about 2000 mya. Photosynthetic activity increases atmosphere oxygen concentration. With increased oxygen, a UV blocking ozone layer begins to form. Quarz-rich sandstone, shale, and limestone (a photosynthetic product) were deposited on continents. Rock and mineral deposit formation included hematite (iron oxide), chromium, lead, uranium, and gold. About 1700 mya iron-containing rock oxidized by atmosphereic oxygen appears.
	Middle			1600 mya	Stromatolites and Protozoa (single celled animals) dominate.
	Late			900 mya	Appearance of algae. Stromatolites begin to decline.
		1.	Vendian	650 mya	Appearance of multicellular Ediacara (soft-body, wormlike animals).
III.	Phanerozoic Eon
	A. Paleozoic Era
		1.	Cambrian	544 mya	Appearance of various marine invertebrates (e.g. sponges, worms, 'moss animals'), animals with mineral skeletons ('armored jawless fish', trilobites, primitive mollusks), and starfish. Plants were only marine including seaweed. A supercontinent in the southern hemisphere called Rodinia splits. The landmass of proto-North America was called Laurentia and was centered near the equator. Two other continents split were Baltica (northern Europe) and Siberia (central Asia). A large remaining portion was called Gondwana and was made up of parts of present-day Africa, S. America, southern Europe, Middle East, India, Australia, and Anarctica.
		2.	Ordovician	505 mya	Appearance of vertebrates (primitive fish), chordates, corals, clams and primitive land plants. Continental movements (Laurentia, Baltica, Siberia) volcanic eruptions, and earthquakes create mountains (e.g. proto - Urals, Norwegian, Appalachians). Gondwana moves to the South Pole. Extinction of ~ 85 % of species (~ 438 mya).
		3.	Silurian	440 mya	Appearance of fish with jaws, coral reefs, air-breathing scorpions and vascular leafless land plants. The supercontinent of Laurussia formed from Laurentia, Baltica, and Siberia.
		4.	Devonian	410 mya	Various forms of fish and mollusks evolve. Appearance of ammonites, sharks, lungfish, land-living vertebrates (tetrapods), and wingless insects. Plants include mosses, horsetails, ferns and woody plants. Seed-ferns appear. Extinction of ~ 82 % of species (~ 367 mya).
		5.	Carboniferous Mississippian	360 mya	Appearance of fern forests. Climate was tropical forming thick peat layers which eventually became layers of sedimentary rock containing bands of coal. Siberia splits fron Laurussia and drifts north.
			Carboniferous Pennsylvanian	325 mya	Insects (including dragonflies) and amphibians flourish. Appearance of land snails, reptiles and seed plants (e.g. ginko trees, conifers). Laurussia and Gondwana move towards each other.
		6.	Permian	286 mya	Reptiles spread and evolve. Laurussia and Gondwana (along with the other proto-continents) combine to form the supercontinent of Pangea. In the process, ancestral North America is pushed north off the equator and up against Siberia creating mountains (Ural, Appalachian, etc). Extinction of ~ 96 % of species (~ 245 mya).
	B. Mesozoic Era
		1.	Triassic	248 mya	Appearance of dinosaurs (including winged lizards) and earliest mammals. Ginkos, conifers, and palms (cycads) dominate. Extinction of ~ 76 % of species (~ 208 mya).
		2.	Jurassic	213 mya	Appearance of birds. Pangea starts to split between proto-North America and the adjacent lands of South America and Africa forming the initial stages of the Gulf of Mexico and the Atlantic. The split results in creating Laurasia in the north. The remaining "Gondwana" moves south of the equator and includes present-day Australia, Anarctica, India, Africa, and S. America. An eastern block (Eurasia) of Laurasia begins turning clockwise (south). A unit including Australia and Anarctica splits.
		3.	Cretaceous	145 mya	Appearance of primates (e.g. marsupials), deciduous trees (e.g. oak, maple) and flowering plants. Also appearing are snakes and meat-eating crustaceans (shrimp, crab, lobster). North America splits off from Laurasia. The western movement creates the Rocky Mountains. India splits and joins Laurasia creating the proto-Himalayas. Africa and S. America separate. Extinction of ~ 76 % of species including dinosaurs (~ 65 mya).
	C. Cenozoic Era
		1.	Tertiary
			a. Paleocene	65 mya	Early mammals, primates, rodents, and insectivores evolve. Large herbivores and carnivores appear. Greenland separates from North America. Australia starts to separate from Anarctica.
			b. Eocene	55.5 mya	Appearance of bats, early horses, cattle, pigs, deer, monkeys, whales, early cats and dogs, modern birds, etc. Gaps between Greenland / Europe and Australia / Anarctica widen.
			c. Oligocene	33.7 mya	Appearance of modern mammalian groups including early mastodons and apes. Himalayas and Alps are further uplifted
			d. Miocene	23.8 mya	Appearance of grasses. Mastodons and other mammals evolve. Mountain uplifting continues.
			e. Pliocene	5.3 mya	Grazing hoofed animals spread and diversify. Early humans appear in Africa. Grasslands dominate. Climate starts to become cooler.
		2.	Quaternary
			a. Pleistocene	1.8 mya	Abundance of large mammals (e.g. buffalo, elephants, mammoths, mastodons). Humans evolve. Ice age begins. Warming begins at the end of the epoch to melt glaciers. Mammoths and mastodons become extinct.
			b. Holocene	8000 ya	Appearance of human civilizations.
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