Ma = millions of years, Ga = billions of years

A selection of significant events in the earth's geologic record.

Holocene epoch

• Humans develop agriculture and civilization
• Sea level rises rapidly as continental glaciers finish melting
• Mastodons and woolly mammoths and other large cold-climate mammals become extinct

Pleistocene epoch

• Large mammal species adapted to cold climates appear
• Widespread glaciation advances and retreats repeatedly in a cyclic pattern of global warming and cooling
• Homo sapiens appear

Pliocene epoch

• General global cooling trend continues

Miocene epoch

• Camels, horses and hippopotamus widespread, including in North America

Oligocene epoch

• Grasses become widespread

Eocene epoch

• Last "hot-house" global climate occurs, long-term overall cooling begins
• Indian subcontinent begins collision with Asia forming the Himalayan Mountains
• Mammals continue to diversify, ancestral horses appear

Paleocene epoch

• Mammal types begin to diversify and become widespread

Cretaceous period

• Period ends with large meteorite impact, demise of last dinosaur species
• Shallow inland ocean transgresses across entire North America except Rocky and Appalachian Mountains
• First flowering plants appear

Jurassic period

• Transgression of inland sea forms shallow marine conditions across interior North America
• Convergent plate boundary with subduction zone becomes established in western North America
• Pangaea finishes rifting into separate continents

Triassic period

• First mammals appear
• First dinosaurs appear
• Pangaea begins rifting into separate continents
• Deserts with sand dunes widespread western North America
• Several terranes that later accrete to North America exist in Tethyan area
• Single world ocean with a single large bay in Pangaea - the Tethys Sea
• Huge Pangaea landmass affects climate, causing widespread aridity

Permian period

• Greatest mass extinction in the fossil record occurs
• Relatives of mammals diverge from reptiles
• Pangaea finishes colliding and assembling
• Shallow seas cover much of western North America

Pennsylvanian period

• Widespread glaciation

Mississippian period

• First reptiles appear
• First dense forests with large primative trees appear

Devonian period

• First horsetails, clubmosses, and ferns appear
• First amphibians appear

Silurian period

• Insects appear on land
• Diverse fish types appear

Ordovician period

• Widespread glaciation for short interval
• First green plants on land appear

Cambrian period

• Several groups of animals appear, including arthropods and early ocean-dwelling vertebrates
• Ocean transgresses across much of interior and western North America, depositing sand, then mud, then lime as water gets deeper

Proterozoic eon

• Glaciation may have been widespread
• Multi-cellular organisms appear
• Single celled organisms, more complex than bacteria appear
• Mats of photosynthesizing bacteria widespread
• Atmosphere becomes oxygenated
• Continental cratons finish forming

Archean eon

• Continental cratons begin to form
• First fossil bacteria appear
• No free oxygen in the atmosphere
• Earliest crust forms
• Earth's mantle and core form

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Geology 101 - Introduction to Physical Geology
Basics Table--Geologic Time Scale
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
updated: 9/11/13

Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.

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