(b). Geologic Time

Geologists and geomorphologists describe the Earth's geologic history through a temporal system known as the geologic time scale (Table 10b-1). On this scale, time is measured using the following four units of time: eons, eras, periods and epochs. All of these temporal subdivisions are established on the occurrence of some important geologic event. For example, Hadean Eon represents the time on Earth when life did not exist. During the Archean Eon life started and was dominated by one-celled prokaryotic life forms. Eukaryotic one-celled organisms became dominant in the Proterozoic Eon. Multicellular organisms ruled the planet during the eon known as the Phanerozoic.

Table 10b-1 describes some of the important geologic events that have occurred since the Earth's formation some 4.6 billion years ago.

 

Table 10b-1: Geologic time scale.

Eon	Era	Period	Epoch	Major Geologic Milestones
Phanerozoic	Cenozoic	Quaternary (0-1.6 million yrs BP)	Holocene (Present-10,000 yrs BP)	Modern humans develop. Pleistocene Ice Age Interglacial.
			Pleistocene (10,000 -1,600,000 yrs BP)	Pleistocene Ice Age. Extinction of many species of large mammals and birds.
		Tertiary	Pliocene (1.6-5.3 million yrs BP)	Development of hominid bipedalism. Cascade Mountains began forming. Climate cooling.
			Miocene (5.3-24 million yrs BP)	Chimpanzee and hominid lines evolve. Extensive glaciation in Southern Hemisphere. Climate cooling.
			Oligocene (24-37 million yrs BP)	Browsing mammals and many types of modern plants evolve. Creation of the Alps and Himalaya mountain chains. Volcanoes form in Rocky Mountains.
			Eocene (37-58 million yrs BP)	Primitive monkeys evolve and Himalayas began forming. Australian plate separates from Antarctica. Indian plate collides with Asia.
			Paleocene (58-65 million yrs BP)	Rats, mice, and squirrels evolve. Shallow continental seas become less widespread.
	Mesozoic	Cretaceous (65-144 million yrs BP)		First flowering plants, greatest dinosaur diversity, Cretaceous Mass Extinction (65 m BP), and Andes Mountains form. Africa and South America begin to separate. Climate cooling because of mountain building. Shallow seas have extensive distribution.
		Jurassic (144-208 million yrs BP)		First birds and mammals appear. Nevadian Mountains form. Large areas of the continents covered by shallow seas. Climate generally warm and stable with little seasonal or latitudinal variation. Shallow seas expanding.
		Triassic (208-245 million yrs BP)		First dinosaurs. Extensive deserts exist in continental interiors. Climate warm. Shallow seas limited in distribution.
	Paleozoic	Permian (245-286 million yrs BP)		Permian Mass Extinction. Reptiles become more diverse. Climate cold at beginning of the Permian then warms. Average elevation of landmasses at their highest shallow seas less extensive.
		Pennsylvanian (286-320 million yrs BP)		First reptiles appear.Winged insects evolve. Occasional glaciation in Southern Hemisphere.
		Mississippian (320-360 million yrs BP)		Primitive ferns and insects evolve. Forests appear and become dominant. Mountain building producing arid habitats in the interior of some continents.
		Devonian (360-408 million yrs BP)		First amphibians and trees appear. Appalachian Mountains form. Extinction of primitive vascular plants. Landmasses generally increasing in altitude. Climate cooling.
		Silurian (408-438 million yrs BP)		Major extinction event occurs. First land plants and insects. Continents are generally flat. Tectonic uplift begins.
		Ordovician (438-505 million yrs BP)		First fish and fungi. Greatest extent of shallow seas. Climate becoming warmer.
		Cambrian (505-551 million yrs BP)		Invertebrates become common. Fossilization of the Burgess Shale. Large areas of shallow seas near the equator. Climate was warm.
Proterozoic (551-2500 million yrs BP)	Also known as Precambrian			Eukaryotic cell organisms develop. First multicellular organisms. Changes in the lithosphere created major land masses and extensive shallow seas.
Archean (2500-3800 million yrs BP)				Slow development of the lithosphere, hydrosphere, and atmosphere. First single-celled prokaryotic organisms.
Hadean (3800-4600 million yrs BP)				Earth's oldest rocks come from the end of this Eon.

 

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