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Geologic Time
Eon, main division of geologic time. Eons are the largest units that geologists use to
measure the age of the earth. Eons are divided into eras. Each era has a number of
periods, each of which is divided into epochs. The eons of the earth are the Archean
(from the formation of the earth to 2.5 billion years before present), the Proterozoic
(2.5 billion to 570 million years before present), and the Phanerozoic (570 million
years before present to present). The Archean and Proterozoic Eons are often called
Precambrian time.
The Archean (Greek for “ancient”) is the earliest eon in the geological record of the
planet. It began when the planet first cooled—about 4.5 to 4.6 billion years before
present. The oldest dated rocks on the earth are about 4 billion years old, but rocks
gathered on the moon and models of the formation of the earth help geologists
estimate that the earth is really about half a billion years older than the oldest rocks so
far recorded on it. Geologists define the end of the Archean Eon as 2.5 billion years
before present. The continents, oceans, and atmosphere formed during this eon.
Archean rocks form the ancient cores of all of the present-day continents. The first life
on the earth appeared during the Archean. Earth scientists have found microscopic
fossils of simple, single-celled organisms that are at least 3.8 billion years old.
The Proterozoic (Greek for “earlier”) Eon is the second eon of the earth’s history,
starting 2.5 billion years before present and lasting to 570 million years before present.
Proterozoic rocks contain abundant life. Early in the Proterozoic, complex cells called
eukaryotes and photosynthesizing blue-green algae called stromatolites appeared. By
the end of the eon, diverse and advanced organisms began appearing. The oxygen
content of the atmosphere increased greatly during the Proterozoic, gradually
changing the world’s surface and paving the way for the variety of animals that
emerged in the Phanerozoic.
The Phanerozoic (Greek for “appearance of life”) Eon is the youngest and shortest
eon in the earth’s history. The eon began 570 million years before present and
continues to the present. Phanerozoic rocks occur on all continents. The fossils in
Phanerozoic rocks illustrate a progression of life from early marine organisms to
terrestrial plants and animals, ending with the wide variety of species on the earth
today.
The fossil record also shows several major extinctions, in which a large percentage of
the number of species on the earth died out and were replaced by new species.
Asteroids crashing into the earth probably caused some of the extinctions, while
others may have resulted from unknown terrestrial forces. Rocks of the Phanerozoic
reveal that our present-day continents are continuously moving. Phanerozoic rocks
are responsible for providing all of our fossil fuels, most sources of groundwater, many
building supplies, and many commercial mineral products.
Contributed By:
Alan V. Morgan1
1"Eon," Microsoft® Encarta® 98 Encyclopedia. © 1993-1997 Microsoft Corporation. All
rights reserved.
Geologic Dating Techniques
The two radioactive decay sequences most useful to geologists are the decay of
carbon-14 into nitrogen-14 and the decay of potassium-40 into argon-40. Carbon-14,
or radiocarbon, dating works for organic materials less than about 50,000 years old.
Geologists measure the amounts of carbon-14 and nitrogen-14 in wood, charcoal,
fabric, paper, fossil seeds and insect remains, shell materials, or even water
containing dissolved carbon. The ratio of carbon-14 to nitrogen-14 provides a good
estimate of the sample’s age.
Geologists can use potassium-argon dating to determine ages of rocks from about
100,000 years old to as old as the earth itself. The ratio of poatassium-40 to argon-40
provides a good estimate of the age of a rock as long as the rock has not been heated
to temperatures above 125° C (257° F). Heat allows argon to escape, making the rock
appear older than it actually is.
A few non-radiometric techniques, such as varve analysis, dendrochronology, and
paleomagnetism, also provide absolute ages. Varves are layers of sediment deposited
yearly in glacial lakes. A thick layer of coarse sediment deposited during the spring by
glacial runoff and a thinner layer of fine sediment that settles out during the calmer
winter make up each varve. Earth scientists extract cores of sediment from these
glacial lakes and can count back the number of years since a certain geologic event
took place.
Dendrochronology is a technique that uses the annual rings of trees in temperate
climates to estimate a tree’s age. Some trees can live for thousands of years, so this
technique is useful in dating trees from 3000 to 4000 years before present, but it has
also been used in fossil trees from further back in Holocene time.
Paleomagnetism involves measuring the angle of magnetic molecules in rocks. When
lava is hot, magnetic minerals in the molten rock orient themselves to the earth’s
magnetic field. As the temperature of cooling lava drops to a certain point, these tiny
magnets lock into place. Because the earth’s magnetic field has switched orientations
several times during the history of the earth, the magnetic orientation of the rocks that
cooled during different times may be different. Scientists know the dates of the
magnetic reversals, so the magnetic orientation of a rock sample can provide an
estimate of its age.
Contributed By:
Alan V. Morgan2
2"Geologic Time," Microsoft® Encarta® 98 Encyclopedia. © 1993-1997 Microsoft
Corporation. All rights reserved.
Geologic Time Scale.
Eon
Era
Period
Cenozoic
Quaternary
(0-1.6 mya)
Mesozoic
Paleozoic
Phanerozoic
Tertiary
Epoch
Holocene
(Present-10,000 yrs BP)
Hadean
(3800-4600 mya)
Modern humans develop. Pleistocene Ice Age Interglacial.
Pleistocene
Pleistocene Ice Age. Extinction of many species of large mammals
(10,000 -1,600,000 yrs BP) and birds.
Pliocene
(1.6-5.3 mya)
Development of hominid bipedalism. Cascade Mountains began
forming. Climate cooling.
Miocene
(5.3-24 mya)
Chimpanzee and hominid lines evolve. Extensive glaciation in
Southern Hemisphere. Climate cooling.
Oligocene
(24-37 mya)
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 mya)
Primitive monkeys evolve and Himalayas began forming. Australian
plate separates from Antarctica. Indian plate collides with Asia.
Paleocene
(58-65 mya)
Rats, mice, and squirrels evolve. Shallow continental seas become
less widespread.
Cretaceous
(65-144 mya)
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 mya)
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 mya)
First dinosaurs. Extensive deserts exist in continental interiors.
Climate warm. Shallow seas limited in distribution. Pangea begins to
break apart 235 mya
Permian
(245-286 mya)
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. All of the
major continents assembled into a giant landmass called Pangea
Pennsylvanian
(286-320 mya)
First reptiles appear.Winged insects evolve. Occasional glaciation in
Southern Hemisphere.
Mississippian
(320-360 mya)
Primitive ferns and insects evolve. Forests appear and become
dominant. Mountain building producing arid habitats in the interior
of some continents.
Devonian
(360-408 mya)
First amphibians and trees appear. Appalachian Mountains form.
Extinction of primitive vascular plants. Landmasses generally
increasing in altitude. Climate cooling.
Silurian
(408-438 mya)
Major extinction event occurs. First land plants and insects.
Continents are generally flat. Tectonic uplift begins.
Ordovician
(438-505 mya)
First fish and fungi. Greatest extent of shallow seas. Climate
becoming warmer.
Cambrian
(505-551 mya)
Invertebrates become common. Fossilization of the Burgess Shale.
Large areas of shallow seas near the equator. Climate was warm.
Eukaryotic cell organisms develop. First multicellular organisms.
Supercontinent of Rodinia was assembled (~1100bya) then split into
2 halves (~750 mya), opening the Panthalassic Ocean. Changes in the
lithosphere created major land masses and extensive shallow seas.
Proterozoic
(551-2500 mya)
Archean
(2500-3800 mya)
Major Geologic Milestones
Also known as Precambrian
Slow development of the lithosphere, hydrosphere, and atmosphere.
First single-celled prokaryotic organisms.
Earth's oldest rocks come from the end of this Eon.