introduction to geology plate tectonics, structural geology drifting continents and spreading...

Chapter 12

Deep Time: How Old is Old?Earth: Portrait of a Planet, 4th editionby Stephen Marshak

© 2011 W.W. Norton & Company

PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202

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Earth: Portrait of a Planet, 4th edition, by Stephen Marshak © 2011, W. W. Norton Earth: Portrait of a Planet, 4th edition, by Stephen Marshak © 2011, W. W. Norton Chapter 12: Deep Time: How Old is Old?Chapter 12: Deep Time: How Old is Old?

Lecture 10: Geological TimeLecture 10: Geological TimeLecture 10: Geological TimeLecture 10: Geological Time

Prepared by:Prepared by:

Ronald L. ParkerRonald L. Parker, , Senior GeologistSenior Geologist

Fronterra Geosciences,Fronterra Geosciences,

Denver, ColoradoDenver, Colorado

Earth: Portrait of a Planet, 4th edition, by Stephen Marshak © 2011, W. W. Norton Earth: Portrait of a Planet, 4th edition, by Stephen Marshak © 2011, W. W. Norton Chapter 12: Deep Time: How Old Is Old?Chapter 12: Deep Time: How Old Is Old?

Geologic TimeGeologic Time

�� Earth has a history that is billions of years old.Earth has a history that is billions of years old.

�� Discovering this was a major step in human history.Discovering this was a major step in human history.

�� It changed our perception of time and the Universe.It changed our perception of time and the Universe.



�� Provides a frame of reference for understanding:Provides a frame of reference for understanding:

�� Rocks.Rocks.

�� Fossils.Fossils.

�� Geologic structures.Geologic structures.

�� Landscapes.Landscapes.

�� Tectonic events.Tectonic events.

�� Change.Change.

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�� Geologic and historical time were thought the same. Geologic and historical time were thought the same.

�� Archbishop James Ussher, of Archbishop James Ussher, of ArmaghArmagh, Ireland,1654:, Ireland,1654:

��Added up generations from the Old Testament. Added up generations from the Old Testament.

��Pronounced that Earth formed on October 23, 4004 Pronounced that Earth formed on October 23, 4004 BB..CC..EE..



�� Scientists found many clues indicating an ancient Earth.Scientists found many clues indicating an ancient Earth.

�� Nicolaus Steno (1638Nicolaus Steno (1638––86), Danish physician.86), Danish physician.

��He observed marine fossils high in the Apennines. He observed marine fossils high in the Apennines.

��He reasoned these to be former animals from an ancient sea.He reasoned these to be former animals from an ancient sea.

��Lithification and uplift suggested long periods of time.Lithification and uplift suggested long periods of time.

Fig. 12.2



�� James Hutton (1726James Hutton (1726––97), Scottish physician and farmer.97), Scottish physician and farmer.

�� He is called “the father of modern geology.” He is called “the father of modern geology.”

�� The first to articulate the “principle of uniformitarianism.”The first to articulate the “principle of uniformitarianism.”

�� Of the abyss of time, Hutton wrote: “we find no vestige of a Of the abyss of time, Hutton wrote: “we find no vestige of a

beginning; no prospect of an end.”beginning; no prospect of an end.”

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�� James Hutton’s principle of uniformitarianism.James Hutton’s principle of uniformitarianism.

�� “The present is the key to the past.”“The present is the key to the past.”

��Processes seen today are the same as those of the past. Processes seen today are the same as those of the past.

��Geologic change is slow; large changes require a long time.Geologic change is slow; large changes require a long time.

�� Therefore, there must have been a long time before humans.Therefore, there must have been a long time before humans.



�� There are two ways of dating geological materials.There are two ways of dating geological materials.

�� Relative agesRelative ages——based upon order of formation.based upon order of formation.

��Qualitative method developed hundreds of years ago.Qualitative method developed hundreds of years ago.

��Permit determination of older vs. younger relationships.Permit determination of older vs. younger relationships.

�� Numerical agesNumerical ages——actual number of years since an event.actual number of years since an event.

��Quantitative method developed recently. Quantitative method developed recently.

��Age is given a number.Age is given a number.

Fig. 12.23


Physical PrinciplesPhysical Principles

�� Sir Charles Lyell wrote Sir Charles Lyell wrote Principles of Geology Principles of Geology in 1830in 1830––33.33.

�� Laid out a set of principles for deciphering Earth history.Laid out a set of principles for deciphering Earth history.

�� Used to establish relative ages of Earth materials.Used to establish relative ages of Earth materials.

�� The principle of uniformitarianismThe principle of uniformitarianism

��Processes observed today were the same in the past. Processes observed today were the same in the past.

��Mudcracks in old sediments formed like mudcracks today.Mudcracks in old sediments formed like mudcracks today.

Fig. 12.4a

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�� The principle of The principle of uniformitarianismuniformitarianism..

�� Processes observed today were the same in the past.Processes observed today were the same in the past.

��We can observe lava flowing and cooling into solid rock.We can observe lava flowing and cooling into solid rock.

��Ancient lava flows are the products of volcanic eruptions.Ancient lava flows are the products of volcanic eruptions.

Fig. 12.4b



�� The principle of original horizontality.The principle of original horizontality.

�� Sediments settle out of a fluid by gravity.Sediments settle out of a fluid by gravity.

�� This causes sediments to accumulate horizontally.This causes sediments to accumulate horizontally.

�� Sediment accumulation is not favored on a slope.Sediment accumulation is not favored on a slope.

�� Hence, tilted sedimentary rocks must be deformed.Hence, tilted sedimentary rocks must be deformed.

Fig. 12.4


�� The principle of superposition.The principle of superposition.

�� In an In an undeformedundeformed sequence of layered rocks:sequence of layered rocks:

��Each bed is older than the one above, andEach bed is older than the one above, and

��Younger than the one below. Younger than the one below.

�� Younger strata are on top; older strata below.Younger strata are on top; older strata below.


Fig. 12.4e

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�� The principle of lateral continuity.The principle of lateral continuity.

�� Strata often form laterally extensive horizontal sheets.Strata often form laterally extensive horizontal sheets.

�� Subsequent erosion dissects once continuous layers.Subsequent erosion dissects once continuous layers.

�� FlatFlat--lying rock layers are unlikely to have been disturbed. lying rock layers are unlikely to have been disturbed.

Fig. 12.4f



�� The principle of crossThe principle of cross--cutting relations.cutting relations.

�� Younger features truncate (cut across) older features.Younger features truncate (cut across) older features.

�� Faults, dykes, erosion, etc., Faults, dykes, erosion, etc., mustmust be younger than the younger than the

material that is faulted, intruded, or eroded.material that is faulted, intruded, or eroded.

�� A volcano cannot intrude rocks that are not there.A volcano cannot intrude rocks that are not there.

Fig. 12.4g



�� The principle of baked contacts.The principle of baked contacts.

�� An igneous intrusion cooks the invaded country rock. An igneous intrusion cooks the invaded country rock.

�� The baked rock must have been there first (it is older).The baked rock must have been there first (it is older).

�� A chilled margin forms at the contact from rapid cooling. A chilled margin forms at the contact from rapid cooling.

Fig. 12.4h

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�� Principle of inclusionsPrinciple of inclusions——a rock fragment within another.a rock fragment within another.

�� Weathering rubble must have come from older rock.Weathering rubble must have come from older rock.

�� Fragments (xenoliths) and igneous intrusion are older.Fragments (xenoliths) and igneous intrusion are older.

�� Inclusions are always older than the enclosing material.Inclusions are always older than the enclosing material.

Fig. 12.4Fig. 12.4i



�� Physical principles allow us to sort out relative age.Physical principles allow us to sort out relative age.

�� This is possible even in complex situations.This is possible even in complex situations.

�� Consider this block of geologic history. We see:Consider this block of geologic history. We see:

�� Folded sediments.Folded sediments.

�� Intrusions.Intrusions.

�� Granite.Granite.

�� Basalt.Basalt.

�� A fault.A fault.

�� Xenoliths.Xenoliths.

�� Inclusions.Inclusions.

�� Baked contact.Baked contact.

�� Easily deciphered!Easily deciphered!

Fig. 12.5a


�� A sequence of horizontal strata accumulates.A sequence of horizontal strata accumulates.

�� Superposition (1 oldest).Superposition (1 oldest).

�� An igneous sill intrudes.An igneous sill intrudes.

�� Inclusions of 4 and 5 in sill Inclusions of 4 and 5 in sill

confirm it is younger.confirm it is younger.

�� Folding, uplift, and erosion take place.Folding, uplift, and erosion take place.

�� Folding occurs before intrusion.Folding occurs before intrusion.

Geologic HistoryGeologic History

Fig. 12.5b

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�� A granitic A granitic plutonpluton intrudes the folded sediments.intrudes the folded sediments.

�� Granite cuts folded layers.Granite cuts folded layers.

�� Heat alters intrusive contact.Heat alters intrusive contact.

�� Xenoliths fall into magma.Xenoliths fall into magma.

�� A fault cuts the granite and folded sediments.A fault cuts the granite and folded sediments.

�� A basalt dyke cuts across the block.A basalt dyke cuts across the block.

�� The dyke cools and is eroded.The dyke cools and is eroded.

Geologic HistoryGeologic History

Fig. 12.5b


�� Fossils are often preserved in sedimentary rocks. Fossils are often preserved in sedimentary rocks.

�� Fossils are time markers useful for relative ageFossils are time markers useful for relative age--dating.dating.

�� Fossils speak of past depositional environments.Fossils speak of past depositional environments.

�� Specific fossils are only found within a limited time range.Specific fossils are only found within a limited time range.

The Principle of Fossil SuccessionThe Principle of Fossil Succession

Fig. 12.6



�� Species evolve, exist for a time, and then disappear. Species evolve, exist for a time, and then disappear.

�� The first appearance, range, and extinction used for dating. The first appearance, range, and extinction used for dating.

�� Fossils succeed one another in a known order.Fossils succeed one another in a known order.

�� A time period is recognized by its fossil content.A time period is recognized by its fossil content.

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�� Fossil rangeFossil range——the first and last appearance.the first and last appearance.

�� Each fossil has a unique range.Each fossil has a unique range.

�� Range overlap narrows time.Range overlap narrows time.

�� Index fossils are diagnostic of Index fossils are diagnostic of

a particular geologic time.a particular geologic time.

�� Fossils correlate strata.Fossils correlate strata.

�� LocallyLocally

�� RegionallyRegionally

�� GloballyGlobally


Fig. 12.7


�� An unconformity is a time gap in the rock record, from:An unconformity is a time gap in the rock record, from:

�� Nondeposition.Nondeposition.

�� Erosion. Erosion.

�� There are three types of unconformity.There are three types of unconformity.

�� Angular unconformity.Angular unconformity.

�� Nonconformity.Nonconformity.

�� Disconformity.Disconformity.

UnconformitiesUnconformities

Fig. 12.8b


Angular UnconformityAngular Unconformity

�� James HuttonJames Hutton——first to recognize angular first to recognize angular unconformities.unconformities.

�� An angular unconformity represents a huge gulf in time.An angular unconformity represents a huge gulf in time.

��Horizontal marine sediments deformed by orogenesis.Horizontal marine sediments deformed by orogenesis.

��Mountains eroded completely away.Mountains eroded completely away.

��Renewed marine invasion.Renewed marine invasion.

��New sediments deposited. New sediments deposited.

Fig. 12.9a

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Angular UnconformityAngular Unconformity

�� Hutton’s Unconformity, Siccar Point, Scotland.Hutton’s Unconformity, Siccar Point, Scotland.

�� A common destination for geologists. A common destination for geologists.

��Vertical beds of Ordovician sandstone.Vertical beds of Ordovician sandstone.

��Overlain by gently dipping Devonian redbeds.Overlain by gently dipping Devonian redbeds.

��Missing time: 50 million years.Missing time: 50 million years.

Fig. 12.8a



�� NonconformityNonconformity——igneous/metamorphic rocks capped by igneous/metamorphic rocks capped by sedimentary rocks. sedimentary rocks.

�� Crystalline igneous/metamorphic rocks were exposed by Crystalline igneous/metamorphic rocks were exposed by

erosion.erosion.

�� Sediment was deposited Sediment was deposited

on this eroded surface. on this eroded surface.

Fig. 12.9bGranite

Sandstone



�� DisconformityDisconformity——parallel strata bounding nondeposition.parallel strata bounding nondeposition.

�� Due to an interruption in sedimentation.Due to an interruption in sedimentation.

��Pause in deposition.Pause in deposition.

��Sea level falls, then rises.Sea level falls, then rises.

��Erosion.Erosion.

�� Sometimes hard to see.Sometimes hard to see.

Fig. 12.9c,d

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�� Earth history is recorded in sedimentary strata.Earth history is recorded in sedimentary strata.

�� The Grand Canyon has thick layers of strata and The Grand Canyon has thick layers of strata and numerous gaps.numerous gaps.

�� Formations can be correlated over long distances.Formations can be correlated over long distances.

Correlating FormationsCorrelating Formations

Fig. 12.10


Stratigraphic CorrelationStratigraphic Correlation

�� Stratigraphic columns depict strata in a region. Stratigraphic columns depict strata in a region.

�� Drawn to scale to accurately portray relative thicknesses.Drawn to scale to accurately portray relative thicknesses.

�� Rock types are depicted by graphical fill patterns. Rock types are depicted by graphical fill patterns.

�� Divided into formations.Divided into formations.

��Mapable rock units.Mapable rock units.

�� Formations are Formations are

separated by contacts. separated by contacts.

Fig. 12.12


Stratigraphic CorrelationStratigraphic Correlation

�� Lithologic correlation (based on rock type) is regional.Lithologic correlation (based on rock type) is regional.

�� SequenceSequence——the relative order in which the rocks occur.the relative order in which the rocks occur.

�� Fossil correlationFossil correlation——based on fossils within the rocks.based on fossils within the rocks.

�� Applicable to much broader areas.Applicable to much broader areas.

Fig. 12.11

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Geologic MapsGeologic Maps

�� In 1793, William “Strata” Smith was the first to note that In 1793, William “Strata” Smith was the first to note that strata could be matched across distances. strata could be matched across distances.

�� Similar rock types in a similar order. Similar rock types in a similar order.

�� Rock layers contained the same distinctive fossils.Rock layers contained the same distinctive fossils.

�� After years of work, he made the first geologic map.After years of work, he made the first geologic map.

Fig. 12.13a,b


The Geologic ColumnThe Geologic Column

�� A composite stratigraphic A composite stratigraphic

column can be constructed.column can be constructed.

�� Assembled from incompleteAssembled from incomplete

sections across the globe.sections across the globe.

�� It brackets almost allIt brackets almost all

of Earth history.of Earth history.

Fig. 12.14


�� The composite column is divided into time blocks.The composite column is divided into time blocks.

�� This is the geologic time scale, Earth’s “calendar.”This is the geologic time scale, Earth’s “calendar.”

�� EonsEons——the largest subdivision of time (hundreds to the largest subdivision of time (hundreds to

thousands Ma).thousands Ma).

�� ErasEras——subdivisions of an eon (65 to hundreds Ma).subdivisions of an eon (65 to hundreds Ma).

�� PeriodsPeriods——subdivisions of an era (2 to 70 Ma).subdivisions of an era (2 to 70 Ma).

�� EpochsEpochs——subdivisions of a period (0.011 to 22 Ma). subdivisions of a period (0.011 to 22 Ma).


Fig. 12.24

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�� Life first appeared on Earth ~3.8 Ga.Life first appeared on Earth ~3.8 Ga.

�� Early life consisted of anaerobic Early life consisted of anaerobic

singlesingle--celled organisms. celled organisms.

�� OO22 from cyanobacteria built from cyanobacteria built

up in atmosphere by 2 Ga.up in atmosphere by 2 Ga.

�� Around 700 Ma, multicellular Around 700 Ma, multicellular

life evolved.life evolved.

�� Around 542 Ma marks the Around 542 Ma marks the

first appearancefirst appearance

of hard shells.of hard shells.

Geologic Time and LifeGeologic Time and Life

�� Shells increased fossil Shells increased fossil

preservation.preservation.

�� Life diversified rapidly,Life diversified rapidly,

the “Cambrian Explosion.”the “Cambrian Explosion.”

Fig. 12.15


�� Names of the Eons.Names of the Eons.

�� PhanerozoicPhanerozoic——”visible life” (542 Ma to present).”visible life” (542 Ma to present).

��Starts at the PreStarts at the Pre--CambrianCambrian––Cambrian boundary.Cambrian boundary.

��Marks the first appearance of hard shells.Marks the first appearance of hard shells.

��Life diversified rapidly afterwards.Life diversified rapidly afterwards.

�� ProterozoicProterozoic——“before life” (2.5 to 0.542 “before life” (2.5 to 0.542 GaGa). ).

��Development of tectonic plates like those of today.Development of tectonic plates like those of today.

��Buildup of atmospheric OBuildup of atmospheric O22; ; multicellularmulticellular life appears.life appears.

�� ArchaeanArchaean——“ancient” (3.8 to 2.5 “ancient” (3.8 to 2.5 GaGa).).

��Birth of continents.Birth of continents.

��Appearance of the earliest life forms. Appearance of the earliest life forms.

�� HadeanHadean——“hell” (4.6 to 3.8 “hell” (4.6 to 3.8 GaGa). ).

��Internal differentiation.Internal differentiation.

��Formation of the oceans and secondary atmosphere.Formation of the oceans and secondary atmosphere.

The Geologic Time ScaleThe Geologic Time Scale

Fig. 12.24



�� Names of the Eras.Names of the Eras.

�� CenozoicCenozoic——“recent life.” “recent life.”

��From 65.5 Ma to present.From 65.5 Ma to present.

��The Age of Mammals.The Age of Mammals.

�� MesozoicMesozoic——“middle life.”“middle life.”

��From 251 to 65.5 Ma.From 251 to 65.5 Ma.

��The Age of Dinosaurs.The Age of Dinosaurs.

�� PaleozoicPaleozoic——“ancient life.”“ancient life.”

��From 542 to 251 Ma.From 542 to 251 Ma.

��Life diversified rapidly.Life diversified rapidly.

Fig. 12.16

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�� TimeTime--scale subdivisions are variously named.scale subdivisions are variously named.

�� The nature of lifeThe nature of life——“zoic” means life (i.e., Proterozoic).“zoic” means life (i.e., Proterozoic).

�� A characteristic of the time period (i.e., Carboniferous).A characteristic of the time period (i.e., Carboniferous).

�� A specific locality (i.e., Devonian).A specific locality (i.e., Devonian).




Fig. 12.24


Numerical AgeNumerical Age

�� Many relative ages can now be assigned actual dates.Many relative ages can now be assigned actual dates.

�� Based on radioactive decay of atoms in minerals.Based on radioactive decay of atoms in minerals.

�� Radioactive decay proceeds at a known, fixed rate.Radioactive decay proceeds at a known, fixed rate.

�� Radioactive elements act as internal clocks. Radioactive elements act as internal clocks.

�� Numerical dating is also called geochronology.Numerical dating is also called geochronology.

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Radioactive DecayRadioactive Decay

�� IsotopesIsotopes——elements that have varying numberss of elements that have varying numberss of neutrons. neutrons.

�� Isotopes have similar but different mass numbers. Isotopes have similar but different mass numbers.

�� StableStable——isotopes that never change (i.e., isotopes that never change (i.e., 1313C).C).

�� RadioactiveRadioactive——isotopes that spontaneously decay (i.e., isotopes that spontaneously decay (i.e., 1414C).C).



�� Radioactive decay progresses along a decay chain. Radioactive decay progresses along a decay chain.

�� Decay creates new unstable elements that also decay.Decay creates new unstable elements that also decay.

�� Decay proceeds to a stable element endpoint. Decay proceeds to a stable element endpoint.

�� Parent isotopeParent isotope——the isotope that undergoes decay.the isotope that undergoes decay.

�� Daughter isotopeDaughter isotope——the product of this decay.the product of this decay.



�� HalfHalf--life (tlife (t½½))——time for half of the unstable nuclei to decay.time for half of the unstable nuclei to decay.

�� The halfThe half--life is a characteristic of each isotope.life is a characteristic of each isotope.

�� After one tAfter one t½½, one half of the original parent remains. , one half of the original parent remains.

�� After three tAfter three t½½, one eighth of the original parent remains., one eighth of the original parent remains.

�� As the parent disappears, the daughter “grows in”.As the parent disappears, the daughter “grows in”.

Fig. 12.18

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Radiometric DatingRadiometric Dating

�� The age of a mineral can be determined by:The age of a mineral can be determined by:

�� Measuring the ratio of parent to daughter isotopes.Measuring the ratio of parent to daughter isotopes.

�� Calculating the amount of time by using the known tCalculating the amount of time by using the known t½½..

�� Geochronology requires analytical precision. Geochronology requires analytical precision.

Fig. 12.19


What Is a Radiometric Date?What Is a Radiometric Date?

�� Radiometric dates give the time a mineral began to Radiometric dates give the time a mineral began to preserve all atoms of parent and daughter isotopes. preserve all atoms of parent and daughter isotopes.

�� Requires cooling below a “closure temperature.” Requires cooling below a “closure temperature.”

�� If rock is reheated, the radiometric clock can be reset.If rock is reheated, the radiometric clock can be reset.

�� Igneous/Metamorphic rocks are best for geochronologic Igneous/Metamorphic rocks are best for geochronologic work.work.

�� Sedimentary rocks cannot be directly dated.Sedimentary rocks cannot be directly dated.


Other Numerical AgesOther Numerical Ages

�� Numerical ages are possible without isotopes. Numerical ages are possible without isotopes.

�� Growth ringsGrowth rings——annual layers from trees or shells. annual layers from trees or shells.

�� Rhythmic layeringRhythmic layering——annual layers in sediments or ice. annual layers in sediments or ice.

Fig. 12.20

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Other Numerical AgesOther Numerical Ages

�� MagnetostratigraphyMagnetostratigraphy——magnetic signatures in strata are magnetic signatures in strata are compared to the global reference column.compared to the global reference column.

Fig. 12.21


Dating the Geologic ColumnDating the Geologic Column

�� Geochronology is less useful for sedimentary deposits.Geochronology is less useful for sedimentary deposits.

�� However, it can constrain these deposits.However, it can constrain these deposits.

�� Sediments can be bracketed by numerical dates.Sediments can be bracketed by numerical dates.

�� Yields age ranges that narrow as data accumulates. Yields age ranges that narrow as data accumulates.

�� Defines major boundaries in the geologic column.Defines major boundaries in the geologic column.

Fig. 12.23


The Age of the EarthThe Age of the Earth

�� The oldest rocks on Earth’s surface date to 3.96 Ga.The oldest rocks on Earth’s surface date to 3.96 Ga.

�� Zircons in ancient sandstones date to 4.1Zircons in ancient sandstones date to 4.1––4.2 Ga.4.2 Ga.

�� Age of Earth is 4.57 Ga based on correlation with:Age of Earth is 4.57 Ga based on correlation with:

�� Meteorites.Meteorites.

�� Moon rocks.Moon rocks.

Fig. 12.25

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Geology at a Glance


�� Read Read MarshakMarshak Chapter 12Chapter 12

�� USGS Geologic Time Online EditionUSGS Geologic Time Online Edition

�� http://pubs.usgs.gov/gip/geotime/http://pubs.usgs.gov/gip/geotime/

�� Geological Society of America (GSA) Printable Time ScaleGeological Society of America (GSA) Printable Time Scale

�� http://www.geosociety.org/science/timescale/http://www.geosociety.org/science/timescale/

�� CHRONOS CHRONOS CyberinfrastructureCyberinfrastructure SiteSite

�� http://www.chronos.org/http://www.chronos.org/

�� Deep Time: A History of the EarthDeep Time: A History of the Earth——Interactive Interactive InfographicInfographic

�� http://deeptime.info/http://deeptime.info/

�� Exploring TimeExploring Time

�� http://exploringtime.org/?page=segmentshttp://exploringtime.org/?page=segments

�� Smithsonian National Museum of Natural History, Geologic TimeSmithsonian National Museum of Natural History, Geologic Time

�� http://paleobiology.si.edu/geotime/index.htmhttp://paleobiology.si.edu/geotime/index.htm

Useful Web ResourcesUseful Web Resources

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