GeoTime vak

8/2/2019 GeoTime vak

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Geologic Time and Earth History

Dr. R. B. Schultz


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Geologic Time

By examining layers of sedimentary rock, geologists developed a time scale for

dividing up earth history.Earlier in the 20th century, radiometric-dating techniques allowed scientists to

put absolute dates on divisions in the geologic time scale.

In this segment, we will learn how geologists:

determine the relative ages of rock units,

determine the divisions of the geologic time scale, and

how radiometric techniques can be used to date some rocks.

How do geologists determine how old rocks are?

1. Relative dating-- determine whether the rock is older or younger than other

rocks2. Absolute dating-- use radiometric dating techniques to determine how longago the rock formed in the exact number of years

*Not all rocks can be dated absolutely, so combinations of techniques are used.


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Example of Relative Age Dating and Correlation


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Relative Age Dating


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Absolute Dating: Radiometric Dating Techniques Use naturally-occurring radioactive isotopes Isotope -- form of an element that has additional neutrons Radioisotope -- isotope that spontaneously decays, giving off radiation

Rate of Radioactive Decay Radioisotopes decay at a constant rate. Rate of decay is measured by half-life Half-life -- time it takes for one-half of the radioactive material to decay.

Decay products Radioisotopes may decay to form a different isotope or a stable isotope. May be a series of radioactive decays before a stable isotope is formed. Stable isotope is called the "daughter" formed from decay of radioactive "parent"

Radiometric Age DatingRadioisotopes are trapped in minerals when they crystallize.

Radioisotopes decay through time, and stable isotopes are formed.Determining the ratio of parent isotope to daughter product reveals the number of half-lives that has elapsed.

Common isotopes used in age datingU-Pb -- half-life of U-238 is 4.5 b.y.K-Ar -- half-life of K-40 is 1.3 b.y.

Rb-Sr -- half-life of Rb-87 is 47 b.y.Carbon 14 -- half-life of C-14 is 5730 yrs


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Thus, by using the appropriate radioactive isotope (knowing its half-lifetime), and measuring the quantity of the isotope present in the rock, onecan deduce how long it has taken to decay down to the present amount in

the rock.

Example: A rock has 0.5 (one-half) of the original carbon 14 material in it.One can deduce that knowing the half-life of carbon 14 is 5730 years, therock must have decayed (lost) 50% of its original carbon 14 material and isnow 5730 years old. In a period of 5730 years from now, the rock will

contain .25 (25%) of its original carbon 14 material. Theoretically, therewill always be some trace of carbon 14 present in the rockit will neverdecay totally.

Absolute Dating Example


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Interpreting the rock record:

Prior to geologic principles, Archbishop James Ussher calculated theage of the Earth at 6000 years. He noted that calculations were

made based on the books of the Bible (namely Genesis) andpinpointed the origin of the Earth to be October 26, 4004 B.C. at9:00 a.m.

Principle of Uniformitarianism:

James Hutton, late 1700s (considered to be "Father of Geology")Hutton realized that most sedimentary layers were deposited from

gradual, day-to-day processes. He realized that it took a long timeto form these rocks. This was far different from what othersbelieved prior to this time.

"Present is the key to the past" -- whatever processes are occurringtoday (plate tectonics, volcanism, mountain building, earthquakes,sedimentation) also occurred in the past and probably at the same(or very comparable) rates.


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Principles associated with Relative Dating

The comparing of rock units to decipher their age relative to one another

Principle of Superposition

Rock layer above is younger than the ones below it. (Oldest on bottom,youngest on top)May not apply to rocks that have been folded (can get turned upside-down).

Principle of Original HorizontalitySedimentary layers are deposited in approximately horizontal sheets.

If layers are folded, episode of deformation must have occurred afterrocks formed. Age of folding is younger than youngest deformed rock unit.

Principle of Crosscutting RelationshipsAny feature (e.g. fault or intrusion) that cuts across rocks is younger thanthe youngest rock that is cut.

Principle of Faunal SuccessionOrganisms have evolved and gone extinct through timeFossil content of rock changes in a systematic way, reflecting evolutionarychangesFossil content can be used to help determine age of rock and correlaterocks.

Paraphrased as "Organisms within rock units change with time".


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Illustration of Relative Age Principles

Superposition

Cross CuttingRelationsOriginal

Horizontality


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Correlation

Correlationis determiningthat rocks are the sameformation (may mean rocksare the same age)


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Illustration of Principle of Faunal Succession


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UnconformitiesUnconformitiesare surfaces in rock that represent periods of erosion or non-

deposition. In other words, time has been left out of the physical geologicrock record.

There are three (3) principal types of unconformities:1. Angular Unconformity

Rocks above and below unconformity have different orientations. Showsthat there was a period of deformation, followed by erosion, and thenrenewed deposition. Easiest of the three types to recognize because theunits are at an angle truncated with the units above them.

2. NonconformityRocks in a horizontal fashion were eroded down to igneous bedrock materialat which time subsequent deposition of sedimentary layers commenced.Shows that there was a period of deformation, followed by erosion, andthen renewed deposition. Represents the greatest amount of time left outof the geologic rock record.

3. DisconformityRocks in a nearly horizontal fashion were eroded and an erosional profileremains covered by subsequent sedimentary deposition. Shows that therewas a period of erosion and then renewed deposition in nearly horizontallayers. Most difficult to recognize because the units are nearly horizontaland only a small discontinuous layer can be observed (rubble zone or soil

profile).


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Unconformity Types Using Grand Canyon as Example


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Geologic Time ScaleDeveloped in 1800s from relative dating of rocksMore recently, radiometric techniques have allowed us to determine

ages of units in years before present.Many of the names relate back to localities in England (Ex: Devonian

from Devonshire)

Divisions of Geologic Time Scale:EonsEras:Paleozoic -- Mesozoic -- CenozoicOldest -----------------> Youngest

Periods of the Phanerozoic: Paleozoic EraPermian (youngest)

Pennsylvanian together with Mississippian are called "Carboniferous" inGreat Britain

Epochsof Tertiary and QuaternaryPaleoceneEoceneOligoceneMiocenePliocenePleistocene


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The Geologic Time Scale

Origin of the Earth4.55 Billion years

First multi-celledorganisms

Age of Fishes

Age of CoalFormation

Major MassExtinction

Age ofDinosaurs

Major MassExtinction

Humansarrive

Most recentIce Age


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What Killed the Dinosaurs?


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Earth During the Silurian (430+ million years ago)


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Earth during:

Devonian ~410

million years ago

Mississippian ~330million years ago

Permian ~250 millionyears ago


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Earth during:

Triassic ~ 200

million years agoJurassic ~190 million

years ago

Cretaceous ~100million years ago

Early Cenozoic ~50million years ago


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Key Terminology

Relative age dating Absolute age datingIsotope Radioisotope

Half-life Daughter product

Parent Material Archbishop Ussher

James Hutton Uniformitarianism (Uniformity)

Superposition Cross cutting relationships

Original horizontality Faunal succession

Correlation Unconformity

Angular unconformity Disconformity

Nonconformity Geologic time scale

Eon Era

Period Epoch


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Dating TechniquesThis site contains information on relative and numeric dating techniques.

EarthTripsExcellent virtual field trips through Earth history from the Paleontological Research Institution.

FossilsAn excellent fossil Web site. Billed as the world's largest Online fossil company, the Extinctions Fossil Company is a business dedicated to

supplying fossils to museums and collectors around the world.Fossils and Fossil Collecting (USGS)

A listing of publications about fossils and fossil collecting available from the United States Geological Survey (USGS), including links toseveral fossil related sites.

Fossils, Rocks, and Time (USGS)

An online book from the United States Geological Survey (USGS) explaining the occurrence and usefulness of fossils .

Geologic Maps (USGS)An explanation of geologic maps from the United States Geological Survey (USGS).

Geologic Time & Fossils Links (NAGT)

An extensive listing of geologic time and fossils links arranged by topic from the National Association of Geology Teachers (NAGT).

Geologic Time LinksThis Central Michigan University site maintained by Dr. Mark Francek lists several good links to Web sites containing information relative to

topics discussed in the chapter.

Geologic Time Links (Houghton Mifflin)Links to several geologic time sites, including class lecture notes, arranged by topic.

Grand Canyon Explorer Home PageAn excellent unofficial Grand Canyon National Park home page that offers information on geology, virtual tours, images, etc. related to the

canyon.

Natural History Museum, LondonLondon's Natural History Museum's Earth galleries, paleontology, and mineralogy exhibits are especially interesting.

Paleobotany LinksAn extensive listing of paleobotany related sites from the University of Wrzburg, Germany.

Paleontology and Fossil LinksAn extensive listing of fossil related sites from the University of Arizona library.

Pertinent Web Sites
http://www2.trincoll.edu/~isample/Geological/outline.htmlhttp://www.priweb.org/ed/earthtrips/earthtrips.htmlhttp://www.extinctions.com/http://geology.er.usgs.gov/eastern/fossils.htmlhttp://pubs.usgs.gov/gip/fossils/contents.htmlhttp://www.wr.usgs.gov/docs/wgmt/aboutmaps.htmlhttp://cw.prenhall.com/bookbind/pubbooks/agi/chapter7/destinations1/deluxe-content.htmlhttp://personal.cmich.edu/~franc1m/geoltime.htmhttp://www.geologylink.com/toc/chap8.htmlhttp://www.kaibab.org/http://www.nhm.ac.uk/http://www.uni-wuerzburg.de/mineralogie/palbot1.htmlhttp://dizzy.library.arizona.edu/users/mount/paleont.htmlhttp://dizzy.library.arizona.edu/users/mount/paleont.htmlhttp://www.uni-wuerzburg.de/mineralogie/palbot1.htmlhttp://www.nhm.ac.uk/http://www.kaibab.org/http://www.geologylink.com/toc/chap8.htmlhttp://personal.cmich.edu/~franc1m/geoltime.htmhttp://cw.prenhall.com/bookbind/pubbooks/agi/chapter7/destinations1/deluxe-content.htmlhttp://www.wr.usgs.gov/docs/wgmt/aboutmaps.htmlhttp://pubs.usgs.gov/gip/fossils/contents.htmlhttp://geology.er.usgs.gov/eastern/fossils.htmlhttp://www.extinctions.com/http://www.priweb.org/ed/earthtrips/earthtrips.htmlhttp://www2.trincoll.edu/~isample/Geological/outline.html


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Paleontological Research Institution

The Paleontological Research Institution site features links to several sites featuring current paleontological events.

Paleontology Study WEB

Links to numerous paleontology topics and sites. The alphabetical listing allows for rapid searching by topic.

Paleontology: Yahoo!

An extensive listing of links to sites on paleontology (the study of fossils) from Yahoo.

Radiocarbon Dating

Online information concerning the radiocarbon dating method from the labs of Waikato and Oxford Universities.

Timeline of the Universe (NASA)

Beginning with the Big Bang, this tutorial follows the 15-billion-year-long history of the Universe.

Time Machine (University of California, Berkeley)

An excellent tour of geological time from the University of California.

Virtual Age

Dating

An interactive exercise about geologic time where you can learn how geologists and archeologists determine the ages of rocksand ancient artifacts.

WebDoGS Time Machine!

A tour of geologic time from the University of Kentucky.
http://www.priweb.org/http://www.studyweb.com/Science/tocbiolpaleo.htmhttp://www.yahoo.com/Science/Earth_Sciences/Paleontology/index.htmlhttp://www2.waikato.ac.nz/c14/webinfo/index.htmlhttp://eis.jpl.nasa.gov/origins/poster/poster.htmlhttp://www.ucmp.berkeley.edu/help/timeform.htmlhttp://vearthquake.calstatela.edu/VirtualDating/http://vearthquake.calstatela.edu/VirtualDating/http://vearthquake.calstatela.edu/VirtualDating/http://www.uky.edu/ArtsSciences/Geology/webdogs/time/time4.htmhttp://www.uky.edu/ArtsSciences/Geology/webdogs/time/time4.htmhttp://vearthquake.calstatela.edu/VirtualDating/http://vearthquake.calstatela.edu/VirtualDating/http://vearthquake.calstatela.edu/VirtualDating/http://www.ucmp.berkeley.edu/help/timeform.htmlhttp://eis.jpl.nasa.gov/origins/poster/poster.htmlhttp://www2.waikato.ac.nz/c14/webinfo/index.htmlhttp://www.yahoo.com/Science/Earth_Sciences/Paleontology/index.htmlhttp://www.studyweb.com/Science/tocbiolpaleo.htmhttp://www.priweb.org/

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