Geologic TimeGeologic Time

Marble demoSome Index FossilsCoin Toss SheetColor Copies of Expected Values for X2 in homeworkTime Scale.doc

Determining geological agesDetermining geological ages

• Relative age dates – placing rocks and geologic events in their proper sequence

• Numerical dates – define the actual age of a particular geologic event (termed absolute age dating)

Principles of relative Principles of relative datingdating

Developed by Nicolaus Steno Developed by Nicolaus Steno in 1669in 1669

• 1. Law of superposition1. Law of superposition• In an undeformed sequence of In an undeformed sequence of

sedimentary or volcanic rocks, sedimentary or volcanic rocks, oldest rocks at base; youngest at oldest rocks at base; youngest at toptop

Steno recognized the organic origin of fossils and sketched a theory of geological

strata, which he used in an attempt to reconstruct Tuscany's geological development Niels Steensen (Nicolas Steno)

Superposition illustrated by Superposition illustrated by strata in the Grand Canyonstrata in the Grand Canyon

Steno’s Steno’s 2nd principle of relative dating2nd principle of relative dating

• Principle of original horizontality

• Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)

Steno’s 3rd principle of relative datingSteno’s 3rd principle of relative dating• Principle of cross-cutting relationships

Chunks of this “country rock” have broken off and are visible in the intrusion

3rd principle of relative dating3rd principle of relative dating• Principle of cross-cutting relationships

This fault shows the offsetof the two sides. See the key beds? Notice this side is lower

Unconformities Unconformities (loss of rock record)(loss of rock record)

• An unconformity is a break in the rock record produced by erosion and/or non-deposition

• Types of unconformities– Nonconformity – sedimentary rocks deposited above

metamorphic or igneous rocks (basement) with time lost– Angular unconformity – tilted rocks overlain by flat-

lying rocks– Disconformity – strata on either side of the

unconformity are parallel (but time is lost)

8_9(a)

(b)

(c)

Layeredsedimentaryrocks

NonconformityMetamorphicrock

Igneousintrusive rock

Youngersedimentaryrocks

Angularunconformity

Older, foldedsedimentaryrocks

Disconformity

Brachiopod(290 million years old)

Trilobite (490 million years old)

Development of a Nonconformity

Pennsylvanian sandstone over Precambrian granite is a nonconformity

Nonconformity in the Grand Canyon - Sediments deposited over Schist

Formation of an angular unconformityFormation of an angular unconformity

Horizontal younger sediments over tilted older sedimentsCambrian Tapeats sandstone over Precambrian Unkar Group

What type of unconformity is this?

Grand Canyon in Arizona

Cross Cutting Relationships in strataZoroaster Granite across Vishnu Schist

Correlation of rock layersCorrelation of rock layers

• Matching strata of similar ages in different regions is

called correlationhttp://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg

Correlation of strata in southwestern United States

Sections are incompleteMatch with fossilsMatching lithology is risky, discussion

8_10

Rock brokento reveal external moldof shell

Rock brokento revealfossil cast

Shellsburied insediment

Mold, or cavity,forms when originalshell materialis dissolved

Cast forms when moldis filled in with mineralwater

Shellssettle onoceanfloor

How impression fossils form (the most common type)

Correlation of rock layers with Correlation of rock layers with fossilsfossils

• Correlation often relies upon fossils• Principle of fossil succession (Wm. Smith)

– fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it

• Index Fossils - useful for correlation– Existed for a relatively brief time– Were widespread and common

http://www.csun.edu/~psk17793/ES9CP/ES9%20fossils.htm

•Most fossils are just impressions. A few may have small amounts of some original tissue

Determining the ages of Determining the ages of rocks using overlap of fossilsrocks using overlap of fossils

Overlap time span is shorterthan that of any one fossil.

Fossils allow correlation in spite of unconformitiesFossils allow correlation in spite of unconformities

Geologic time scaleGeologic time scale

• The geologic time scale – a “calendar” of Earth history

• Subdivides geologic history into units

• Originally created using relative dates

• Structure of the geologic time scale

•Eon, Era, Period, Epoch

Geologic Timescale

Divisions based on fossilsEon, Era, Period, Epoch

HomeworkLearn Timescale.doc less Epochs

Origin of Period Names

Geologic time scaleGeologic time scale• Structure of the geologic time scale

• Names of the eons– Phanerozoic (“visible life”) – the most recent

eon, began about 545 million years ago– PreCambrian (Cryptozoic)

• PreCambrian subdivisions:• Proterozoic – begins 2.5 billion years ago• Archean – begins 3.8 bya• Hadean – the oldest eon begins 4.6 bya

Read from bottom to top – Oldest to Youngest

Geologic time scaleGeologic time scale

• Precambrian time• Nearly 4 billion years prior to the Cambrian period• Long time units because the events of Precambrian

history are not know in detail – few fossils, most rock modified

• Immense space of time (Earth is ~ 4.6 Ga)• PreCambrian spans about 88% of Earth’s history

Geologic time scaleGeologic time scale

• Structure of the geologic time scale• Era – subdivision of an eon• Eras of the Phanerozoic (visible life”) eon

– Cenozoic (“recent life”) begins ~ 65 mya

– Mesozoic (“middle life”) begins ~ 248 mya

– Paleozoic (“ancient life”) begins ~ 540 mya

• Eras are subdivided into periods• Periods are subdivided into epochs

Using radioactivity in datingUsing radioactivity in dating

• Importance of radiometric dating•Allows us to calibrate geologic timescale

•Determines geologic history

•Confirms idea that geologic time is immense

• Included in some sediment from NW Australia, detrital grains of the mineral Zircon that are 3.96 billion years old. The dates are based on datable Uranium in the Zircons.

•Similar dates are known from Yellow Knife Lake, NWT, Canada

•Claims of older zircons 4.4 by.

Radiometric Age Determinationsshow Earth not as old as Moon, Meteorites

Radiometric Age DeterminationsRadiometric Age Determinations of the Earth of the Earth

• However, the age of the Earth is thought to be about 4.5 - 4.6 billion years

• Based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time.

Recall Isotopes

• The number of protons in an atom's nucleus is called its atomic number –defines “element”

• Protons + neutrons called atomic weight

• The number of neutrons can vary

• Atoms of the same element with different numbers of neutrons are called isotopes. Some are radioactive

p

p

p

Atomic mass not changedmuch; atomic numberincreases by 1 becauseNeutron becomes proton

(b) Beta decayBeta particle

Radioactiveparent nucleus

Decay process Daughternucleus

Atomic mass not changed much;atomic numberdecreases by 1

(c) Electron capture

Beta particle

Atomic mass decreasesby 4; atomic numberdecreases by 2

(a) Alpha decay

Alpha particle

ProtonNeutron

pp

pp

pp

pp

pp

ppp

p

pp

ppp

p

pp

pp

pp

pp

ppp

pp

pp

ppp

pp

p

pp

p

p

ppp

p

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Emission of 2 protons and 2 neutrons (alpha particle)

An electron (beta particle) is ejected from the nucleus

electron combines with a proton to form a neutron

Using radioactivity in datingUsing radioactivity in dating

• Parent – an unstable radioactive isotope

• Daughter product – stable isotopes resulting from decay of parent

• Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product

A radioactive decay curveA radioactive decay curve

1/2 = 50% parent: 1 half-life has passed1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed

MARBLE DEMO

Uranium to Lead used for granites; Potassium to Argon used for basalts

How do we actually “date” a rock?

1. Collect sample

2. Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids

3. Measure parent/daughter ratio of target isotopes - mass spectrometer

4. Substance heated – Ions – move in Electrical Field, curved in Magnetic

Mineralcrystal

Mineral crystalformed in igneousrock

Parentatoms

Daughteratoms

1

Igneous rock

buried beneathyounger rocks;daughter atomsformed bynormal decay

2

Dating a crystal

(3) We calculate age based on half-life

Usual Case

8_22bDeep burial andmetamorphismduring mountainbuilding causesdaughter atomsto escape fromcrystal

3

After mountainbuilding ends,accumulation ofdaughter atomsin crystalresumes

4

Heat

Resets the clock

But IF:

Easily recognized,useful in studyingmetamorphism

Rock looks as if it just formed: it looks young

Age found dates from metamorphic event

Metamorphism Case

Dating sediments without fossils: Superposition, Cross-cutting

Radiometric Dating with Igneous RocksOr Bracket between fossiliferous layers

Morrison Fm older than 160 my(superposition)

Wasatch Fm. younger than 66 myMancos Shale and Mesa Verde Fm.

older than 66 myRule of Cross-cutting

Basalt Lava flow 2200 mya

Lava flow 1209 mya

We can bracket thislimestone’s age between 209 and 200 mya

Even better: we get lucky. A layer we need to date is between two datable bedsSo we have and upper and lower bound on the age of this limestone:

Dating with carbon-14 (Carbon Dating)• Half-life only 5730 years• Used to date very young rocks• Carbon-14 is produced in the upper atmosphere• Useful tool for geologists who study very recent

Earth history

Atoms split intosmaller particles,among them neutrons

Neutrons strikenitrogen atoms

Nitrogen atom gains a neutron and loses aProton; becomes carbon-14

C-14 mixes with atmospheric oxygento produce CO2

CO2 taken upby plants, water

C-14 absorbedby livingorganisms

CO2 dissolvedin water

C-14 intake ceases when organismdies; C-14 concentration decreases

Cosmic raysbombardatmospheric atoms

Carbon-14

Sediment layerswith tree logs tobe collected fordendrochronology

Annual-ring similaritiesshow correlation Current year

Years of age

50 100 150 200400

500

Buried treelogs

Treegrowthrings

A

A

B

B

C

C

D

D

Tree Rings both modern and past 2000 years

8_28

Turbid water

Heavyrunoffintolake

Very little or norunoff

Summer layer(coarse, thick, andlight-colored)

Clear water

Winter layer(fine, thin, and dark-colored)

Summer Winter

Ice

Dating with Lake Varves

Lake deposits, fossil plants C14. Fossil tree pollen track climate.

Southern lakes track glaciation

http://bcornet.tripod.com/Cornet94/Cornet94.htm

http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html

Hanneke Bos

End of Geologic End of Geologic Time LectureTime Lecture