1 Copyright (c) 2005 Pearson Education Canada, Inc. Chapter 8: Geologic Time PowerPoint Presentation...

1Copyright (c) 2005 Pearson Education Canada, Inc.

Chapter 8: Geologic Chapter 8: Geologic TimeTime

PowerPoint PresentationPowerPoint PresentationStan Hatfield . Stan Hatfield . SW Illinois CollegeSW Illinois CollegeKen Pinzke . Ken Pinzke . SW Illinois CollegeSW Illinois CollegeCharles Henderson . Charles Henderson . University University

of Calgaryof CalgaryTark Hamilton . Tark Hamilton . Camosun Camosun

CollegeCollege

Geological Geological Time is of Time is of

Vast Vast DurationDuration

Sediments Sediments were buried were buried

under a under a Mountain Mountain

Range, Range, metamorphosmetamorphosed, half the ed, half the crust was crust was uplifted & uplifted &

eroded, eroded, glaciers glaciers

carried it, carried it, lake storms lake storms removed all removed all

but the but the biggest rocks.biggest rocks.

Geologic TimeGeologic Time Relative age dates – placing rocks and Relative age dates – placing rocks and

events in their proper sequence of events in their proper sequence of formation. e.g. Tertiary is younger than formation. e.g. Tertiary is younger than Cretaceous Cretaceous (rocks, fossils, climate)(rocks, fossils, climate)

Numerical dates – specifying the actual Numerical dates – specifying the actual number of years that have passed since number of years that have passed since an event occurred an event occurred (known as absolute age (known as absolute age dating using isotope clocks)dating using isotope clocks)

Geologic time scale – Earth’s history is Geologic time scale – Earth’s history is ~4.5 Ga long and written ~4.5 Ga long and written disproportionately by the rocks formed disproportionately by the rocks formed and processes which operated at different and processes which operated at different times and places.times and places.

Ordovician Strata: Chute Ordovician Strata: Chute Vernal, PQVernal, PQ

Relative Dating – Key Relative Dating – Key PrinciplesPrinciples

Law of SuperpositionLaw of Superposition

Developed by the Danish physician Developed by the Danish physician Nicolaus StenoNicolaus Steno working in Italy in 1669 working in Italy in 1669

In an undeformed sequence of In an undeformed sequence of sedimentary rocks (or layered igneous sedimentary rocks (or layered igneous rocks), the oldest rocks are on the rocks), the oldest rocks are on the bottombottom

Most sedimentary & volcanic rocks are Most sedimentary & volcanic rocks are deposited in sequences of essentially deposited in sequences of essentially flat lying bedsflat lying beds

Principle of Original HorizontalityPrinciple of Original Horizontality Layers of sediment are generally deposited Layers of sediment are generally deposited in a horizontal positionin a horizontal position

Rock layers that are flat have not been Rock layers that are flat have not been disturbeddisturbed

Principle of Cross-Cutting RelationshipsPrinciple of Cross-Cutting Relationships Younger features cut across older featuresYounger features cut across older features

Principle of InclusionsPrinciple of Inclusions Younger rocks/features include older onesYounger rocks/features include older ones All 3 types of rocks can include older rocks All 3 types of rocks can include older rocks and mineralsand minerals

Relative Dating – Key Relative Dating – Key PrinciplesPrinciples

Relative Dating – Key Relative Dating – Key Principles: Horizontality & Principles: Horizontality &

Law of SuperpositionLaw of Superposition

Permian Strata, South Rim

Correlation

Lower Paleozoic Strata: Lower Paleozoic Strata: Devon, UKDevon, UK

Deposition, DeformationAt the Closing of Iapetus

Fold Hinge

Formation of Sedimentary Formation of Sedimentary InclusionsInclusions

UnconformitiesUnconformitiesTypes of UnconformitiesTypes of Unconformities

DisconformityDisconformity – strata on either – strata on either side of the unconformity are side of the unconformity are parallel parallel (least missing time)(least missing time)

Angular unconformityAngular unconformity – tilted – tilted rocks are overlain by flat-lying rocks are overlain by flat-lying rocks rocks (10’s of Ma missing)(10’s of Ma missing)

NonconformityNonconformity – metamorphic or – metamorphic or igneous rocks in contact with igneous rocks in contact with sedimentary strata sedimentary strata (most missing time, (most missing time, eroded down to crystalline basement rocks)eroded down to crystalline basement rocks)

Relative Dating – Key Relative Dating – Key PrinciplesPrinciples

Siccar Point, Scotland &The development of an angular unconformity.

cd

e f

gh

ik l

m

Figure out the sequence of geological events,Oldest on the bottom, youngest on the top.

j

What kind ofunconformity is “i”?

What kind ofContact is “m”?

cd

e f

gh

ik l

m

Figure out the sequence of geological events,Oldest on the bottom, youngest on the top.

j

“i” is a disconformity“m” is intrusive,

& baked margins.

Dyke ADyke BIntr-l

Fault BkjI

Fault Ahgfedc

Which type of unconformity here at theSouth Rim represents the greatest amount

of missing time and why?

Correlation of Rock LayersCorrelation of Rock Layers Matching of rocks of similar ages in Matching of rocks of similar ages in

different regions is known as correlationdifferent regions is known as correlation Correlation often relies upon fossilsCorrelation often relies upon fossils

William Smith (late 1700s and early William Smith (late 1700s and early 1800s) noted that sedimentary strata in 1800s) noted that sedimentary strata in widely separated areas could be identified widely separated areas could be identified and correlated by their distinctive fossil and correlated by their distinctive fossil contentcontent

Other than fossils we often rely on Other than fossils we often rely on widespread instantaneous events like widespread instantaneous events like volcanic ash fallsvolcanic ash falls

Unconformities bound packages of Unconformities bound packages of stratigraphystratigraphy

CorrelatiCorrelation on

acrossacrossslightly slightly overlappoverlapp

ingingstratigrastratigra

phyphyArizona Arizona to Utah.to Utah.

Correlation often relies upon Correlation often relies upon fossilsfossils

Principle of Fossil (Faunal) Succession – Principle of Fossil (Faunal) Succession – fossil organisms succeed one another in fossil organisms succeed one another in a definite and determinable order that a definite and determinable order that documents the evolution of life; documents the evolution of life; therefore any time period can be therefore any time period can be recognized by its fossil contentrecognized by its fossil content

Index fossils – represent best fossils for Index fossils – represent best fossils for correlation; they are widespread correlation; they are widespread ecologically & geographically and are ecologically & geographically and are limited to a short time span (limited to a short time span (i.ei.e., they ., they evolved rapidly)evolved rapidly)

Correlation of Rock LayersCorrelation of Rock Layers

Time from Concurrent Range Zones

Reviewing Basic Atomic StructureReviewing Basic Atomic StructureIsotope (only a few of these are Isotope (only a few of these are radioactive)radioactive)

Variant of the same parent atom, same Variant of the same parent atom, same atomic numberatomic number

Differs in the number of neutrons (weight)Differs in the number of neutrons (weight) Results in a different mass number than Results in a different mass number than the common type of atom for this elementthe common type of atom for this element

Too many additional neutrons makes a Too many additional neutrons makes a nucleus unstable and prone to radioactive nucleus unstable and prone to radioactive decaydecay

Dating with RadioactivityDating with Radioactivity

3 kinds3 kindsofof

radioactiradioactiveve

decaydecay

Parent – an unstable radioactive Parent – an unstable radioactive isotopeisotope

Daughter product – the isotopes Daughter product – the isotopes that result from the decay of a that result from the decay of a parentparent

Half-Life – the time required for Half-Life – the time required for one-half of the radioactive nuclei one-half of the radioactive nuclei in a sample to decayin a sample to decay

Half-life differs for every Parent-Half-life differs for every Parent-Daughter pair of isotopesDaughter pair of isotopes

Dating with RadioactivityDating with Radioactivity

Exponential Decay of Parent Exponential Decay of Parent IsotopesIsotopes

(& exponential growth of daughters)(& exponential growth of daughters)The DecayEquation:

A = A0 e –λt

A0 is the initialamount of theparent isotope

Radiometric DatingRadiometric DatingPrinciple of Radioactive DatingPrinciple of Radioactive Dating

The percentage of radioactive The percentage of radioactive atoms that decay during one half-atoms that decay during one half-life is always the same (50 percent)life is always the same (50 percent)

However, the actual number of However, the actual number of atoms that decay continually atoms that decay continually decreasesdecreases

Comparing the ratio of parent to Comparing the ratio of parent to daughter yields the age of the daughter yields the age of the samplesample

Dating with RadioactivityDating with Radioactivity

Radiometric datingRadiometric datingUseful radioactive isotopes for Useful radioactive isotopes for providing radiometric agesproviding radiometric ages

Rubidium-87 Rubidium-87 Strontium-87 ~Precambrian Strontium-87 ~Precambrian Feldspar, micas, amphibolesFeldspar, micas, amphiboles

Thorium-232 Thorium-232 Lead-207 ~Pc Lead-207 ~Pc ZirconZircon

Two isotopes of uranium (235 and 238) to Lead Two isotopes of uranium (235 and 238) to Lead (208 & 206), 4.5 Ga & 713 Ma respectively, ~Pc to (208 & 206), 4.5 Ga & 713 Ma respectively, ~Pc to MesozoicMesozoic

ZirconZircon Potassium-40 Potassium-40 Argon-40 or relative to Argon-39 Argon-40 or relative to Argon-39

>2 Ma>2 Ma Feldspar, micas, volcanic glassFeldspar, micas, volcanic glass

Dating with RadioactivityDating with Radioactivity

Slow USlow U238238U U 210210PbPb

(½ life = 4.5 (½ life = 4.5 Ga)Ga)

Used forUsed forAncientAncientZirconsZircons

&&<200a<200a

210210PbPb

Rules for Radiometric DatingRules for Radiometric DatingMineral contains both parent & Mineral contains both parent & daughter as for Zircon with daughter as for Zircon with 238238U U and and 206206Pb or feldspar with Pb or feldspar with 8787Rb and Rb and 8787SrSr

Formed at time of event to be Formed at time of event to be dated as for a lava flow, dyke or dated as for a lava flow, dyke or contact metamorphismcontact metamorphism

The mineral is a closed system and The mineral is a closed system and has neither gained nor lost parent has neither gained nor lost parent & daughter& daughter

Dating with RadioactivityDating with Radioactivity

Radiometric datingRadiometric datingSources of errorSources of error

A closed system is requiredA closed system is requiredIf temperature is too high, If temperature is too high, daughter products may be lostdaughter products may be lost

To avoid potential problems, To avoid potential problems, only fresh, unweathered rock only fresh, unweathered rock samples should be usedsamples should be used

Dating with RadioactivityDating with Radioactivity

Production of Production of 1414C in the upper C in the upper atmosphereatmosphere

(& decay once organisms die)(& decay once organisms die)Cosmic rays

Expel neutrons fromAtmospheric gases (N,O).

This expels a protonFrom nitrogen

Forming radioactive 14C.

The extra neutronin radioactive 14C

decays back to 14NWith a ½ life of

5730 years.

Dating with carbon-14 Dating with carbon-14 (radiocarbon dating)(radiocarbon dating)

Half-life of only 5730 yearsHalf-life of only 5730 yearsUsed to date very recent eventsUsed to date very recent eventsCarbon-14 is produced in the Carbon-14 is produced in the upper atmosphereupper atmosphere

Useful tool for anthropologists, Useful tool for anthropologists, archaeologists, historians, and archaeologists, historians, and geologists who study very recent geologists who study very recent Earth historyEarth history

Only works for plant or animal Only works for plant or animal tissue, not rockstissue, not rocks

Dating with RadioactivityDating with Radioactivity

The Geological Time Scale

Structure of the geologic Structure of the geologic time scaletime scale

Names of the eonsNames of the eons Phanerozoic (“visible life”) – the Phanerozoic (“visible life”) – the most recent eon, began just over most recent eon, began just over 540 million years ago540 million years ago

ProterozoicProterozoic ArcheanArchean Hadean – the oldestHadean – the oldest eoneon

Geologic Time ScaleGeologic Time Scale

The Geological Time Scale

Structure of the geologic Structure of the geologic time scaletime scale

EraEra – subdivision of an eon – subdivision of an eonEras of the Phanerozoic eonEras of the Phanerozoic eon

Cenozoic (“recent life”)Cenozoic (“recent life”)Mesozoic (“middle life”)Mesozoic (“middle life”)Paleozoic (“ancient life”)Paleozoic (“ancient life”)

Eras are subdivided into periodsEras are subdivided into periodsPeriodsPeriods are subdivided into are subdivided into EpochsEpochs

Geologic Time ScaleGeologic Time Scale

Precambrian timePrecambrian timeNearly 4 billion years prior to Nearly 4 billion years prior to the Cambrian periodthe Cambrian period

Not divided into smaller time Not divided into smaller time units because the events of units because the events of PrecambrianPrecambrian history are not history are not known in great enough detailknown in great enough detail

Also, first abundant fossil evidence Also, first abundant fossil evidence does not appear until the beginning does not appear until the beginning of the Cambrianof the Cambrian

Difficulties in dating the Difficulties in dating the geologic time scalegeologic time scale

Not all rocks can be dated by Not all rocks can be dated by radiometric methodsradiometric methods

Grains comprising detrital sedimentary Grains comprising detrital sedimentary rocks are older than the rock in which rocks are older than the rock in which they formedthey formed

The age of a particular mineral in a The age of a particular mineral in a metamorphic rock may not necessarily metamorphic rock may not necessarily represent the time when the rock represent the time when the rock formed as porphyroblasts may grow for formed as porphyroblasts may grow for many Mamany Ma

The rock needs minerals with the right The rock needs minerals with the right parent and daughter isotope pairs and parent and daughter isotope pairs and in the right age span to be measureable.in the right age span to be measureable.

Difficulties in dating the Difficulties in dating the geologic time scalegeologic time scale

Datable materials (such as Datable materials (such as volcanic ash beds and igneous volcanic ash beds and igneous intrusions) are often used to intrusions) are often used to bracket various episodes in Earth bracket various episodes in Earth history and arrive at ageshistory and arrive at ages

Dates change as brackets become Dates change as brackets become narrower and methods refined; narrower and methods refined; e.g.,e.g., base of Triassic is now 252 base of Triassic is now 252 Ma, base of Permian is now 299 Ma, base of Permian is now 299 Ma, base of Cambrian is 543 Ma, base of Cambrian is 543 Ma…Ma…

Igneous events permitAbsolute dating

Of sedimentary rocks.

Dating Terrains On other Dating Terrains On other WorldsWorlds

CraterCratercountscounts& cross& crosscuttingcuttingrelatiorelatio

nsns mostlymostly

PrecambriaPrecambriann

Dendrochronology (wiggle matching)For wood in 14C realm <70Ka

Extinctions: Dinosaur Extinctions: Dinosaur GraveyardsGraveyards

Bolide impactBolide impact& Deccan & Deccan

TrapTrapVolcanism Volcanism

bothbothoccur at occur at 65Ma.65Ma.

Biodiversity Biodiversity dwindled for dwindled for

10 Ma prior to 10 Ma prior to this.this.

The K/T The K/T massive massive

extinction is extinction is debated.debated.