Geologic TimeGeologic TimeDeciphering Earth History and Creating a Time ScaleDeciphering Earth History and Creating a Time Scale

• Interpreting Earth history is a prime goal among scientists.• Geologists seek clues from the rock record.• The history of Earth needs a calendar.

• The Geologic Time Scale – “The Earth’s history book”

Let’s take a field trip and decipher Earth history.

UniformitarianismThink back to chapter 1 – Intro to Geology,and write down the philosophical meaningbehind the concept of uniformitarianism.

As we take our “virtual” field trip, ponder the concept of uniformitarianism.

““The Present is the Key to the Past”The Present is the Key to the Past”““The Present is the Key to the Past”The Present is the Key to the Past”

Let’s take a field trip and decipher some Earth history.

Which stratigraphic layer is oldest?

AA

BB

CC

DD

EEFormation(mappable units)

FormationContacts

FormationsFormations and and ContactsContacts

Let’s take a field trip and decipher some Earth history.

Fault (displacing strata)Fault (displacing strata)

Is the fault younger or older than the strata?

11 11

22

22

33

33

Geologists use two types of dating methodsGeologists use two types of dating methodsto interpret Earth history.to interpret Earth history.

Relative Dating TechniquesRelative Dating Techniques::• Geologic events are arranged in Geologic events are arranged in chronological sequences using relative datingchronological sequences using relative dating principles ----- which came first?principles ----- which came first? No numerical values are applied. No numerical values are applied.

Absolute Dating TechniquesAbsolute Dating Techniques::• Radioactive isotopes (unstable elements) decayRadioactive isotopes (unstable elements) decay into stable atoms ----- rate of decay is measureableinto stable atoms ----- rate of decay is measureable with a numerical valuewith a numerical value

An actual number (numerical age) can beAn actual number (numerical age) can be applied. applied.

I I I I Geologic Dating. Geologic Dating.

I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.

1.1.Describe the difference Describe the difference between:between:

Relative Dating Relative Dating techniquestechniques

Absolute Dating Absolute Dating techniquestechniques

2. What is meant by John 2. What is meant by John Powell’s phrase:Powell’s phrase:

““The Grand Canyon is the Book of The Grand Canyon is the Book of Revelation ?”Revelation ?”

3. What role does 3. What role does uniformatarianism playuniformatarianism play

when interpreting the when interpreting the previous field tripprevious field trip

slides? slides?

I>clicker question

Uniformatarianism’s role in relatively dating a sequenceof strata is:

a.a. All strata is datable.All strata is datable.

b.b. Layers of strata that formed in the past are subject to Layers of strata that formed in the past are subject to erosive forces.erosive forces.

c. Earthquake faults displacing strata are always olderc. Earthquake faults displacing strata are always older than the displaced strata.than the displaced strata.

d. Observing geological processes today, ensures the d. Observing geological processes today, ensures the accuracy of dating a sequence of strata that has formedaccuracy of dating a sequence of strata that has formed in the geological past.in the geological past.

Relative Dating - placing the geologic occurrence in the proper sequence

Relative Dating - placing the geologic occurrence in the proper sequence

Which came first and WHY?Which came first and WHY?

To construct a “relative” geologic time scale, rules were To construct a “relative” geologic time scale, rules were established (principles of relative dating).established (principles of relative dating). Nicholas Steno Nicholas Steno (1636-1686)(1636-1686)

• Principle of Original HorizontalityPrinciple of Original Horizontality• Law of SuperpositionLaw of Superposition• Principle of Cross-Cutting RelationsPrinciple of Cross-Cutting Relations• Principle of InclusionsPrinciple of Inclusions• Principle of UnconformitiesPrinciple of Unconformities

Which came first and WHY?Which came first and WHY?

To construct a “relative” geologic time scale, rules were To construct a “relative” geologic time scale, rules were established (principles of relative dating).established (principles of relative dating). Nicholas Steno Nicholas Steno (1636-1686)(1636-1686)

• Principle of Original HorizontalityPrinciple of Original Horizontality• Law of SuperpositionLaw of Superposition• Principle of Cross-Cutting RelationsPrinciple of Cross-Cutting Relations• Principle of InclusionsPrinciple of Inclusions• Principle of UnconformitiesPrinciple of Unconformities

Let’s unravel some geologic history from observations of various formationsLet’s unravel some geologic history from observations of various formationsand their contacts.and their contacts.

Nicholas Steno – 1669 proposed the following relative dating principles:Nicholas Steno – 1669 proposed the following relative dating principles:

The Principle of Original Horizontality:•Sedimentary rock layers are deposited as horizontal strata.

•Any observed non-horizontal strata have been disturbed.

basin

Sediment inputSediment input

AA

BB

CC

Limestone (ls)

Shale (sh)

Sandstone (ss)

granitic rock

Original Horizontal Strata

The Principle of The Principle of SuperpositionSuperposition

In any undisturbed sequence of strata, In any undisturbed sequence of strata, the oldest stratum is at the bottom the oldest stratum is at the bottom of the sequence, and the youngest of the sequence, and the youngest stratumstratum

is on top.is on top.

The Principle of The Principle of SuperpositionSuperposition

In any undisturbed sequence of strata, In any undisturbed sequence of strata, the oldest stratum is at the bottom the oldest stratum is at the bottom of the sequence, and the youngest of the sequence, and the youngest stratumstratum

is on top.is on top.

1

2

3

4

5

Unit 1 = oldestUnit 1 = oldestUnit 5 = youngestUnit 5 = youngest

Which strata is oldest?

1

2

3

4

554321 oldest

youngest

The principle of The principle of Cross-Cutting RelationshipsCross-Cutting Relationships

•Any geologic feature that cuts across another Any geologic feature that cuts across another geologic feature is geologic feature is younger.younger.

1

2

3

4

5

6

Unit 1 = oldestUnit 6 = youngest

Which came first:Which came first:Unit 5 or Unit 6?Unit 5 or Unit 6?

Which is older, the faultor volcanic layer?

Which is older, the faultor volcanic layer?

Volcanic layerVolcanic layer

faultfault

Which is younger, the dikeor country rock?

Which is younger, the dikeor country rock?

country rockcountry rock

dikedike

Determine the relative age of the two dikes.Determine the relative age of the two dikes.

1122

The Principle of The Principle of Inclusions:Inclusions:

•A piece of rock (clast) that has become “included” A piece of rock (clast) that has become “included” in another rock body is in another rock body is olderolder than the rock body than the rock body it has become part of – why?it has become part of – why?

Rock body Rock body AA

Intrusion of pluton BIntrusion of pluton B

AA A

Older (Rock A was there first.)

Which “granites” are older and younger?Which “granites” are older and younger?

OLDEROLDERYOUNGERYOUNGER

Which rock body is older?:Which rock body is older?:

AA

BB

CC??

??

Can you identify the inclusionsfound in this Sierra NevadaMountain batholitic material?

Can you identify the inclusionsfound in this Sierra NevadaMountain batholitic material?

YoungestYoungest

OldestOldest

SuperpositionSuperposition

Original HorizontalityOriginal Horizontality

Cross-Cutting RelationshipCross-Cutting Relationship

A B C Asp Vn

Principle of Inclusions

Principle of Inclusions

Which granite is older?Which granite is older?

OlderOlder YoungerYounger

I I I I this geology class. this geology class. this geology class. this geology class.

I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.

3. Explain the concept of relative dating.4. Draw a diagram, and explain each of the following dating principles:

• Original Horizontality• Superposition• Cross-Cutting Relations• Inclusion Principle

3. Explain the concept of relative dating.4. Draw a diagram, and explain each of the following dating principles:

• Original Horizontality• Superposition• Cross-Cutting Relations• Inclusion Principle

I>clicker

In the diagram below, unit D is younger than ______In the diagram below, unit D is younger than ______

A.Units B and C because of original horizontality

B. Units E and F because of cross-cutting relations

C. Units E and F because of inclusions principle

D. Units B and A because of cross-cutting relations

A.Units B and C because of original horizontality

B. Units E and F because of cross-cutting relations

C. Units E and F because of inclusions principle

D. Units B and A because of cross-cutting relations

Ok – given the principles, what is wrong with this stratigraphic section?

123567

Missing time – or does time really stop?

oldest

youngest

The principle of The principle of UnconformitiesUnconformities

•rock surface that represents a period of erosion or rock surface that represents a period of erosion or non- depositionnon- deposition•referred to as “missing time”referred to as “missing time”•three major types of unconformities:three major types of unconformities:

•disconformitydisconformity•angular unconformityangular unconformity•non-conformitynon-conformity

Unconformity

disconformitydisconformity – unconformity in non-disturbed unconformity in non-disturbed sedimentary layerssedimentary layers

angular unconformityangular unconformity – uncon. lies between angled – uncon. lies between angled strata and overlying strata and overlying horizontal stratahorizontal strata

non-conformity non-conformity – sedimentary strata– sedimentary strata overlies crystallineoverlies crystalline rocks (ig and met)rocks (ig and met)

Igneous or metamorphic rockIgneous or metamorphic rock

Xln rocksXln rocks

Sedimentary rocksSedimentary rocks

DisconformityDisconformity

AngularUnconformity

AngularUnconformity

NonconformityNonconformity

DisconformityDisconformity

Angular UnconformityAngular Unconformity

NonconformityNonconformity

TimeTimeFormation of an angular unconformity

Deposition of layersDeposition of layers(original horizontality)(original horizontality)

Tectonic activity producesuplift of strata causing tilted

layering.

Active erosional processes Active erosional processes shaping the surfaceshaping the surface

A period of erosion creatingA period of erosion creatingthe unconformitythe unconformity

Continued deposition buryingContinued deposition buryingthe erosional surface to createthe erosional surface to create

an angular unconformityan angular unconformity

Grand Canyon StratigraphyTypes of Unconformities

I I I I this geology class. this geology class. this geology class. this geology class.

I will get an A on my exams and quizzesI will get an A on my exams and quizzes

5. Describe an unconformity and what5. Describe an unconformity and what what it represents regarding geologicwhat it represents regarding geologic history.history.

6. Diagram pictures that represent the 6. Diagram pictures that represent the three types of three types of unconformitiesunconformities

Fossils – evidence of past life or “time pieces,” the remains or traces of prehistoric life

Paleontology – study of fossils

How do we get a fossil? – preservation of past lifeHow do we get a fossil? – preservation of past life

• 2 conditions must exist for preservation2 conditions must exist for preservation• rapid burialrapid burial• possession of hard partspossession of hard parts

Prehistoric bug

Bug dies Bug soft parts areeaten or dissolved

Rapid burial ofsediment coversthe bug – fossil

Fossils – evidence of past life or “time pieces,”

the remains or traces of prehistoric lifePreservation of fossils

• Small percentage of fossils preserved throughout geologic time – WHY?

Most organisms composed of soft parts.

Organisms with hard parts and within a sedimentary environment are favored.

Very rare to see vast array of other life forms

How do fossils help scientists relatively date layersof rock (strata)?

William Smith – Principle of Fossil Succession

Fossil organisms succeed one another in a definite and determinable order, and ,therefore,any time period can be recognized by its fossilcontent.

“Fossils are arranged according to their age by using the law of superposition.”

Fossil succession:• allows geologists to age date wide geographicalallows geologists to age date wide geographical areasareas• documents the evolution of life

• Age of mammals• Age of reptiles• Age of fish Oldest

Youngest

How do fossils help date rocks?1200 miles

Which fossils are the youngestand oldest?

DisconformityDisconformity

11

22

33

55

66

77

22

33

44

6677

I I I I this geology class. this geology class. this geology class. this geology class.

I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.

7. Give 2 reasons why many organisms 7. Give 2 reasons why many organisms are not fossilized.are not fossilized.

8. Explain the law of fossil succession and8. Explain the law of fossil succession and how this law allows dating of strata.how this law allows dating of strata.9. How has fossil succession helped geologists9. How has fossil succession helped geologists unravel earth history?unravel earth history?

I>clickerI>clicker

The principle of fossil succession states:

a. Fossils discovered in lower sections of rock layers are younger than fossils above.

b. Most fossil soft parts are preserved.

c. The age of fossils is marked by the preservation of all hard parts.

d. Fossils can be dated by using the principle of superposition.

The principle of fossil succession states:

a. Fossils discovered in lower sections of rock layers are younger than fossils above.

b. Most fossil soft parts are preserved.

c. The age of fossils is marked by the preservation of all hard parts.

d. Fossils can be dated by using the principle of superposition.

OK – We have relative dating and fossils – How dowe get “absolute” ages on the rocks (numbers)?

Radiometric dating – applying a number• radioactive atoms (isotopes) decay at a constant rate over time

Review of the atom:• Atomic number = • Atomic mass = • Isotope =

number of protons (+) chargednumber of protons + neutrons

unstable atoms with varying numbers of neutrons (atomic masses)

Unstable atoms will decay into stable atoms by losing neutrons and protons at a constant rate.

U238 Pb206DecaysDecays

• Isotopes decay at a fixed rate.• Decay rate is measureable.• Isotope decay is not influenced by weathering.• One isotope will decay into another isotope.

Radio active decay U238 Pb206

Alpha emissionAlpha emissionMass # reduced by 4Mass # reduced by 4

Atomic # reduced by 2Atomic # reduced by 2

Beta emissionBeta emissionMass # remains unchangedMass # remains unchanged

Atomic # increases by 1Atomic # increases by 1

How does radiometric dating work, and where does the age (number) come from?

Half life: the time required for ½ of the parent to decay into the daughter element

Parent element: the “beginning” element that contains 100% of radioactive particles

Daughter element: the element that the parent element decays into (or turns into over time)

Parent Isotope

Stable Daughter Product

Currently Accepted Half-Life Values

Uranium-238 Lead-206 4.5 billion years

Uranium-235 Lead-207 704 million years

Thorium-232 Lead-208 14.0 billion years

Rubidium-87 Strontium-87 48.8 billion years

Potassium-40 Argon-40 1.25 billion years

Samarium-147 Neodymium-143 106 billion years

U-3U235 Pb207

704 m.y.704 m.y.

1.4 b.y.1.4 b.y.

2.1 b.y.2.1 b.y.

1 half life = 704 million years

1/2 1/4 1/8

Daughterelement

Parentelement

I I I I absolute dating. absolute dating. absolute dating. absolute dating.

I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.

11. Define the following absolute dating terms:11. Define the following absolute dating terms: parent/daughter elements, half-lifeparent/daughter elements, half-life

12. Explain how the half-life is used to 12. Explain how the half-life is used to calculate an absolute age.calculate an absolute age.

I>Clicker

A certain material contains 1/64 of parent material. IfA certain material contains 1/64 of parent material. Ifthe half-live for decay is 2.3 million years, what is thethe half-live for decay is 2.3 million years, what is theage of the material?age of the material?

A. 11.6 m.y.A. 11.6 m.y.

B. 13.8 m.y.B. 13.8 m.y.

C. 10.5 m.y.C. 10.5 m.y.

D. 3.18 b.yD. 3.18 b.y..

What is the importance of radiometric dating?• produced thousands of dates for earth events

• rocks have been dated at more than 3 b.y.• granite in South Africa dated at 3.2 b.y.• granite contains inclusions of quartzite• quartzite inclusions must be older• Acasta gneiss in Northern Canada – 4.0 b.y.• Earth believed to be 4.55 (4.6) b.y. old

Radiometric dating:• vindicated the ideas of Hutton, Steno, and others• consistent with relative dating techniques• allowed “absolute” dating on the Geologic Time Scale

Lets make a Geologic Time Scale!Lets make a Geologic Time Scale!

The Geologic Time Scale:• It combines both relative and absolute dating.

• Created during the nineteenth century in Western Europe and Great Britain

• Sub-divides the 4.6 billion-year history of the earth• Eons• Eras• Periods• Epochs

Relative dating + Absolute datingRelative dating + Absolute dating

Hadean• represents the earth’s time of formation• no rocks are represented• “hellish” conditions

Hadean• represents the earth’s time of formation• no rocks are represented• “hellish” conditions

Archean• Single-cell life developed• most “ancient” rocks found• preserved rocks at the base of the Archean

Archean• Single-cell life developed• most “ancient” rocks found• preserved rocks at the base of the Archean

Proterozoic• Multi-celled, soft body organisms• “early life”

Proterozoic• Multi-celled, soft body organisms• “early life”

Precambrian

Phanerozoic• “visible life”• fossil record becomes more detailed• animals have hard shells and skeletons

Phanerozoic• “visible life”• fossil record becomes more detailed• animals have hard shells and skeletons

Building the Geologic Time Scale

Paleozoic Era• known as ancient life

• life progressed from marine invertebrates to fish, amphibians, and reptiles

Paleozoic Era• known as ancient life

• life progressed from marine invertebrates to fish, amphibians, and reptiles

Mesozoic Era• marks the rise in dinosaurs

• dominant vertebrates

• first flowering plants

• first shrew-like mammals

Mesozoic Era• marks the rise in dinosaurs

• dominant vertebrates

• first flowering plants

• first shrew-like mammals

Cenozoic Era• birds and mammals flourished

• appearance of man

Cenozoic Era• birds and mammals flourished

• appearance of man

Periods based on:

• fossil types• massive extinctions• geographical locations• characteristics of strata

Periods based on:

• fossil types• massive extinctions• geographical locations• characteristics of strata

Cambrian period• animals with hard shells• diversification of life “the Cambrian explosion”

Cambrian period• animals with hard shells• diversification of life “the Cambrian explosion”

Cretaceous, Jurassic, Triassic• age of reptiles• dinosaurs dominant• massive dinosaur extinction at 65 m.y. –Cretaceous• “Jurassic Park”

Cretaceous, Jurassic, Triassic• age of reptiles• dinosaurs dominant• massive dinosaur extinction at 65 m.y. –Cretaceous• “Jurassic Park”

EpochsEpochs

• not defined by extinctionnot defined by extinction events, but % of fossilsevents, but % of fossils still livingstill living

• plants and animals foundplants and animals found in the Pliocene epoch in the Pliocene epoch have living species todayhave living species today

• Eocene-few species Eocene-few species surviving todaysurviving today

• HoloceneHolocene• human’s timehuman’s time

Age ofReptiles

Amphibians

Age of fish

Invertebrates

How accurate is the Geologic Time Scale?How accurate is the Geologic Time Scale?

I I I I the Geologic Time Scale. the Geologic Time Scale. the Geologic Time Scale. the Geologic Time Scale. 13. You should be able to draw the 13. You should be able to draw the GeologicGeologic Time ScaleTime Scale and label it with the following: and label it with the following:

Eons, Eras, Periods, and Cenozoic/Eons, Eras, Periods, and Cenozoic/ Tertiary epochs.Tertiary epochs.

14. List major characteristics of each14. List major characteristics of each period.period.

15. How did the strength of both absolute and15. How did the strength of both absolute and relative dating techniques contribute to therelative dating techniques contribute to the development of the Geologic Time Scale?development of the Geologic Time Scale?

44

I>clicker

What time “slots” below arrange the geologic timescale from largest to smallest divisions?

a. period, era, eon, epoch

b. era, eon, period, epoch

c. eon, period, epoch, era

d. eon, era, period, epoch

What time “slots” below arrange the geologic timescale from largest to smallest divisions?

a. period, era, eon, epoch

b. era, eon, period, epoch

c. eon, period, epoch, era

d. eon, era, period, epoch

Cenozoic, Mesozoic, PaleozoicEras

The Geologic Time Scale – How much of Earth history isrepresented?

12%12%

Precambrian EonPrecambrian Eon88%88%

Geologic Time Scale

Difficulties in Dating the Geologic Time ScaleDifficulties in Dating the Geologic Time Scale

• Not all rocks can be dated radiometrically.Not all rocks can be dated radiometrically.• all minerals must contain 100% parent atoms.all minerals must contain 100% parent atoms.

• Sedimentary rocks can only rarely be dated.Sedimentary rocks can only rarely be dated.• some parent atoms come from pre-existing rockssome parent atoms come from pre-existing rocks that have been weathered and transported.that have been weathered and transported.• sedimentary rocks are dated in proximity of sedimentary rocks are dated in proximity of igneous bodies.igneous bodies.

• Metamorphic rocks are challenging.Metamorphic rocks are challenging.• some minerals do not necessarily represent thesome minerals do not necessarily represent the time when the rock was formedtime when the rock was formed