Geologic TimeGeologic Time
1
Goals for understanding geologic timeGoals for understanding geologic time
• Relative dating vs. Absolute datingRelative dating vs. Absolute dating
• Relative dating principles – how Relative dating principles – how geologists date layers of rockgeologists date layers of rock
• The dynamics of fossilsThe dynamics of fossils
• Techniques of radiometric datingTechniques of radiometric dating
• Why the earth is believed to be 4.6 b.y.Why the earth is believed to be 4.6 b.y.
• The Geologic Time ScaleThe Geologic Time Scale• what problems exist with the what problems exist with the geologic time scalegeologic time scale
Goals for understanding geologic timeGoals for understanding geologic time
• Relative dating vs. Absolute datingRelative dating vs. Absolute dating
• Relative dating principles – how Relative dating principles – how geologists date layers of rockgeologists date layers of rock
• The dynamics of fossilsThe dynamics of fossils
• Techniques of radiometric datingTechniques of radiometric dating
• Why the earth is believed to be 4.6 b.y.Why the earth is believed to be 4.6 b.y.
• The Geologic Time ScaleThe Geologic Time Scale• what problems exist with the what problems exist with the geologic time scalegeologic time scale
2
• Which geologic event took place first andWhich geologic event took place first and when?when?
• Which rock layer is older, and how is earthWhich rock layer is older, and how is earth history deciphered?history deciphered?
• How do we assign actual years to rock layers?How do we assign actual years to rock layers?
• Which geologic event took place first andWhich geologic event took place first and when?when?
• Which rock layer is older, and how is earthWhich rock layer is older, and how is earth history deciphered?history deciphered?
• How do we assign actual years to rock layers?How do we assign actual years to rock layers?
Consider:Consider:1111
2222
3333
4444
5555Sandstone (ss)Sandstone (ss)
Limestone (LS)Limestone (LS)
Shale (sh)Shale (sh)
ssss
Geology needs aTime Scale Geology needs aTime Scale
3
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” time scale, rules were To construct a “relative” 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
Which came first and WHY?Which came first and WHY?
To construct a “relative” time scale, rules were To construct a “relative” 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
11
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
12
Limestone (ls)
Shale (sh)
Sandstone (ss)
granitic rock
Original Horizontal Strata
13
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 = oldUnit 5 = youngUnit 1 = oldUnit 5 = young
14
Which strata is older?
1
2
3
4
554321 oldest
youngest
15
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.
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 = olderUnit 6 = youngestUnit 1 = olderUnit 6 = youngest
Which came first:Unit 5 or Unit 6?Which came first:Unit 5 or Unit 6?
16
Which is older, the faultor volcanic layer?
Which is older, the faultor volcanic layer?
Volcanic layerVolcanic layer
faultfault
What type of fault is this?
What type of fault is this?
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
NormalNormal
17
The Principle of The Principle of InclusionsInclusions
•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.)
18
Which “granites” are older and younger?Which “granites” are older and younger?
OLDEROLDERYOUNGERYOUNGER
19
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?
20
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 YoungerYounger21
I I I I this geology science class. this geology science class. this geology science class. this geology science 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.
Discuss with a friend:Discuss with a friend:Discuss with a friend:Discuss with a friend:
1.1.Explain the concept of Explain the concept of relative dating.relative dating.2.2. Draw a diagram, and explain each of theDraw a diagram, and explain each of the following dating principles:following dating principles:
Original HorizontalityOriginal HorizontalitySuperpositionSuperpositionCross-Cutting RelationsCross-Cutting Relations Inclusion PrincipleInclusion Principle
1.1.Explain the concept of Explain the concept of relative dating.relative dating.2.2. Draw a diagram, and explain each of theDraw a diagram, and explain each of the following dating principles:following dating principles:
Original HorizontalityOriginal HorizontalitySuperpositionSuperpositionCross-Cutting RelationsCross-Cutting Relations Inclusion PrincipleInclusion Principle
22
Ok – given the principles, what is wrong with this stack of rock (strata)
123567
Missing time – or does time really stop?
oldest
youngest
23
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 24
Xln rocksXln rocks
Sedimentary rocksSedimentary rocks
disconformitydisconformity
angularunconformity
angularunconformity
nonconformitynonconformity
25
I I I I this geology science class.this geology science class. this geology science class.this geology science class.
I will get an A on my exams and quizzesI will get an A on my exams and quizzesDiscuss with a friendDiscuss with a friend
1.1.Explain what an unconformity is andExplain what an unconformity is and what it representswhat it represents
2. Diagram pictures that represent the 2. Diagram pictures that represent the three types of three types of unconformitiesunconformities
26
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 dissolve
Rapid burial ofsediment coversthe bug – fossil
27
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)? 28
William Smith – Principle of Fossil Succession
Fossil organisms succeed one another in a definite and determinable order, and thereforeany 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
29
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
30
31
I I I I this geology science class. this geology science class. this geology science class. this geology science class.
I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.Discuss with a friend:Discuss with a friend:
1.1. Give 2 reasons why many Give 2 reasons why many organisms areorganisms are
are not fossilized.are not fossilized.
2.2.Explain the law of fossil Explain the law of fossil succession andsuccession and
how this law allows dating of how this law allows dating of strata.strata.
3.3.How has fossil succession How has fossil succession helped geologistshelped geologists
unravel earth history?unravel earth history?32
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
33
Radioactive decay of anunstable isotope atom
++
++++
++++
++ ++
++
++
++++
Decay process
• The time of decay can be measured.• Isotope decay does not vary under various weathering conditions.• Isotopes decay at a fixed rate.• One isotope will decay into another isotope.
• The time of decay can be measured.• Isotope decay does not vary under various weathering conditions.• Isotopes decay at a fixed rate.• One isotope will decay into another isotope.
U238 (Uranium)
Pb206 (lead)
34
How does radiometric dating work, and where does theage (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
Terms:
35
U-3U-235 Pb 207
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 1/2 1/2
Daughterelement
Parentelement
36
I I I I geology science class. geology science class. geology science class. geology science class.
I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.
1.1.Specifically define the differences Specifically define the differences between relative and absolute dating between relative and absolute dating techniques.techniques.
2. Define the following absolute dating terms:2. Define the following absolute dating terms: parent/daughter elements, half-lifeparent/daughter elements, half-life
3. Explain how the half-life is used to 3. Explain how the half-life is used to calculate an absolute age.calculate an absolute age.
37
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, Darwin, and others• consistent with relative dating techniques• allowed “absolute” dating on the Geologic Time Scale
38
Lets make a Geologic Time ScaleLets 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
39
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
4040
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
41
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”
42
Epochs
• not defined by extinction events, but % of fossils still living
• plants and animals found in the Pliocene epoch have living species today
• Eocene-few species surviving today
• Holocene• human’s time
Epochs
• not defined by extinction events, but % of fossils still living
• plants and animals found in the Pliocene epoch have living species today
• Eocene-few species surviving today
• Holocene• human’s time
Age ofReptiles
Amphibians
Age of fish
Invertebrates
How accurate is the Geologic Time Scale?How accurate is the Geologic Time Scale?
43
I I I I the geologic Time Scale. the geologic Time Scale. the geologic Time Scale. the geologic Time Scale. 1.1.You should be able to draw the 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.
2. List major characteristics of each2. List major characteristics of each period.period.
3. How did the strength of both absolute and3. How did the strength of both absolute and relative dating techniques contribute therelative dating techniques contribute the development of the geologic time scale?development of the geologic time scale?
44
Cenozoic, Mesozoic, PaleozoicEras
The Geologic Time Scale – How much of Earth history isrepresented?
12%12%
Precambrian EonPrecambrian Eon88%88%
Geologic Time Scale
45
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
46
How is the age of the Earth determined?How is the age of the Earth determined?
Why is it difficult to determine the ageof the earth? (think rock cycle)
• The external and internal forces constantly recycle earth material, obliterating rock clues to the earth’s past.
Why is it difficult to determine the ageof the earth? (think rock cycle)
• The external and internal forces constantly recycle earth material, obliterating rock clues to the earth’s past.
What evidence suggests a 4.6 b.y. old earth?What evidence suggests a 4.6 b.y. old earth?• Precambrian rocks (Acasta gneiss, northern Canada) date at 4.0 billion years.
• Mineral grain found in sedimentary rock (Australia) dates at 4.4 billion years.
• What does the mineral grain in a sedimentary rock indicate about the 4.4 b.y. age relationship?
• Precambrian rocks (Acasta gneiss, northern Canada) date at 4.0 billion years.
• Mineral grain found in sedimentary rock (Australia) dates at 4.4 billion years.
• What does the mineral grain in a sedimentary rock indicate about the 4.4 b.y. age relationship?
47
Acasta gneiss, northern Canada• known as the Acasta gneiss complex
• dated at the Hadean Eon (4.0 billion years old)
• part of the Canadian Slave craton
Acasta gneiss, northern Canada• known as the Acasta gneiss complex
• dated at the Hadean Eon (4.0 billion years old)
• part of the Canadian Slave craton
48
Evidence from space to age date the earth--Evidence from space to age date the earth--moon dust and meteorites:
• moon dust from Apollo astronauts dated at 4.55 billion years
• the Allende Meteorite:
• a carbonaceous chondrite meteorite that was found in Chihuahua, Mexico, 1969
• contains unaltered material from the formation of the solar system
• composed of tiny amounts of carbon that form the compounds of amino acids (essential for life)
• age dating of this and other meteorites is around 4.55 billion years
• based on earth rocks and interstellar space objects, earth is believed to be around 4.6 billion years old.
moon dust and meteorites:• moon dust from Apollo astronauts dated at 4.55 billion years
• the Allende Meteorite:
• a carbonaceous chondrite meteorite that was found in Chihuahua, Mexico, 1969
• contains unaltered material from the formation of the solar system
• composed of tiny amounts of carbon that form the compounds of amino acids (essential for life)
• age dating of this and other meteorites is around 4.55 billion years
• based on earth rocks and interstellar space objects, earth is believed to be around 4.6 billion years old.49
Allende Meteorite, Chihuahua, Mexico, Feb. 8, 1969Allende Meteorite, Chihuahua, Mexico, Feb. 8, 1969
carbonaceous chondritecarbonaceous chondrite
• unaltered material from our solar system
• contains carbon (3 parts/1000)
• some carbon compounds in the form of amino acids
dark areas – olivine withtrace amounts of iron andcarbon
dark areas – olivine withtrace amounts of iron andcarbon
calcium and aluminum oxidecompounds
• first matter to form during solar system formation
• older than earth
calcium and aluminum oxidecompounds
• first matter to form during solar system formation
• older than earth50
I I I I what geology can do for me. what geology can do for me. what geology can do for me. what geology can do for me.
I will get an A on my exams and quizzes.I will get an A on my exams and quizzes.
Discuss with a friend:Discuss with a friend:
1.1.Identify at least 2 problems with theIdentify at least 2 problems with the accuracy of the Geologic Time Scale.accuracy of the Geologic Time Scale.
2. Why does the Geologic Time Scale only2. Why does the Geologic Time Scale only represent about 12% of the earth’s geologicrepresent about 12% of the earth’s geologic
history (assuming the earth is 4.6 b.y. old)? history (assuming the earth is 4.6 b.y. old)?
51