topic 8: geologic time scale early attempts at estimating geologic time relative dating principles:...
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Topic 8: Geologic Time ScaleTopic 8: Geologic Time Scale
Early Attempts at Estimating Geologic Time
Relative Dating Principles:
- What is Relative Dating?- Relative Dating Principles:
=> Principle of Superposition=> Principle of Original Horizontality
=> Principle of Cross-Cutting Relationships
=> Principle of Fossil Succession- What are Unconformities?
Topic 8: Geologic Time ScaleTopic 8: Geologic Time Scale
Fossils: Evidence of Past Life:- Types of Fossils- Conditions Favoring Preservation- Fossils and Rock Correlation
What is Absolute Dating?- Understanding Radioactivity- Radiometric Dating:
=> Potassium-Argon Dating=> Dating with Carbon-14
- Importance of Radiometric Dating
Topic 8: Geologic Time ScaleTopic 8: Geologic Time Scale
The Geologic Time Scale:
- Structure of the Geologic Time Scale- Precambrian Time- Difficulties in Dating Geologic Time Scale
Early Ideas of Geologic TimeEarly Ideas of Geologic Time
Earth’s rocks and processes show that Earth is very old
But Earth’s true age remained subject of great debate between 15th and 19th centuries
Catastrophists argued for a shortened Earth history of less than 6000 years
Early Ideas of Geologic TimeEarly Ideas of Geologic Time
Whereas, Uniformitarians argued for immensity of geologic time spanning millions of years
Though the evidence of Earth’s history is concealed in its rocks, both group had no way to estimate its true age
However, geologic time had been estimated in a number of ways based on:- salinity of ocean water:
(90-100million yrs by John Joly in 1899)
Early Ideas of Geologic TimeEarly Ideas of Geologic Time
- heat loss: (20-40 million years old, but ignored radioactive addition of heat)
- thickness of fossiliferous layers: (3 million to 1.5 billion yrs or 500million yrs assuming a sedimentation rate of 0.3m/1000yrs)
None of these early efforts produced any reliable numerical dating of Earth’s age
Early Ideas of Geologic TimeEarly Ideas of Geologic Time
However, later discovery of radioactivity in 1896 by Henri Becquerel allowed scientists to accurately determine numerical dates or age of rocks
Before then, geologists first relied on relative dating techniques for arranging geologic events in their chronological order of occurrence
Early Ideas of Geologic TimeEarly Ideas of Geologic TimeHence, the development of the geologic time
scale is in two phases:- relative dating of rock layers & features- radiometric dating utilizing the radioactive clocks in rocks
But, interpreting geologic time from rock layers remained a big challenge because:- rock layers are often discontinuous, as indicated by unconformities in rocks- whereas, geologic time is continuous
Types of UnconformitiesTypes of Unconformities
Rock layers are said to be conformable if the layers were deposited over a large span of time without interruption
But no place on Earth has a complete set of conformable strata
Unconformities mark discontinuities or missing gaps in rock records
Types of UnconformitiesTypes of Unconformities
Therefore, unconformities represent periods:
- when deposition ceased- when uplift and erosion may have removed existing rock layers - when subsidence and another phase of sedimentation resumed
Unconformities help us to identify the length of time not represented by existing rock layers
Types of UnconformitiesTypes of Unconformities
There are three types of unconformities:
- Angular unconformity- Disconformity- Non-Conformity
All three types of unconformities can be identified along the walls of the Grand Canyon
Angular UnconformityAngular Unconformity Involves 2 sedimentation phases
1st sedimentation followed by strata deformation and exposed strata subjected to erosion phase
2nd sedimentation phase on top of the deformed 1st set of strata
The 2 sets of sedimentation separated by erosional surface called angular unconformity
Angular UnconformitiesAngular Unconformities
Angular unconformity occurs where the older strata dip at an angle different from that of the younger strata
Angular UnconformityAngular Unconformity
DisconformityDisconformity Involves 2 sedimentation phases
New set of layers deposited and uplifted without deformation
Exposed layers subjected to erosion leading to loss of layers
Another set of layers deposited on top of the 1st set of strata and the erosional surface separating them is called Disconformity
DisconformityDisconformity Disconformity occurs when
rock strata on both sides of the unconformity are essentially parallel
Non-conformityNon-conformity
Occurs when layers of sedimentary rocks are unconformably deposited over the surface of a non-sedimentary rock like igneous intrusion
It occurs where the break separates older metamorphic or igneous rocks from younger sedimentary rock strata
Cross Section of the Grand Canyon Cross Section of the Grand Canyon (A Trip Through Time)(A Trip Through Time)
Modern Geologic Time ScaleModern Geologic Time Scale
The development of modern geologic time scale relied on two methods:- Relative time method
- Absolute time Method
Relative Time:- determines the chronology or time sequence of geologic events and strata
Relative TimeRelative Time
- determines which geologic event or strata is older without knowing the actual age in years
- Chronological ordering of geologic eventsor strata is based on a number of principles:
=> Principle of Superposition=> Principle of Cross-Cutting
Relationships=> Principle of Inclusion=> Principle of Faunal Succession
Principle of SuperpositionPrinciple of Superposition
Nicolaus Steno is credited to have used this simple principle to sequence rock layers of a rock outcrop in Italy
It states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it
Principle of SuperpositionPrinciple of Superposition
[1]
[2]
[3]
[4]
[5]
Principle of Original HorizontalityPrinciple of Original Horizontality
It states that water laid sediment layers are generally deposited in horizontal position
This means that flat-laid layers have not been disturbed and retain their original horizontality
But, crustal deformation causes the layers to be moved to become folded or inclined
Distortion of Rock Layers into FoldDistortion of Rock Layers into Fold
Principle of Cross-Cutting RelationshipsPrinciple of Cross-Cutting Relationships
It states that geologic features are always younger than the rocks they cut across
Hence, the rock intrusion (F) is younger than the sedimentary rock layers and sill (D) it cuts across
Principle of Cross Cutting Relationships
Principle of InclusionPrinciple of Inclusion
It states that a fragment of a rock found within another rock is older than the host rock
Relative DatingRelative Dating
We can use the principles of relative dating to arrange the rock features on the next slide from the oldest to the youngest
But we are not able to determine their actual ages in years
By correlating the rocks from one region to another, a more comprehensive view of the geologic history of a region emerges
A
B
Relative DatingRelative Dating
We can use the principles of relative dating to arrange the rock features on the next slide from the oldest to the youngest
But we are not able to determine their actual ages in years
By correlating the rocks from one region to another, a more comprehensive view of the geologic history of a region emerges
Relative DatingRelative Dating
When great distances separate big regions (e.g. continents), geologists must rely on fossil records for rock correlation to effectively reveal a more comprehensive view of the geologic history an entire continent or between continents
Fossils are time indicators and useful in rock correlation
Types of FossilsTypes of Fossils
Petrified Wood
Trilobite Fossils (mold & Cast)
Carbon Film
Fish Impression Insect in Amber (hardened tree resins
Coprolite(Fossil Dung)
Principle of Fossil SuccessionPrinciple of Fossil Succession
According to William Smith, distant rock strata could be identified and correlated by their distinctive fossil content
The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order, therefore any time period can be recognized by its fossil content
fossils document the evolution of life through time, hence fossils help to correlate rocks of similar age
Overlapping Ranges of Fossil GroupsOverlapping Ranges of Fossil Groups
Principle of Fossil SuccessionPrinciple of Fossil SuccessionIndex fossils are best for matching rocks of the same
age (i.e. rock correlation) because:- they have a wide geographical spread- they are limited to a short span of geologic time
But rock formations may not contain index fossils, hence an assemblage or group of fossils are best for dating rocks
The end product of relative dating method is the production of the standard geologic column
The Standard Geologic ColumnThe Standard Geologic Column
The standard geologic column is a composite columnar section showing the chronological order of known strata fitted together on the basis of fossils or other principles of relative age
Sequencing of geologic events based on the study of rock strata in Europe in mid-19th century
Rock strata from other parts of the world were matched with their appropriate units based on fossil similarity
The Standard Geologic ColumnThe Standard Geologic Column
The Period of each major units is named after the geographic area where the rock layer was exposed and/or first described
The time units of the geologic time scale are:- Eons - Era- Periods- Epoch
The Standard Geologic ColumnThe Standard Geologic Column
The Eons is the largest time interval, divided into: Phanerozoic, Proterozoic, Archean and Hedean
The vast of time after 542 million years is often referred to as Precambrian and accounting for over 88% of Earth’s history
- Phanerozoic (visible life): most recent with plentiful evidence of past life and well preserved hard parts. Divided into 3 Eras, Paleozoic, Mesozoic and cenozoic
The Standard Geologic ColumnThe Standard Geologic Column
- Proterozoic (earlier life): evidence of multi-cellular organisms without preservable parts
- Archean (ancient): oldest rocks known on earth with microscopic life forms
- Hadean is the oldest eon with no rock record on Earth. Rocks of this age are present in
other planets
The Standard Geologic ColumnThe Standard Geologic ColumnEons are subdivided into Eras on the basis of life
forms found in their corresponding rock layers
Phanerozoic Eon is subdivided into three Eras and their names refer to important differences in dominant life-forms:
- Cenozoic (recent life) - Mesozoic (middle life)- Paleozoic (old life)
GEOLOGIC TIME SCALE
The Standard Geologic ColumnThe Standard Geologic ColumnERA PERIOD EPOCH ORIGIN OF NAMES
Cenozoic(recent life)
QuaternaryHolocene Greek for wholly recent
Pleistocene Greek for most recent
Tertiary
Pliocene Greek for more recent
Miocene Greek for less recent
Oligocene Greek for slightly recent
Eocene Greek for dawn of the recent
Paleocene Greek for early dawn of the recent
Mesozoic(middle life)
Cretaceous Latin for chalk, after chalk cliff of Southern England and France
Jurassic Jura Mountains, Switzerland/France, first studied
Triassic Threefold character of these rocks in Germany
Paleozoic(ancient life)
Permian Province of Perm, Russia, first studied
Pennsylvanian State of Pennsylvania, rock yielding much coal
Mississippian Mississippi River, rocks well exposed
Devonian Devonian Province, county of SW England, first studied
Silurian Silures, ancient Celtic tribe of Wales
Ordovician Ordovices, ancient Celtic tribe of Wales
Cambrian Cambria, Roman name for Wales, 1st complex life
The Standard Geologic ColumnThe Standard Geologic Column
Each Era of the Phanerozoic eon is divided into smaller time units called Periods - Paleozoic Era has 7;- Mesozoic Era has 3; - Cenozoic Era has 3
Each Period is further subdivided into still smaller time units called Epochs
Radiometric Dating Radiometric Dating (Absolute Time Dating)(Absolute Time Dating)
It pinpoints the specific time when geologic events took place
This is how the age of Earth is determined to be 4.5 billion years old using radiometric dating techniques
It involves the application of radioactivity in dating
Radiometric Dating Radiometric Dating
Therefore, understanding of radioactivity allows us to accurately determine the number of years a geologic event to place
Every atom has a nucleus that contains protons and neutrons
Each proton is positively charged and a neutron is neutral because it contains a positive & a negative charge
Radiometric Dating Radiometric Dating
Electron is negatively charged and orbits the nucleus
Atomic number is the number of protons in the nucleus and identifies each chemical element
For example: atomic number of carbon is 6 and Zinc is 30
Radiometric Dating Radiometric Dating
The mass of an atom is the sum of its protons and neutrons and called atomic mass number
The number of neutrons in the nucleus can vary and the varieties are called isotopes
For example: Carbon has two isotopes, Carbon-12 and Carbon-14
Radiometric Dating Radiometric Dating
The nuclei of some isotopes are unstable and undergo spontaneous change to form new daughter product
This process is called radioactive decay
Unstable isotopes can decay in three ways:- Emission of alpha particle (Alpha Emission)- Emission of beta particle (Beta (Electron) Emission)- Electron capture
Common Types of Radioactive DecayCommon Types of Radioactive Decay
Radioactive Decay: Alpha EmissionRadioactive Decay: Alpha Emission
It emits alpha particle made of 2 protons and 2 neutrons, hence:- mass number is reduced by 4- atomic number is reduced by 2
Radioactive Decay: Beta (Electron) EmissionRadioactive Decay: Beta (Electron) Emission
It emits an electron that was part of a neutron, hence:- no change in the atomic mass number- atomic number is increased by 1
Radioactive Decay: Electron CaptureRadioactive Decay: Electron Capture
It captures an electron that combines with a proton to form a neutron, hence:- no change in the atomic mass number- atomic number is reduced by 1
Radioactive DecayRadioactive Decay
In all three types of radioactive decay, the number of proton in the nucleus (i.e. atomic number) will change
Question: When Uranium-238 decays to Thorium-234, what type of decay must take place?
U238 Decays Th234
Radioactive DecayRadioactive Decay
Question: When Thorium-234 decays to Protactinium-234, what type of decay must occur?
Th234 Decays Pa234
When Uranium-238 decays, it emits 8 alpha particles and 6 beta particles before finally becoming the stable daughter end product (Pb-206)
Radioactive DecayRadioactive Decay
Before the stable end product, Lead-206 (Pb-206) is reached, many different isotopes are produced as intermediate steps
Radioactive DecayRadioactive Decay
Why we use radioactivity in determining accurate absolute numerical dates is that the rate of decay for each radioactive isotope is constant and can be accurately determined
The rate of radioactive decay for each unstable isotope is expressed as its half-life
Half-life is the time for one-half of the nuclei in an unstable isotope to decay
Isotopes Half-Life of Parent
(years)
Effective Dating Range (years)
Minerals & other materials that can be Dated
Parent Decay System
Daughter
Uranium-238Uranium-235Thorium-232
α + β decayα + β decayα + β decay
Lead-206Lead-207Lead-208
4.5 billion710 million14 billion
10m – 4.6b10m – 4.6b10m – 4.6b
Zircon & UraniniteZircon & UraniniteZircon & Uraninite
Potassium-40 β captureβ decay
Argon-40Calcium-40
1.3 billion 50,000-4.6b Muscovite, BiotiteHornblende, whole volcanic rock
Rubidium-87 β decay Strontium-87 47 billion 10m -4.6b Muscovite, biotite,Potassium feldspar,Whole metamorphicor igneous rock
Carbon-14 β decay Nitrogen-14 5,730 ± 30 100-70,000 Wood, charcoal, peat, grain, plant materials, glacier ice, Bone, tissue, animal materials, cloth, shell, Ocean water, Groundwater, stalactites
Radioactive DecayRadioactive Decay
After one half-life, half of the radioactive atoms have decayed to the stable daughter product
Hence the number of radioactive parent atoms and stable daughter product atoms is the same (1:1)
After a second half-life, half the remaining radioactive atoms will decay
The Radioactive-Decay Curve The Radioactive-Decay Curve (Exponential Change)(Exponential Change)
Radioactive DecayRadioactive Decay
Compared to the number of radioactive atoms at the beginning, there are now only ¼ (one-quarter) as many (parent-daughter ratio is 1:3)
After the 3rd half life, the traction of the original radioactive atoms remaining will be ⅛
The percentage of radioactive atoms that decay during one half-life is always the same (50%)
Radioactive DecayRadioactive Decay
But the actual number of atoms that decay with the passage of each half-life continues to decrease
Radioactive dating is not good for dating sedimentary rocks because the rock particles are from rocks of different ages
Radiometric dating is best for igneous rock because its crystals form at the same time
Radioactive DecayRadioactive Decay
The numerical dates for sedimentary layers are usually determined by examining their relationships to the surrounding igneous rocks
Radioactive DecayRadioactive Decay
Through a combination of relative and radiometric dating techniques, 20th century geologists have fitted a scale of absolute time to the geologic column worked out in the 19th century
The 19th century standard geologic columns established by the ordering of rock strata into relative ages has been fully confirmed by radiometric dating
Radiocarbon DatingRadiocarbon Dating
Carbon-14 is continuously created in the atmosphere through the bombardment of Nitrogen-14 by neutrons created by cosmic radiation
Radiocarbon DatingRadiocarbon Dating
Carbon-14 decays back to nitrogen-14 by beta (electron) emission
The proportion of Carbon-14 is nearly constant in the atmosphere and in balanced proportion
Once living organisms die, the balance is upset and begins to decrease
Radiocarbon DatingRadiocarbon Dating
Radiocarbon dating involves the determination of the proportion of the unstable Carbon-14 has decayed
With a short half-life of only 5730 years, it is best for dating of organic materials and ice-sheets
Isotopes Half-Life of Parent
(years)
Effective Dating Range (years)
Minerals & other materials that can be Dated
Parent Decay System
Daughter
Uranium-238Uranium-235Thorium-232
α + β decayα + β decayα + β decay
Lead-206Lead-207Lead-208
4.5 billion710 million14 billion
10m – 4.6b10m – 4.6b10m – 4.6b
Zircon & UraniniteZircon & UraniniteZircon & Uraninite
Potassium-40 β captureβ decay
Argon-40Calcium-40
1.3 billion 50,000-4.6b Muscovite, BiotiteHornblende, whole volcanic rock
Rubidium-87 β decay Strontium-87 47 billion 10m -4.6b Muscovite, biotite,Potassium feldspar,Whole metamorphicor igneous rock
Carbon-14 β decay Nitrogen-14 5,730 ± 30 100-70,000 Wood, charcoal, peat, grain, plant materials, glacier ice, Bone, tissue, animal materials, cloth, shell, Ocean water, Groundwater, stalactites
Review Questions for Topic 8Review Questions for Topic 8
1. __________ dates pinpoint the time in history when something took place.
A. Relative B. chronologicalC. numerical
2. How many protons (p) and neutrons (n) comprise an alpha particle?
A. 1p, 1n B. 3p, 2n C. 2p, 2nD. 29, 1n
3. Which of the following presents the eras of the Phanerozoic eon in the correct order, from most recent to oldest?
Review Questions for Topic 8Review Questions for Topic 8
A. Mesozoic, Cenozoic, PaleozoicB. Cenozoic, Paleozoic, MesozoicC. Paleozoic, Mesozoic, CenozoicD. Cenozoic, Mesozoic, Paleozoic
4. The number of _________ in an atom gives the atom its atomic number.
A. protons B. electrons C. positionsD. neutrons
5. The number of protons plus neutrons in an atom’s nucleus is the atom’s __________ number.
A. mass B. isotope C. key
Review Questions for Topic 8Review Questions for Topic 8
D. atomic
6. Atoms which have the same atomic numbers, but different mass numbers, are referred to as __________.
A. ions B. catalysts C. isotopesD. variants
7. When a beta particle is emitted from the nucleus of an atom:
A. neutron becomes a protonB. an electron changes to a neutron
C. a proton changes to a neutron
Review Questions for Topic 8Review Questions for Topic 8
D. a proton changes to a neutron
8. After the third half-life, the fraction of the original radioactive atoms remaining will be __________.
A. one-half B. one-fourth C. one-eighthD. one-sixteenth
9. Collectively, the Hadean, Archean, and Proterozoic eons are often referred to as the __________.
A. Paleozoic B. Mesozoic C. PleistoceneD. Precambrian
Review Questions for Topic 8Review Questions for Topic 8
10. Isotopes of the same atom will have different numbers of __________.
A. protons B. ions C. electronsD. neutrons
11. The rate of radioactive decay for each unstable radioactive isotope is expressed as its __________.
A. half-life B. degeneration curveC. deep span D. radioactive term
12. No place on Earth has a complete set of conformable strata.
Review Questions for Topic 8Review Questions for Topic 8
A. True B. False
13. Each era is divided into smaller units called __________, which may be further divided into __________.
A. stages; period B. epochs; agesC. ages; stages D. periods; epochs
14. __________ dating is the task of placing rock units and geologic events in their proper sequence.
A. Numerical B. RelativeC. Sequence D. Radioactive
Review Questions for Topic 8Review Questions for Topic 8
15. In the geologic time scale, era names represent important differences in __________.
A. continental upliftB. mountain building episodesC. volcanic eventsD. dominant life-forms
16. With each type of radioactive decay, the atomic number of the radioactive atom changes.
A. True B. False
Review Questions for Topic 8Review Questions for Topic 8
17. When we observe strata that are inclined at a steep angle, we can conclude that they were tilted sometime after their deposition by applying this relative dating principle.
A. superposition B. cross-cuttingC. inclusion D. original horizontality
18. Breaks or gaps in the rock record in which strata on both sides of the unconformity are parallel is:
A. a disconformity B. a nonconformityC. an angular unconformity D. a conformity
Review Questions for Topic 8Review Questions for Topic 8
19. On the geologic time scale, __________ represent the greatest expanses of time.
A. epochs B. eras C. eonsD. periods E. none of the above
20. We are presently living in the __________ era.
A. Hadean B. Paleozoic C. CenozoicD. Mesozoic
21. The spontaneous breaking apart of unstable atomic nuclei is called radioactivity.
A. True B. False
Review Questions for Topic 8Review Questions for Topic 8
22. Which one of the following is NOT a type of unconformity?
A. transitional unconformityB. disconformityC. nonconformityD. angular unconformity
23. The percentage of radioactive atoms that decay during each half-life is always fifty percent; however the actual number of atoms that decay continually decreases.
A. True B. False
Review Questions for Topic 8Review Questions for Topic 8
24. Which one of the following is NOT a type of radioactive decay?
A. alpha capture B. beta emissionC. neutron captureD. electron capture
25. What type of unconformity consists of folded sedimentary rocks overlain by younger, more flat-lying strata?
A. nonconformity B. reversed conformityC. disconformity D. angular unconformity
Review Questions for Topic 8Review Questions for Topic 8
26. Which is the subdivision of an epoch?A. Era B. epoch C. eon D.period E. none
27. Mesozoic is an example of …………..
A. Era B. epoch C. eon D.period E. none
28. Eons are divided into one of these: A. Era B. epoch C. eon D.period
Review Questions for Topic 8Review Questions for Topic 8E. none
29. Which is the smallest time span on the geologic time scale?A. Era B. epoch C. eon D. period E.
none
30. Cambrian and Jurassic are examples of …….A. Era B. epoch C. eon D. period E.
none
31. The age of the Earth is about ………….A. 4.5 million yrs B. 540 million yrsC. 4.5 billion yrs D. 12.6 billion yrs
Review Questions for Topic 8Review Questions for Topic 8
32. The Phanerozoic era began about … years agoA. 4.5 billion yrs B. 2.5 billion yearsC. 540 million yrs D. 248 million years
33. The Hadean, Archean, and Proterozoic are:A. Eons that collectively comprise PrecambrianB. epochs of the quaternary PeriodC. eras of the Phanerozoic eonD. Periods of the Paleozoic era
Review Questions for Topic 8Review Questions for Topic 8
34. Which era of the Phanerozoic eon has the most Periods?A. Mesozoic B. Paleozoic C. CenozoicD. all have the same number
35. Eocene, Pleistocene, and Holocene are examples of _____
A. eons B. Periods C. ErasD. Epochs
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