Topic 8: Geologic Time Scale Early Attempts at Estimating Geologic Time Relative Dating Principles:...

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?

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

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

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)

- 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

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

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

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

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

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

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 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 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

- 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

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

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

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

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

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)

Before the stable end product, Lead-206 (Pb-206) is reached, many different isotopes are produced as intermediate steps

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

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)

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%)

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

The numerical dates for sedimentary layers are usually determined by examining their relationships to the surrounding igneous rocks

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

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 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?

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

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

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

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.

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

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

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

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

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

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

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.

30. Cambrian and Jurassic are examples of …….A. Era B. epoch C. eon D. period E.

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

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

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

Topic 8: Geologic Time Scale Early Attempts at Estimating Geologic Time Relative Dating Principles:...

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