Chapter 12: Deep Time: How Old Is Old? Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak...

Chapter 12: Deep Time: How Old Is Old? Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 12Deep Time: How Old Is Old?

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

Deep Time: How Old Is Old?Deep Time: How Old Is Old?

Prepared by

Ronald Parker Earlham College Department of Geosciences

Richmond, Indiana

Prepared by

Ronald Parker Earlham College Department of Geosciences

Richmond, Indiana


Geologic TimeGeologic Time Discovering the magnitude of the Earth’s past was Discovering the magnitude of the Earth’s past was

a momentous discovery in the history of humanity.a momentous discovery in the history of humanity. This discovery forever altered our perception of This discovery forever altered our perception of

ourselves within nature and the Universe.ourselves within nature and the Universe.


Geologic TimeGeologic Time Understanding time permits assigning an age to…Understanding time permits assigning an age to…

Rocks.Rocks. Fossils.Fossils. Geologic structures.Geologic structures. Landscapes.Landscapes. Tectonic events.Tectonic events.


Geologic TimeGeologic Time Deep time – The immense span of geologic time.Deep time – The immense span of geologic time. It is so vast that it is difficult for people to grasp.It is so vast that it is difficult for people to grasp.

We think of time in terms of our lives…We think of time in terms of our lives…The lives of our parents and grandparents. The lives of our parents and grandparents. The lives of our children or grandchildren.The lives of our children or grandchildren.

Human history is tiny compared Human history is tiny compared

to geologic time. to geologic time.


Geologic TimeGeologic Time Prior to the late 17Prior to the late 17thth century, geologic time was century, geologic time was

thought to be the same as historical time. thought to be the same as historical time. Archbishop James Ussher of Armagh, Ireland, 1654.Archbishop James Ussher of Armagh, Ireland, 1654.

He added up generations from the Old Testament. He added up generations from the Old Testament. He determined that Earth formed on October 23, 4004 BCE.He determined that Earth formed on October 23, 4004 BCE.


Geologic TimeGeologic Time Scientists began to find clues to an ancient Earth.Scientists began to find clues to an ancient Earth.

Nicolaus Steno (1638–1686) – Danish physician.Nicolaus Steno (1638–1686) – Danish physician.Observed marine fossils high in the Apennines. Observed marine fossils high in the Apennines. Deduced that these were ancient animals in loose sediment.Deduced that these were ancient animals in loose sediment.Lithification and uplift suggested long periods of time.Lithification and uplift suggested long periods of time.


Geologic TimeGeologic Time James Hutton (1726-1797) – Scottish physician. James Hutton (1726-1797) – Scottish physician.

Called “the Father of Modern Geology.” Called “the Father of Modern Geology.” First to articulate the “Principle of Uniformitarianism.”First to articulate the “Principle of Uniformitarianism.” Of the abyss of time, Hutton wrote: “we find no vestige of Of the abyss of time, Hutton wrote: “we find no vestige of

a beginning; no prospect of an end.”a beginning; no prospect of an end.”


Geologic TimeGeologic Time James Hutton’s Principle of Uniformitarianism.James Hutton’s Principle of Uniformitarianism.

““The present is the key to the past.”The present is the key to the past.”Processes seen today are the same as those of the past. Processes seen today are the same as those of the past.

Ancient mudcracks formed as mudcracks do today.Ancient mudcracks formed as mudcracks do today.

Geologic change is slow; large changes require large times.Geologic change is slow; large changes require large times.


Geologic TimeGeologic Time There are two ways of dating geological materials.There are two ways of dating geological materials.

Relative ages – Based upon order of formation.Relative ages – Based upon order of formation.Qualitative method developed 100s of years ago.Qualitative method developed 100s of years ago.Permit determination of older vs. younger relationships.Permit determination of older vs. younger relationships.

Numerical ages – Actual number of years since an event.Numerical ages – Actual number of years since an event.Quantitative method developed recently. Quantitative method developed recently. Age is given a number.Age is given a number.


Relative vs. AbsoluteRelative vs. Absolute Relative ages Relative ages

assign order assign order

to events.to events. Numerical ages Numerical ages

assign exact assign exact

dates to events.dates to events.


Relative AgeRelative Age Logical tools are useful for defining relative age.Logical tools are useful for defining relative age.

Principle of uniformitarianism.Principle of uniformitarianism. Principle of superposition.Principle of superposition. Principle of original horizontality.Principle of original horizontality. Principle of original continuity.Principle of original continuity. Principle of cross-cutting relationships.Principle of cross-cutting relationships. Principle of inclusions.Principle of inclusions. Principle of baked contacts.Principle of baked contacts.


Geologic TimeGeologic Time Uniformitarianism – The present is key to the past.Uniformitarianism – The present is key to the past.

Physical processes that we observe today operated in the Physical processes that we observe today operated in the same way in the geological past. same way in the geological past.

Modern processes help us understand ancient events. Modern processes help us understand ancient events.


Defining Relative AgeDefining Relative Age Superposition. Superposition.

In an undeformed sequence of layered rocks… In an undeformed sequence of layered rocks… Each bed is older than the one above and…Each bed is older than the one above and…Younger than the one below. Younger than the one below.

Younger strata are on top; older strata below.Younger strata are on top; older strata below.


Relative AgeRelative Age Horizontality and continuity.Horizontality and continuity.

Strata often form laterally extensive horizontal sheets.Strata often form laterally extensive horizontal sheets. Subsequent erosion dissects once-continuous layers.Subsequent erosion dissects once-continuous layers. Flat-lying rock layers are unlikely to have been disturbed. Flat-lying rock layers are unlikely to have been disturbed.


Relative AgeRelative Age Cross-cutting relations.Cross-cutting relations.

Younger features truncate (cut across) older features.Younger features truncate (cut across) older features. Faults, dikes, erosion, etc., Faults, dikes, erosion, etc., mustmust bebe younger than the younger than the

material that is faulted, intruded, or eroded.material that is faulted, intruded, or eroded. A volcano cannot intrude rocks that aren’t there yet.A volcano cannot intrude rocks that aren’t there yet.


Relative AgeRelative Age Inclusions – A rock fragment within another.Inclusions – A rock fragment within another.

Igneous xenoliths – Country rock that fell into magma.Igneous xenoliths – Country rock that fell into magma. Weathering rubble – Debris from pre-existing rocks.Weathering rubble – Debris from pre-existing rocks.

The inclusion is older than the material enclosing The inclusion is older than the material enclosing it.it.


Relative AgeRelative Age Baked contacts.Baked contacts.

Thermal metamorphism occurs when country rock is Thermal metamorphism occurs when country rock is invaded by a plutonic igneous intrusion. invaded by a plutonic igneous intrusion.

The baked rock must have been there first (it is older). The baked rock must have been there first (it is older).


Relative AgeRelative Age Determining Determining relativerelative ages empowers geologists to ages empowers geologists to

easily unravel complicated geologic histories.easily unravel complicated geologic histories.


Geologic HistoryGeologic History

Deposition of horizontal strata below sea level in Deposition of horizontal strata below sea level in order 1, 2, 3, 4, 5, 6, 7, and 8 (oldest to youngest).order 1, 2, 3, 4, 5, 6, 7, and 8 (oldest to youngest).



Igneous intrusion of a sill.Igneous intrusion of a sill.


Tectonic compression and folding. Tectonic compression and folding. Beds had to be present to be folded.Beds had to be present to be folded.

Uplift above sea level and erosion.Uplift above sea level and erosion.



Intrusion of a granitic igneous pluton.Intrusion of a granitic igneous pluton.



Extensional normal faulting.Extensional normal faulting. Faulting cross-cuts the older granitic pluton.Faulting cross-cuts the older granitic pluton.



Intrusion of a dike.Intrusion of a dike. Dike cross-cuts the normal fault.Dike cross-cuts the normal fault.



Erosion to present landscape configuration.Erosion to present landscape configuration. Erosion removed the volcano and cross-cuts the dike.Erosion removed the volcano and cross-cuts the dike.



Geologic HistoryGeologic History Relative ages help to unravel a complicated history.Relative ages help to unravel a complicated history. Simple rules permit one to decipher this diagram. Simple rules permit one to decipher this diagram.


Fossil SuccessionFossil Succession Fossil remnants, or traces of once living organisms, Fossil remnants, or traces of once living organisms,

are often preserved in sedimentary rocks. are often preserved in sedimentary rocks. Fossil are useful for relative age determination.Fossil are useful for relative age determination.

Several types of fossils will occur as an assemblage.Several types of fossils will occur as an assemblage. Fossils are time markers.Fossils are time markers.


Fossil SuccessionFossil Succession Species evolve, exist for a time, and then go extinct. Species evolve, exist for a time, and then go extinct. First appearance, range, and extinction dates rocks. First appearance, range, and extinction dates rocks. Fossils succeed one another in a known order.Fossils succeed one another in a known order. A time period is recognized by its fossil content.A time period is recognized by its fossil content.


Fossil range – First and last appearance.Fossil range – First and last appearance. Each fossil has a unique range.Each fossil has a unique range. Overlapping ranges provide Overlapping ranges provide

distinctive time markers.distinctive time markers.

Permit correlation of strata.Permit correlation of strata. Locally.Locally. Regionally.Regionally. Globally.Globally.

Fossil SuccessionFossil Succession


UnconformitiesUnconformities An unconformity is a time gap in the rock record.An unconformity is a time gap in the rock record.

Nondeposition.Nondeposition. Erosion. Erosion.

There are three types of unconformity.There are three types of unconformity.


UnconformitiesUnconformities Three unconformity types: Three unconformity types:

Disconformity – Parallel strata bracketing nondeposition.Disconformity – Parallel strata bracketing nondeposition.Due to an interruption in sedimentation.Due to an interruption in sedimentation.May be difficult to recognize.May be difficult to recognize.

DisconformitiesDisconformities



Nonconformity – Metamorphic or igneous rocks overlain Nonconformity – Metamorphic or igneous rocks overlain by sedimentary strata. by sedimentary strata. Crystalline ig/met rocksCrystalline ig/met rocks

were exposed by erosion.were exposed by erosion.Sediment was deposited Sediment was deposited

on this eroded surface. on this eroded surface.

NonconformityNonconformity



Angular unconformity – Represents a huge gulf in time.Angular unconformity – Represents a huge gulf in time.Horizontal marine sediments deformed by orogenesis.Horizontal marine sediments deformed by orogenesis.High mountains are eroded away to High mountains are eroded away to belowbelow sea level. sea level.Sediments deposited horizontally on the erosion surface. Sediments deposited horizontally on the erosion surface.


Angular UnconformityAngular Unconformity James Hutton was the 1James Hutton was the 1stst to realize the enormous to realize the enormous

time-significance of angular unconformities.time-significance of angular unconformities. Mountains created.Mountains created. Mountains completely erased.Mountains completely erased. New sediment deposition.New sediment deposition.

Incomprehensible time.Incomprehensible time.


Angular UnconformityAngular Unconformity ““Hutton’s Unconformity” on Siccar Point, Scotland, Hutton’s Unconformity” on Siccar Point, Scotland,

is a common destination for geologists. is a common destination for geologists.


UnconformitiesUnconformities Earth history is Earth history is

recorded in strata.recorded in strata. Missing strata = Missing strata =

missing history. missing history. The Grand Canyon.The Grand Canyon.

Thick layers of strata.Thick layers of strata. Numerous gaps.Numerous gaps. A partial record of A partial record of

geological history. geological history.


Stratigraphic CorrelationStratigraphic Correlation Stratigraphic columns depict strata in a region. Stratigraphic columns depict strata in a region.

Drawn to scale to accurately portray relative thicknesses.Drawn to scale to accurately portray relative thicknesses. Rock types are depicted by graphical fill patterns. Rock types are depicted by graphical fill patterns. Divided into formations.Divided into formations.

Mapable rock units.Mapable rock units. Formations are Formations are

separated by contacts. separated by contacts.


Stratigraphic CorrelationStratigraphic Correlation In 1793, William “Strata” Smith was the first to note In 1793, William “Strata” Smith was the first to note

that strata could be matched across distances. that strata could be matched across distances. Similar rock types in a similar order. Similar rock types in a similar order. Rock layers contained the same distinctive fossils.Rock layers contained the same distinctive fossils.

After years of work, he made the 1After years of work, he made the 1stst geologic map. geologic map.


Stratigraphic CorrelationStratigraphic Correlation Lithologic correlation is based on rock type.Lithologic correlation is based on rock type.

Sequence – The relative order in which the rocks occur.Sequence – The relative order in which the rocks occur. Limited to correlation between nearby regions.Limited to correlation between nearby regions.

Fossil correlation – Based on fossils within rocks.Fossil correlation – Based on fossils within rocks. Applicable to much broader areas.Applicable to much broader areas.


Stratigraphic CorrelationStratigraphic Correlation National parks of Arizona and Utah.National parks of Arizona and Utah.

Formations can be traced long distances.Formations can be traced long distances. Overlap is seen in the sequences of rock types.Overlap is seen in the sequences of rock types. Overlapping rock columns are used to build a composite. Overlapping rock columns are used to build a composite.

Correlation among rock strata in 3 national parks.Correlation among rock strata in 3 national parks.


The Geologic ColumnThe Geologic Column A composite stratigraphic column exists.A composite stratigraphic column exists.

Constructed from incomplete sections across the globe.Constructed from incomplete sections across the globe. It brackets almost the entirety of Earth history.It brackets almost the entirety of Earth history.


The composite column is divided into time blocks.The composite column is divided into time blocks. This is the geologic time scale, Earth’s “calendar.”This is the geologic time scale, Earth’s “calendar.”

The structure of the geologic time scale.The structure of the geologic time scale.Eons – The largest subdivision of time (100s to 1000s Ma).Eons – The largest subdivision of time (100s to 1000s Ma).Eras – Subdivisions of an eon (65 to 100s Ma).Eras – Subdivisions of an eon (65 to 100s Ma).Periods – Subdivisions of an era (2 to 70 Ma).Periods – Subdivisions of an era (2 to 70 Ma).Epochs – Subdivisions of a period (0.011 to 22 Ma). Epochs – Subdivisions of a period (0.011 to 22 Ma).

Geologic TimeGeologic Time


Time scale subdivisions are variously named.Time scale subdivisions are variously named. The nature of life (“zoic” means life); i.e., Proterozoic.The nature of life (“zoic” means life); i.e., Proterozoic. A characteristic of the time period; i.e., Carboniferous.A characteristic of the time period; i.e., Carboniferous. A specific locality; i.e., Devonian.A specific locality; i.e., Devonian.



The Geologic Time The Geologic Time ScaleScale


Geologic Time and LifeGeologic Time and Life Life first appears on Earth ~ 3.8 Ga.Life first appears on Earth ~ 3.8 Ga. Early life consisted of anaerobic Early life consisted of anaerobic

single-celled organisms. single-celled organisms. Oxygen from cyanobacteria Oxygen from cyanobacteria

built up by 2 Ga.built up by 2 Ga. ~ 700 Ma, multicellular ~ 700 Ma, multicellular

life evolved.life evolved. ~ 542 Ma marks the ~ 542 Ma marks the

11stst appearance appearance

of hard shells.of hard shells. Shells increased fossil Shells increased fossil

preservation.preservation.

Life diversified rapidly Life diversified rapidly ––the “Cambrian Explosion.”the “Cambrian Explosion.”


Names of the eons.Names of the eons. Phanerozoic – “Visible life” (542 Ma to the present).Phanerozoic – “Visible life” (542 Ma to the present).

Started 542 Ma at the Precambrian – Cambrian boundary.Started 542 Ma at the Precambrian – Cambrian boundary.Marks the 1st appearance of hard shells.Marks the 1st appearance of hard shells.Life diversified rapidly afterward.Life diversified rapidly afterward.

Proterozoic – “Before life” (2.5 to 0.542 Ga). Proterozoic – “Before life” (2.5 to 0.542 Ga). Development of tectonic plates like those of today.Development of tectonic plates like those of today.Buildup of atmospheric OBuildup of atmospheric O22; multicellular life appears.; multicellular life appears.

Archean – “Ancient” (3.8 to 2.5 Ga).Archean – “Ancient” (3.8 to 2.5 Ga).Birth of continents.Birth of continents.Appearance of the earliest life forms. Appearance of the earliest life forms.

Hadean – “Hell” (4.6 to 3.8 Ga). Hadean – “Hell” (4.6 to 3.8 Ga). Internal differentiation.Internal differentiation.Formation of the oceans and secondary atmosphere.Formation of the oceans and secondary atmosphere.

The Geologic Time ScaleThe Geologic Time Scale


The Geologic Time ScaleThe Geologic Time Scale Names of the eras.Names of the eras.

Cenozoic – “Recent life.” Cenozoic – “Recent life.” 65.5 Ma to present.65.5 Ma to present.The “Age of Mammals.”The “Age of Mammals.”

Mesozoic – “Middle life.”Mesozoic – “Middle life.”251 to 65.5 Ma.251 to 65.5 Ma.The “Age of Dinosaurs.”The “Age of Dinosaurs.”

Paleozoic – “Ancient life.”Paleozoic – “Ancient life.”542 to 251 Ma.542 to 251 Ma.Life diversified rapidly.Life diversified rapidly.


Numerical AgeNumerical Age The relative age of geologic events is established.The relative age of geologic events is established. Based on radioactive decay of atoms in minerals.Based on radioactive decay of atoms in minerals.

Radioactive decay proceeds at a known, fixed rate.Radioactive decay proceeds at a known, fixed rate. Radioactive elements act as internal clocks. Radioactive elements act as internal clocks.

Numerical dating is also called geochronology.Numerical dating is also called geochronology.


Radioactive DecayRadioactive Decay Isotopes – Atoms with the same # of protons, Isotopes – Atoms with the same # of protons,

different # of neutrons. different # of neutrons. Isotopes have similar but different mass numbers. Isotopes have similar but different mass numbers.

Stable – Isotopes that never change (i.e., Stable – Isotopes that never change (i.e., 1313C).C). Radioactive – Isotopes that spontaneously decay.Radioactive – Isotopes that spontaneously decay.


Radioactive DecayRadioactive Decay Radioactive decay progresses along a decay chain. Radioactive decay progresses along a decay chain.

Decay creates new unstable elements that also decay.Decay creates new unstable elements that also decay. Decay proceeds to a stable element endpoint. Decay proceeds to a stable element endpoint.

Parent isotope – The isotope that undergoes decay.Parent isotope – The isotope that undergoes decay. Daughter isotope – The product of this decay.Daughter isotope – The product of this decay.


Radioactive DecayRadioactive Decay Half-life (tHalf-life (t½½) – Time for ½ unstable nuclei to decay.) – Time for ½ unstable nuclei to decay.

tt½½ is a characteristic of each isotope. is a characteristic of each isotope.

After one tAfter one t½½ half of the original parent remains. half of the original parent remains.

After three tAfter three t½½ an eighth of the original parent remains. an eighth of the original parent remains.

As the parent disappears, the daughter “grows in”.As the parent disappears, the daughter “grows in”.


Radiometric DatingRadiometric Dating The age of a mineral can be determined by…The age of a mineral can be determined by…

Measuring the ratio of parent to daughter isotopes.Measuring the ratio of parent to daughter isotopes. Calculating the amount of time by using the known tCalculating the amount of time by using the known t½½..

Must pick the right mineral and the right isotope. Must pick the right mineral and the right isotope. Geochronology requires analytical precision. Geochronology requires analytical precision.


What Is a Radiometric Date?What Is a Radiometric Date? Radiometric dates give the time a mineral began to Radiometric dates give the time a mineral began to

preserve all atoms of parent and daughter isotopes. preserve all atoms of parent and daughter isotopes. Requires cooling below a “blocking temperature.” Requires cooling below a “blocking temperature.” If rock is reheated, the radiometric clock can be reset.If rock is reheated, the radiometric clock can be reset.

Ig / Met rocks are best for geochronologic work.Ig / Met rocks are best for geochronologic work. Sedimentary rocks cannot be directly dated.Sedimentary rocks cannot be directly dated.


Other Numerical AgesOther Numerical Ages Numerical ages are possible without isotopes. Numerical ages are possible without isotopes.

Growth rings – Annual layers from trees or shells. Growth rings – Annual layers from trees or shells. Rhythmic layering – Annual layers in sediments or ice. Rhythmic layering – Annual layers in sediments or ice.


Other Numerical AgesOther Numerical Ages Magnetostratigraphy – Magnetic signatures in Magnetostratigraphy – Magnetic signatures in

strata are compared to the global reference strata are compared to the global reference column.column.


Other Numerical AgesOther Numerical Ages Fission-track analysis – Measuring decay paths. Fission-track analysis – Measuring decay paths.

Radioactive decay particles scar crystals. Radioactive decay particles scar crystals. The number of scars is proportional to age.The number of scars is proportional to age.


Dating the Geologic ColumnDating the Geologic Column Geochronology less useful for sediment deposits.Geochronology less useful for sediment deposits. It can, however, constrain these deposits.It can, however, constrain these deposits. Sediments can be bracketed by absolute dates.Sediments can be bracketed by absolute dates.

Yields age ranges that improve as data accumulates. Yields age ranges that improve as data accumulates. Defines major boundaries in the geologic column.Defines major boundaries in the geologic column.


The Age of the EarthThe Age of the Earth Before radioactivity-based Before radioactivity-based

dating methods…dating methods… 20 Ma – From Earth cooling.20 Ma – From Earth cooling. 90 Ma –Ocean salinization.90 Ma –Ocean salinization.

Assumed oceans were initially Assumed oceans were initially freshwater.freshwater.

Measured the mass of Measured the mass of dissolved material in rivers.dissolved material in rivers.

Uniformitarianism and Uniformitarianism and evolution indicated an Earth evolution indicated an Earth older than ~100 Ma.older than ~100 Ma.


The Age of the EarthThe Age of the Earth The oldest rocks on Earth’s surface date to 3.96 Ga.The oldest rocks on Earth’s surface date to 3.96 Ga. Zircons in ancient sandstones date to 4.1-4.2 Ga.Zircons in ancient sandstones date to 4.1-4.2 Ga. Age of Earth is 4.57 Ga based on correlation with…Age of Earth is 4.57 Ga based on correlation with…

Meteorites.Meteorites. Moon rocks.Moon rocks.


Geologic TimeGeologic Time The immensity of time is beyond comprehension.The immensity of time is beyond comprehension. Metaphors illustrate the scale of time.Metaphors illustrate the scale of time.

The age of Earth (4.6 Ga) can be compared to pennies.The age of Earth (4.6 Ga) can be compared to pennies. Lined up, 4.6 billion pennies would be 87,400 km long.Lined up, 4.6 billion pennies would be 87,400 km long.

More than twice around Earth. More than twice around Earth.


Chapter 12Deep Time: How Old Is Old?

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

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Chapter 12: Deep Time: How Old Is Old? Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak...

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