Physical Geology Lecture 12 Time.pdf

8/12/2019 Physical Geology Lecture 12 Time.pdf

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Geologic TimeGeologic Time

Geology 211, Physical Geology Copyright, 2004, Ron ParkerGeology 211, Physical Geology Copyright, 2004, Ron Parker

March 2, 2004March 2, 2004

View from Grand Canyon Lodge, North Rim Ron Parker, 1991View from Grand Canyon Lodge, North Rim Ron Parker, 1991

Geosciences 211, Physical Geology Ron Parker, 2004

Geologic Time

Discovering the magnitude of the Earths past was

a momentous discovery in the history of humanity.

This discovery forever altered our perception of

ourselves within nature and the universe.


Geologic Time 1664, James Ussher, Archbishop of Armagh,

(Ireland)

Counted generations in the bible.

Determined that the earth had been formed:

In 4004 B.C upon the entrance of the

night preceding the 23rd day of October

Acceptance of this short time fostered a belief in

sudden and massive change to explain geologic

complexity.

This idea was known as catastrophism.

ArmaghPublic LibraryArmaghPublic Library


Geologic Time James Hutton

Scottish physician and farmer often calledthe Father of Modern Geology

Recognized the slow pulse of geologic change.He observed

The slow rate of soil loss from his fields

Hadrians Wall had not changed in 1600 years.

Geologic changesrequired a very old Earth.

Presented his Theory of the Earth in 1785.

Of the abyss of time, Hutton wrote:

The result, therefore, of our presentinquiry is that we find no vestige of abeginning - no prospect of an end

http://www.science.siu.edu/geology/people/ferre/hutton.html

http://www.science.siu.edu/geology/people/ferre

/hutton.html


Geologic Time Sir Charles Lyell

Scottish lawyer turned geologist

Published Principles of Geology in 3 volumes (1830-1833)

Provided abundant evidence for the Principle ofUniformitarianism

All past Earth changes can be explained by modern processesoperating over immense spans of time.

Uniformitarianism may be summarized by the phrase:

The present is the key to the past.

Sir Charles Lyell

Scottish lawyer turned geologist

Published Principles of Geology in 3 volumes (1830-1833)

Provided abundant evidence for the Principle ofUniformitarianism

All past Earth changes can be explained by modern processesoperating over immense spans of time.

Uniformitarianism may be summarized by the phrase:

The present is the key to the past.

Rick Miller SDSU Geologic Time Web pagehttp://www-rohan.sdsu.edu/~rhmiller/geologictime/GeologicTime.htm

Rick Miller SDSU Geologic Time Web pagehttp://www-rohan.sdsu.edu/~rhmiller/geologictime/GeologicTime.htm


Uniformitarianism The present is the key to the past.

The Earth has been shaped by uniform forces operating the

same way today as they did in the past.

Modern mudcracks, San Pedro River, SE Arizona Ron Parker, 2003 Cambrian mudcracks, Winooski, Vermont Ron Parker, 2002


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Relative Dating Ages

assigned to rocks and events

based upon their proper

sequence of formation.

Absolute Dating Numerical

ages specifying the actual

number of years that have

passed since an event

occurred.

Geologic Time

There are two ways of dating geological materials.

W. W. Norton W. W. Norton

Relative Absolute



Principle of Uniformitarianism

Principle of Superposition

Principle of Original Horizontality

Principle of Original Continuity

Principle of Cross-Cutting Relationships

Principle of Inclusions

Principle of Baked contacts

Principle of Uniformitarianism

Principle of Superposition

Principle of Original Horizontality

Principle of Original Continuity

Principle of Cross-Cutting Relationships

Principle of Inclusions

Principle of Baked contacts

Principles of Relative Dating

Ron Parker, 1991Geosciences 211, Physical Geology Ron Parker, 2004

Superposition

Ron Parker, 1991

In an undeformed sequence of layered rocks

Each bed is older than the one above, and

younger than the one below.

In an undeformed sequence of layered rocks

Each bed is older than the one above, and

younger than the one below.



Sediment deposits form extensive horizontal sheets.

Flat rock layers have not been disturbed.

Subsequent erosion dissects once continuous layers

Sediment deposits form extensive horizontal sheets.

Flat rock layers have not been disturbed.

Subsequent erosion dissects once continuous layers


Original Horizontality and Continuity

View from Toroweap PointRon Parker, 1991Geosciences 211, Physical Geology Ron Parker, 2004

Younger features cut across

older features.

Faults, dikes, erosion, etc.

Cross-Cutting RelationsCross-Cutting Relations



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The fault is younger than

the strata it cuts across.

The fault is younger than

the strata it cuts across. An inclusion is a piece of rock enclosed within another.

Igneous xenoliths

Weathering rubble.

The inclusion is older than the material enclosing it.

Principle of InclusionsPrinciple of Inclusions

Stanley Shale inclusion in Nepheline Syenite, Magnet Cove, Arkansas Ron Parker, 2003

W.W.Norton

W.W.Norton


Relative dating principles allow geologists to easily unravel

complicated geologic histories.

Relative Dating


An unconformity is a gap in the rock record produced by

Erosion and/or

Non-deposition.

Unconformities

An unconformity is

significant; it represents

missing TIME.

There are several types of

unconformity.


Unconformity types:

Disconformity Strata on either side of the unconformity

are parallel.

Nonconformity Metamorphic or igneous rocks in contact

with sedimentary strata.

Angular unconformity Tilted rocks are overlain by flat-

lying rocks.

Unconformity animation

Relative Dating


Angular UnconformityAngular Unconformity Represents a huge gulf in time

Sediments deposited horizontally belowsea-level.

Sediments deformed by orogenesis

Mountains eroded totally away.

Mountain remnants flooded by the sea.

Younger sediments deposited horizontallyon the deformed sediments.


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Angular Unconformity The most easily recognized.

The time-significance of angular unconformities was first

recognized by Hutton at Siccar Point, Scotland.

Relative Dating

W. W. Norton W. W. Norton Stephen Marshak Stephen Marshak DIGIT, 2001 DIGIT, 2001



Species evolve, exist for a time and then become extinct.

Rocks are dated by fossil appearance, range and extinction.

Fossil organisms succeed one another in a definite order.

A time period is readily recognized by its fossil content.

Principle of Fossil Succession

Sue, the largest and most complete Tyrannosaur yet discovered, Field Museum, Chicago, Illinois Ron Parker, 2001 W. W. Norton W. W. Norton

Life first appears on Earth ~ 3.8 Ba.

During 90% of Earth history, life

consisted of single-celled organisms.

~ 700 Ma, multicellular life evolved.

~ 540 Ma, hard skeletons evolved

leading to a great increase in the

preservation of fossil remains.

Earliest life was anaerobic.

Oxygen built up by 2 Ba.

This is known as the Cambrian Explosion Faunal evolution proceeded at a rapid pace

thereafter.


The first and last appearance of a

specific fossil type is its range.

Ranges are time markers.

Each fossil type has a unique range.

Overlapping faunal ranges provide

distinctive time markers that

permit correlation of strata

Locally

Regionally

Globally


In 1793, William Smith was the first to

note that strata in widely separated

regions could be matched.

He correlated rocks all over England

and produced the first geologic map*.

Stratigraphic Correlation

*(His efforts are described in the 2001 National Bestseller The Map That Changed theWorld: William Smith and the Birth of Modern Geology by Simon Winchester, HarperCollins Publisher).



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This map led to the coal, oil and mining

industries and the development of

highways and railroads.

This map led to the coal, oil and mining

industries and the development of

highways and railroads.

Stratigraphic Correlation

Matching of rocks of similar ages in different regions is known asstratigraphic correlation.



Establishes the actual numerical age of a geologic material.

Refines the relative dating scale.

A complex procedure that requires precise measurement.

The oldest rocks have been dated at more than 4 Ba.

Confirms the idea that geologic time is immense.

Radioactivity

Spontaneous changes (decay) in the structure of atomic nuclei

Parent An unstable radioactive isotope

Daughter product Isotopes resulting from the decay of a parent.

Half-life Time required for one-half of the radioactive nuclei in a

sample to decay.

Radioactive Dating


Atomic number

An elements identifying number

Equal to the number of protons in the atoms nucleus

Mass number

Sum of the number of protons and neutrons in an atoms nucleus

Isotope

Variant of the same parent atom.

Differs in the number of neutrons.

Results in a different mass number than the parent atom.

Radioactive Dating


Radiometric dating During one half-life, the same amount of decay occurs (50 %).

The actual number of atoms that decay decreases exponentially.

Comparing the ratio of parent to daughter yields the sample age.

Radioactive Dating



Radiocarbon (Carbon 14) dating

Half-life of only 5730 years.

Used to date very recent events (limited to


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Names of the eons.

Phanerozoic Visible life The most recent eon,

began about 540 million years ago.

ProterozoicBefore life We now know that life

was present during the proterozoic.

Archean - Old

Hadean The oldest eon. Named after Hades (Hell).


Precambrian

4 billion years prior to the Cambrian period.

Not divided because Precambrian events are

murky, obscured by the haze of the past

First abundant fossil evidence does not appear

until the beginning of the Cambrian (the

Cambrian Explosion).

Precambrian

4 billion years prior to the Cambrian period.

Not divided because Precambrian events are

murky, obscured by the haze of the past

First abundant fossil evidence does not appear

until the beginning of the Cambrian (the

Cambrian Explosion).


Eras of the Phanerozoic

Cenozoic (recent life)

Mesozoic (middle life)

Paleozoic (ancient life)

Geosciences 211, Physical Geology Ron Parker, 2004 W. W. Norton W. W. Norton

Second Exam

Next Class:

Differential weathering in the Silurian Cedarville Dolomite, Clifton Gorge, Ohio Ron Parker, 2004Differential weathering in the Silurian Cedarville Dolomite, Clifton Gorge, Ohio Ron Parker, 2004

Physical Geology Lecture 12 Time.pdf

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