Lecture 23 geologic history and time

to Historical Geology 1 Geologic Time Scale 2 Relative age dating principles 3 Unconformities 4 Correlation of rock units 5 Absolute dating using radioactivity 6 Development of the Geol

Chapter 12: History Before History (Part 1) Chapter 12: History Before History (Part 1)

Class Announcements:

 Midterm exam results should

be ready on Wednesday Updated grade record will be posted then.

 Exam key posted in glass case

outside lecture hall.

 Field trips this week Be sure to read

instructions on website and bring

a pencil and eraser!

 Extra Credit exercise on “The Core”

is due Friday.

 An updated course schedule on

website after class today.

TODAY’s LECTURE:

Chapter 12 :

Intro to Historical Geology

1) Geologic Time Scale

2) Relative age dating principles 3) Unconformities

4) Correlation of rock units

5) Absolute dating using

radioactivity 6) Development of the Geologic

Time Scale

Built upover time by correlation andrelative dating

of rocks from aroundthe world!

Where did the

geologic time

scale come from?

Telling time in Geology:

❖ Earth’s history is recorded in rocks

of the crust…

❖ Geology seeks to unravel this history!

❖ Two basic time keeping methods:

Relative dating - Place events in their proper

order.

Absolute dating - Determine, in years before the

present, when event actually occurred.

Relative Dating: Key principles

Relative Dating: Main Principles

A bed of rock is older than that above, younger than that below

Law of superposition:

Law of superposition: In a sedimentary sequence In a sedimentary sequence

that has not been overturned, the oldest rock units are always at the bottom.

Original lateral continuity

Original horizontality

Cross-cutting relationships

Rule of inclusions

Fig 12.04c

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Relative Dating: Main Principles Relative Dating: Main Principles

Oldest Youngest

Law of superposition

Principle of original lateral continuity: Most

sedimentary rock units were originally deposited

over a large area, but later became isolated by

Fig 12.04fg

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Fig 12.11c

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Note that geologic units can also disappear between localities because of a lateral transition

in environments if deposition.

Original horizontality

Sedimentary units deposited ~ flat and parallel to the earth’s surface.

Principle of Original Horizontality

So, what

happened

here?

Principle of Original Horizontality

What happened here?

Principle of cross-cutting relationships:

If one rock unit cuts across another, it is younger!

Rule of Inclusions

Principles of Relative Dating

Fig 12.04h

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If something cuts something else,

it has to be younger than what it cuts.

Principle of cross-cutting relationships

Principle of Cross-cutting relationships

Which vein is youngest, ‘A’ or ‘B’?

‘A’

‘B’

Law of superposition

Lateral of original continuity

Principle of original horizontality

Principle of cross-cutting relationships

Rule of Inclusions:

Rule of Inclusions: If a unit contains a clast,

or inclusion of another rock unit, the clast

or inclusion is older.

Principles of Relative Dating

Most of the rock

Fig 12.11b

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Example of an unconformity…

Local removal of Rufus Lms by erosion

Rock sections at three different localities: A, B and C

Principles of Relative Dating

Conformable layers: Layers in a sequence thatwere deposited w/out noticeable interruption

Conformable layers:Groups of conformable layers define packages of sedimentary units bounded

by unconformities

Principles of Relative Dating

Tilted and/or folded sedimentary rocks, overlain by

younger layers that are more flat-lying.

Fig 12.08ab

Stephen Marshak

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Angular Unconformities

Fig 12.09a

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Layers on either side of

unconformity are parallel.

No tilting, just erosion.

Fig 12.09c

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Fig 12.04j

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Step 1 in the creation of a nonconformity

Instrusion of magma and contact metamorphism…

Fig 12.09b

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Making a nonconformity…

Erosion surface separating sedimentary layers and

metamorphic or plutonic igneous rocks

Erosion surface developed on igneous or metemorphic basement before sedimentary units deposited….

Chapter 12: History Before History (Part 1) Chapter 12: History Before History (Part 1)

Class Announcements:

 Midterm exam results Updated

grade record will be posted today

on website.

 Field trips this week Be sure to read

instructions on website and bring

a pencil and eraser!

 Extra Credit exercise on “The Core”

TODAY’s LECTURE:

Chapter 12 :

Intro to Historical Geology

1) Animations to review principles

of relative dating 2) Correlation of rock units

3) Absolute dating using

radioactivity 4) Development of the Geologic

Time Scale

In-Class Problem: Place each labeled rock

unit in its proper place in a relative time sequence

from oldest to youngest

Fig 12.05

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Homework Problem: Place each labeled rock unit

in its proper place in a relative time sequence, from

oldest to youngest

Name: _Student ID

Correlation of Rock Units

◆ Physical Criteria:

➨ Position of a bed in sequence.

➨ Distinctive lithologies (rock types,

mineralogy, sedimentary structures, color, etc.).

➨ Unique paleoenvironment implied

by above

Correlation based on rock type…

Fig 12.04b

Stephen Marshak

Correlation using distinctive associations of

sedimentary structures (paleoenvironments)

Ancient mudcracks

Fig 12.04a

Stephen Marshak

Modern

mudcracks

Correlation

◆ Fossils and Biostratigraphy

➨ Principle of Fossil Succession:

In sedimentary sequences, fossil species succeed one another in and orderly, systematic fashion,

reflecting cycles of evolution and Extinction

➨ Fossils can be used to assign relative ages

to rock units based on the fact that each fossil species lived for a only brief time and then died out

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Index zones

and correlation

using fossils.

Principle of Fossil Succession

Biostratigraphic correlation

1 Kansas

2 Indiana

3 Ohio

4 Pennsylvania

# neutrons can vary:

“isotopes”

Model of an atom:

Absolute Age Dating Using Radioactivity

Overheads

Half-life Concept:

The half life of a radioactive isotope

is the time it takes for half of the parent isotope to decay to the

daughter product

Fig 12.17a

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Radioactive decay occurs at a constant rate…

Example: An isotope with a half life of 1 year…

Number of atoms

Fig 12.19

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Errors in age dates can arise

if the rock gains or loses parent

of daughter isotopes…

Absolute Dating Using Radioactivity

These systems are useful for dating

very old rocks For some systems,

the half-life exceeds even the age

of the Universe

Fig 12.22b

Absolute dating using density of fission tracks…

Defects

in crystalsformed

by fissionParticles

Fig 12.21

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Correlation and dating using

Built upover time by correlation andrelative dating

of rocks from aroundthe world!

Where did the

geologic time

scale come from?

Fig 12.14a

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Geologic Time Scale

Eons

Phanerozoic - Last 570 million years

Cenozoic - recent life Mesozoic - age of middle life Paleozoic - ancient life

Precambrian - from birth of Earth up to before

complex life forms developed

Eras

0-66.4 m.y.a 66.4-245 m.y.a 245-570 m.y.a

(mammals, humans) (dinosaurs, 1st scrawny mammals)

(fish, trilobites, clams, corals, ferns )

(algae, bacteria, some fossils without shells like jellyfish)

Absolute ages have been added to the time scale mostly by dating igneous rocks that bracket unit

boundaries.

Next time:

Chapter 14: Resources!