Interpreting Geologic

History

THE PRINCIPLE OF UNIFORMITY:

Geologists can infer events of the past by

Looking at features of rocks androck outcrops

Looking at features of rocks androck outcrops

Uniformitarianis

m(Principle of

Uniformitarianis

m(Principle of states that the forces that acted upon the

___________ crust…

Uniformity)Uniformity)

Earth’s Earth’s

in the __________ are the same as those that are ____________

today.

pastpast

activeactive

**THE KEY TO THE PASTIS THE PRESENT****THE KEY TO THE PASTIS THE PRESENT**

Relative Dating Techniques

5 Basic Laws: 1. Law of Original

Horizontality2. Law of Superposition

3. Law of Inclusions4. Law of Cross-Cutting

Relationships5. Law of Original Lateral

Continuity

Law of Original Horizontality

Strata is originally deposited in flat horizontal

layers because sedimentary particles settle from air and

water under the influence of gravity

Law of Original Horizontality

If strata are ___________, then they must have suffered some kind of disturbance after they were deposited.

Grand Canyon

Western Iran

Steeply Inclined

THE LAW OF SUPERPOSITION:

THE LAW OF SUPERPOSITION:

the principle that the _________ layers in a sequence of rock

strata must have been deposited __________ the layers above, unless the rock strata

have been ___________ or___________

bottombottom

beforebefore

disturbeddisturbeduplifteduplifted

The _______ rocks are found at the bottom.

Geologists can date the

_________ ages of the strata from

________ to __________

olderolder

relativerelative

oldestoldestyoungestyoungest

oldest

youngest

Law of Superposition

Therefore the order of deposition is from the bottom upward.

Law of Inclusions

A rock must first exist in order to be weathered, deposited and cemented as a _____ in another rock. Therefore…

clast

If rock is composed of _____________, the rock fragments must be ___________ than the rock in which they are found.

fragmentsfragments

olderolder

Law of Inclusions

Which is older the Granite or the Sandstone? In figure A? In figure B?

Sandstone is older

Granite is older

The law of inclusions also applies to fossils preserved in the bedrock.

_________ are any naturally preserved remains or impressions of living things.

FossilsFossils

They are found in _______________ because

_____________________

____________

They are found in _______________ because

_____________________

____________

Sedimentary rockSedimentary rock

Heat & pressure in igneous and metamorphic rock destroys them

Heat & pressure in igneous and metamorphic rock destroys them

FOSSILS GIVE US INFORMATION ABOUT

THE ANCIENT ENVIRONMENT AND CLIMATE

Law of Cross-Cutting Relationships

Any __________ or ______, must be younger than all rocks through which it cuts. Simply put, the body of rock that is cross-cut had to be there first in order to be cut by an intruding igneous body or fault.

Igneous rock fault

In general rock is always_________ than the process that changed it.

In general rock is always_________ than the process that changed it.

olderolder

Some Processes Include:

foldsfaultstiltsintrusionsextrusions

foldsfaultstiltsintrusionsextrusions

Sedimentary layersSedimentary layers(the law of original horizontality)

Sedimentary layersSedimentary layers

Sedimentary layersSedimentary layers

Sedimentary layersSedimentary layers

Sedimentary layersSedimentary layers

The fault came after the rock was formed

Sedimentary layersSedimentary layersThe tilt came after the the rock was formed

The extrusion came afterthe rock was formedThe extrusion came afterthe rock was formed

1

2

3

4

5Contactmetamorphism

1

2

3

4

5

6

The extrusion came after the lowerlayers were formed but….Before the top layer

1

2

3

4

5

This intrusion came after all the layers

1

2

3

4

5

6

The extrusion came after the lowerlayers were formed but….Before the top layer

1

2

3

4

5

This intrusion came afterall the layers

These changes can lead to exceptions to the Law of Superposition:

a.An __________ is an igneous rock that formed from lava on the surface of the crust.

extrusionextrusion

An __________ must be younger than the strata below it, but ________ than any layers above.

extrusionextrusion

olderolder

1

2

3

4

5

6

The extrusion came after the lowerlayers were formed but….Before the top layer

b. __________ are created when molten rock (________) is injected into older rock layers in the crust.

intrusionsintrusions

magmamagma

_____________ are_____________ thanall the rock layers in contact with them.

IntrusionsIntrusionsyoungeryounger

1

2

3

4

5

This intrusion came after all the layers

c. _______ are bends in the rock strata. ________ can overturn rock strata so that ________ rock lies on top of _________ rock.

FoldsFolds

foldingfolding

olderolder

youngeryounger

d. _______ are cracksin rock strata. _______ produce offset layers.

FaultsFaults

FaultsFaults

d. _______ are cracksin rock strata. _______ produce offset layers.

FaultsFaults

FaultsFaults

Rock strata must be

________ than the process that changed it.

olderolder

_________, ________ and ________ ___________

are features created after rock or sediment has been deposited.

crackscracks veinsveinsnaturalnatural cementcement

Law of Original Lateral Continuity

. When sediment is dumped by an agent of erosion, strata extends from the source until it gradually thins to zero, or until it reaches the edges of the basin of __________.

deposition

Law of Original Lateral Continuity

Law of Original Lateral Continuity

erosion

This concept enables us to correlate outcrops of strata that has been dissected by processes of ________.

CORRELATION OF ROCK STRATA:

Correlation is Matching similar rock strataat different locations to see if they formed at the same time

Matching similar rock strataat different locations to see if they formed at the same time

Ways to correlate rockformations:

“Walking the outcrop”

is done by Walking from end to endWalking from end to end

This is correlation by

continuitycontinuity

You can match the rock strata in one location with rock

strata in more distant locations by

Comparing ,

Comparing ,

color texturecomposition

Time correlation compares ____________ contained in the

rock strataindex fossilsindex fossils

1

2

3

4

456

3

The best index fossils:

a. _________________

b. _________________

Exist for a brief period of time

are widespread

Exist for a brief period of time

are widespread

Which fossil would make the best index fossil?Which fossil would make the best index fossil?

Found in only 1 layer (short lived)Found in all samples (widespread)

Another way of correlating layers by time is through

___________________Volcanic ash fallsVolcanic ash falls

These ash falls are very ________ events. A single

layer of ______ can be found over a large area, this allows

geologists to make a__________________ from one location to another

at the position of a common ash fall.

briefbriefashash

time correlationtime correlation

GEOLOGIC TIME SCALE:

A. Geologists noticed that rock _________ can be identified by the fossils they contained.

formationsformations

They also found that certain __________ were consistently located ________ or _________ other formations.

formationsformations

aboveabovebelowbelow

From these observations they established a

______ ____ ______with a sequence of fossil groups from ______ to

______________

relative time scalerelative time scale

oldestoldestyoungestyoungest

Each of these groups was named for a location where its ____________ ______ could be observed in the rocks.

Each of these groups was named for a location where its ____________ ______ could be observed in the rocks.

Characteristic fossilCharacteristic fossil

Example:Devon fossil “Devonian” found in Devon England

Example:Devon fossil “Devonian” found in Devon England

Further observations from around the world established a

________ _____ _____Geologic time scaleGeologic time scale

Based on __________________________________

and ________________________ _____ _____

Inferred positions of Earth’s Landmasses Inferred positions of Earth’s Landmasses

Major Geologic Events(ex. Ice ages & Orogenys)Major Geologic Events(ex. Ice ages & Orogenys)

An ________ is the process of mountain building

TURN TO PAGES 8 & 9 IN YOU ESRT!

OrogenyOrogeny

GEOLOGIC EVENTS OF THE PAST:

_________ causes gaps in the geologic record.

ErosionErosion

When a new layer of rock is laid down on a surface that has been _______ it forms a buried erosional surface or an

___________________

Eroded,Eroded,

unconformityunconformity

NEWBOTTOMLAYER APPEARS(EMERGES)

LAYER CIS MISSINGEROSION

EROSION

THE UNCONFORMITY IS THE BURIED EROSIONAL SURFACEBETWEEN B AND D

ACTIVITYWHICH IS OLDER

VII. RADIOACTIVE DATING:

A. Fossils enabled geologists to give ___________ time,relativerelative

Relative Time

Compares rock ages to _______________.

Ex: The Limestone is older than the Sandstone.

each othereach other

However,

B. Measurements of natural ___________in ( metamorphic and igneous) rocks have allowed the _________ time scale to be an ________ time scale.

geologicgeologic

absoluteabsolute

radioactivityradioactivity

The _________ _____ of an object is measured in years.

Ex: The limestone formed 5 mya and the sandstone formed 2 mya

The _________ _____ of an object is measured in years.

Ex: The limestone formed 5 mya and the sandstone formed 2 mya

absolute ageabsolute age

C. Chemical elements often have several forms called _______________isotopesisotopes

ISOTOPE:

An unstable element with different number of neutrons than a normal (stable) element. (Its unstable so wants to change to stable)

An unstable element with different number of neutrons than a normal (stable) element. (Its unstable so wants to change to stable)

EX.

CC CC66 66

1212 1414

6 protons 6 protons6 neutrons6 protons 6 protons6 neutrons

Unstable

8 neutrons8 neutrons

EX.EX.

CC CC66 66

1212 1414

6 protons 6 protons6 neutrons 8 neutrons6 protons 6 protons6 neutrons 8 neutrons

unstableunstable

D. If the nucleus has more or fewer than the

normal number of ____________, the isotope

may be ____________

neutronsneutrons

radioactiveradioactive(unstable)(unstable)

E. A radioactive isotope will break down naturally into a lighter element called

_____ ________ which is stable.

decay productdecay product

This process is called…

Radioactive Decay

Radioactive Decay

RADIOACTIVE DECAY:

WHEN AN UNSTABLE ________ ELEMENT CHANGES INTO A COMPLETELY DIFFERENT (BUT STABLE) __________ ELEMENT

DAUGHTERDAUGHTER

PARENTPARENT

F. A sample starts out at “Time zero” with _______ Percent of radioactive material.

100100

Time Zero: when the sample is originally formed by cooling or solidification of igneous or metamorphic rock

As time goes by and the sample gets older, the radioactive element decay, and _______ radioactive atoms remain in the sample.

fewerfewer

Therefore, the higher the ratio of decay product to the radioactive element, the _____ the sample.

olderolder

The ratio between the mass of the radioactive element and its decay product in a sample is the _______________

decay product ratiodecay product ratio

G. The decay of the parent atoms in a sample to daughter atoms is a _________ process…

randomrandom

That happens at _____________ rates for different radioactive elements. Lets model this with pennies…

differentdifferent

H. The rate of decay of a radioactive element is measured by its’ _______ _________half lifehalf life

HALF-LIFE:HALF-LIFE:THE AMOUNT OF TIME IT TAKES FOR

HALF OF THE UNSTABLEATOMS IN A SAMPLE TO CHANGE TO THE STABLE DECAY PRODUCT

THE AMOUNT OF TIME IT TAKES FOR

HALF OF THE UNSTABLEATOMS IN A SAMPLE TO CHANGE TO THE STABLE DECAY PRODUCT

Original=100%

Decay product=0%

100/0Or 1 to 0

Original=50%

Decay product=50%

50/50 Or 1 to 1

After one Half-life:

Original=25%

Decay product=75%

25/75Or 1:3

After two Half-lives:

Original=12.5%

Decay product=87.5%

12.5/87.5

After three Half-lives:

Original=6.25%

Decay product=93.75%

6.25/93.75

After four Half-lives:

1 Carbon-14 C14

2 Potassium-40 K40

3 Uranium-238 U238

4 Rubidium-87 Rb87

N14

Ar40

Pb206

Sr87

5.7 x 103

1.3 x 109

4.5 x 109

4.9 x 1010

Element Decay Product Half-life

H. Calculating the age of a rock:

1. What would be the age of the rock if it has equal amounts of C-14 and its decay product N-14?

One half life has gone by

One half life has gone by

5.7 x 103 yearsor 5,700 years

5.7 x 103 yearsor 5,700 years

2.What % of the sample is radioactive after the following half-lives,

1 half-life2 half-lives

3 half-lives

2.What % of the sample is radioactive after the following half-lives,

1 half-life2 half-lives

3 half-lives

50%50%25%25%12.5%12.5%

3. After 11,200 years how much C-14 would remain in a 10 gram sample?

25% or 2.5 grams1/4 of the original amount

25% or 2.5 grams1/4 of the original amount

I. Selecting the Best Radioactive Element:

1. Under 50,000 years

2. Over 50,000 years

Use Carbon-14Use Carbon-14

Use Uranium-238Use Uranium-238

Carbon 14 is used for dating

organic materialAnd ancient wood fires

Carbon 14 is used for dating

organic materialAnd ancient wood fires

“Time Zero” for carbon dating begins when the organism ___________ or when the ________ burns out.

diesdieswood firewood fire

THE END