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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