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
_________________
Uniformity)Uniformity)
crust crust
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
Principle 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 ______________
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
Rock __________included in another rock must be ______ than the rock that its is apart of.
A rock must first exist in order to be weathered, deposited and cemented as a _____ in another rock.
fragments
older
clast
Law of Inclusions
Which is older the Granite or the Sandstone? In figure A? In figure B?
Sandstone is older
Granite is older
Principle of Cross-Cutting Relationships
Any __________, ______, 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 intrusion came afterthe rock was formedThe intrusion 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
Exceptions to the Law of Superposition:1.An __________ is an igneous rock that formed from lava at 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
2. __________ 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
3. _______ are bends in the rock strata. ________ can overturn rock strata so that ________ rock lies on top of _________ rock.
FoldsFolds
foldingfolding
olderolder
youngeryounger
4. _______ are cracksin rock strata. _______ produce offset layers.
FaultsFaults
FaultsFaults
4. _______ 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
If rock is composed of _____________, the rock fragments must be ___________ than the rock in which they are found.
fragmentsfragments
olderolder
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 ________.
_________ 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
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
B. Measurements of natural ___________in rocks have allowed the _________ time scale to be an ________ time scale.
geologicgeologicabsoluteabsolute
radioactivityradioactivity
The _________ _____ of an object is measured in years.
The _________ _____ of an object is measured in years.
absolute ageabsolute age
C. Chemical elements often have several forms called _______________isotopesisotopes
ISOTOPE:
Different number of neutrons (unstable)
Different number of neutrons (unstable)
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. Radioactive isotopes
will break down naturally into a lighter element called _____ ________
decay productdecay product
(stable)(stable)
Use the reference table to complete:
Use the reference table to complete:
C14
K40
U238
C14
K40
U238
N14
Ar40
Pb206
N14
Ar40
Pb206
F. The rate of decay of a radioactive element is measured by its’ _______ _________half lifehalf life
HALF-LIFE:HALF-LIFE:
TIME IT TAKES FOR HALF OF AN ELEMENTSATOMS IN A SAMPLE TO CHANGE TO A DECAY PRODUCT
TIME IT TAKES FOR HALF OF AN ELEMENTSATOMS IN A SAMPLE TO CHANGE TO A DECAY PRODUCT
G. As the radioactive element decays _______ radioactive atoms remain in the sample.
fewerfewer
The higher the ratio of decay product to the radioactive element, the _____ the sample.
The ratio between the mass of the radioactive element and its decay product in a sample is the _______________
olderolder
decay product ratiodecay product ratio
Original=100%
Decay product=0%
100/0
Original=50%
Decay product=50%
50/50
After one Half-life:
Original=25%
Decay product=75%
25/75
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
*Add to your notes:
Carbon 14 is used for dating
organic material
Carbon 14 is used for dating
organic material
THE END