GEOL 101
Lecture 8:
Introduction to Geological
History
Chapter 8
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• 14 C lab
• 19th Cent smoke stacks
• A-Bomb
Geologic Time Scale
Relative Dating (Logic) &
Absolute (Radiometric) Dating
1
General Rules for Interpreting
Sedimentary Rocks
•
•
•
•
•
Law of Original Horizontality
Law of Superposition
Law of Cross-Cutting Relationships
The Meaning of Unconformities
Concept of Facies = (Environments
of Formation)
• Correlation Tools
Original Horizontality
• Horizontal Beds vs. Vertical Beds
Up
Up
What’s It All Mean?
Horizontal Bedding: U.S. Rt. 19,
Powell Mtn., WV
J.S. Kite Photo
2
GEOL 101 Lecture #
General Rules for Interpreting
Sedimentary Rocks
•
•
•
•
•
Law of Original Horizontality
Law of Superposition
Law of Cross-Cutting Relationships
The Meaning of Unconformities
Concept of Facies = (Environments
of Formation)
3
Superposition
• Stratified Sediments or Rocks
• Younger “Rocks” - Added to Top
of a Sequence
• Oldest “Rocks” at the Bottom of
Sequence
Horizontal Bedding: U.S. Rt. 19,
Powell Mtn., WV
J.S. Kite Photo
Superposition
• Oldest “Rocks” are at the Bottom.
• Younger Rocks are at the top.
Youngest
Oldest
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Breached (Eroded) Folds & Age of
Rocks. Superposition: Youngest
on Top
Younger Rocks
Older Rocks
Where are older rocks exposed at the surface? Younger Rocks
•
•
•
•
•
General Rules for Interpreting
Sedimentary Rocks
Law of Original Horizontality
Law of Superposition
Law of Cross-Cutting
Relationships
The Meaning of Unconformities
Concept of Facies = (Environments
of Formation)
Cross-Cutting Relationships
• Younger Intrusions or Faults
Cut Across Older Rocks .
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Fault Parts
Cross-Cutting Relationships - Plutons
Explain the geological history
represented by an outcrop of
basalt that is intruded by a dike of
rhyolite porphyry. The 5-10 cm of
rock that was basalt along the dike
is now hornfels.
Which rock was there first?
Which was there second?
What caused the hornfels?
6
•
•
•
•
•
General Rules for Interpreting
Sedimentary Rocks
Law of Original Horizontality
Law of Superposition
Law of Cross-Cutting Relationships
The Meaning of Unconformities
Concept of Facies = (Environments
of Formation)
Unconformity
• Buried Erosion Surface .
Unconformity, Ouray Colorado
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Unconformity, Ouray Colorado
Late Devonian sandstone
Elbert Formation ~360 million years old
Unco
nform
ity: ~
1,000
,
000,0
0
0 yea
rs m
issin
g
Precambrian shale, siltstone & sandstone
(Uncompahgre formation ~1.5 billion years old)
Vince Matthews, Colorado G.S. source for text information.
General Rules for Interpreting
Sedimentary Rocks
•
•
•
•
•
Law of Original Horizontality
Law of Superposition
Law of Cross-Cutting Relationships
The Meaning of Unconformities
Concept of Facies =
(Environments of Formation)
8
Maturity
Immature Sediments
Mature Sediments
Facies
Tools to Reconstruct Relative
Ages in Earth History
• Correlation .
Correlation:
• Marker Beds
–Volcanic Ash
–Iridium Layer
•(Bolide Impact) .
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Volcanic Ash Marker Beds
Iridium Layer
0
,0
65
00
0,
0
ye
s
ar
d
ol
Great Relative Dating Tool: Fossils
Trilobite
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Correlation with Fossils
Strata = Layers
Stratigraphy =
Study of Layered Rocks
Stratigraphic Units
• Groups
• Formations
•
Members
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Formation
• Smallest Mappable Stratigraphic Unit
• (vs. Outcrop)
Morgantown Stratigraphy
•
•
•
•
Monongahela Group
Conemaugh Group
Allegheny Group
Pottsville Group
Bits of Local Stratigraphy
• Monongahela Group
•
Waynesburg Formation
•
Uniontown Formation
•
Pittsburgh Formation
•
Pittsburgh Coal - $$$
• Conemaugh Group
•
Clarksburg Shale
•
Morgantown Sandstone
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Local Stratigraphy
• Pottsville Group
• Connequenessing Formation
•
Upper Connequenessing Sandstone
Crops Out at Coopers Rock
Absolute Dating
• E.G.
Radiometric Dating
• * Table 8.3, p. 195 *
• Plummer et al. 10th ed
• Expect Questions
Meadowcroft Rock
Shelter, Avella, PA:
Occupied
17,000 years ago
How Do we Know That?
13
How Do we Know That?
Carbon-14 Dating
= Radiocarbon Dating
Carbon Isotopes
Common, Stable
Uncommon, Stable
Uncommon,
Radioactive
Cosmic Rays “Zap” a Nitrogen-14
Proton into a Neutron to Form C-14
Beta Capture
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C-14 Dating
• C-14 mixed thru Atmosphere
• Plants take in C-14
• Animals Eat Plants
• Other Animals Eat Animals ... etc.
• All living things include C-14
C-14 Dating
• Dead Things Do Not Take in
New C-14 (or C-12 or C-13)
• C-14 Decays w/ Time
• C-12, C-13 Does Not.
Carbon - 14 is Radioactive
• Decays back into N-14 at Known Rate
• Half-Life is 5730 yr
N-14
C-14
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C-14 Dating
• In 5730 years (1 Half-Life), Half of
C-14 will Decay
• In next 5730 years, Half of what
was left will Decay
–3/4ths will decay in 11,460 yr
• 7/8ths will decay in 17,190 yr
• etc....
C-14 Dating
• <0.1 % of C-14 is Left After
57,000 yr
• Most Labs “poop out” at
>40,000 to >50,000 yr
C-14 Dates
•
•
•
•
Years Before Present (yr B.P.)
Present = A.D. 1950
+/- Measurement Error
example: 22,500 +/- 450 yr B.P.
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What Can C-14 Dating Be
Used For?
• Dating Dead Things
• <100,000 years
Bristlecone Pine
Tree-Ring
Calibration of
C14 Dating
http://www.sonic.net/bristlecone/Images2.html
Dendrochronology
Another Absolute Dating Method
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Carbon - 14
Basic Assumptions
# 1: Predictable Decay Rate
# 2: Constant C-14 Production Rate
# 3: All Living Things have Same
Portions of C-14, C-12, & C-13
Obvious Problems w/ C-14 Dating
• No Good for Most Minerals
• Useless on Really Old Stuff
–Find Another Method
Problems w/ C-14 Dating
• Basic Assumption # 3 :
– All Living Things DO
NOT have Same % of
C-14, C-12, & C-13
• Bottom Waters
• Antarctic Ocean
Diluted by Glacial
Meltwaters
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Problems w/ C-14 Dating
Basic Assumption # 2 :
–C-14 not Constant in Atmosphere
–Sun’s Cosmic Ray Output not
Constant
–More C-14 in Past
–10,000 yr BP = 11,500 Calendar yr
Man’s Impact
• Industrial Revolution - Fossil
Fuels diluted C-14
• Atmospheric Nuclear Testing
increased C-14
Problems w/ C-14 Dating
• Biggest Problem =
–Stupid Geologist -
• 2nd Biggest Problem =
–Contamination of Samples
• Young Carbon
• Old Carbon
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Geologic Cross Section - Hawaii
Highlights of Earth History and
Absolute Geologic Time
• ~13.7 BY (BY = billion years): Universe forms
• 4.55 - 4.60 BY: Earth forms
• (Rest of Solar System, too)
– Molten Magma Surface,
– Methane, Ammonia, Hydrogen Atmosphere
– High UV Radiation
– Amino Acids etc.
Geologic History
Zircon Photo: J. Valley
• 4.4 BY
– Oldest Mineral (Zircon xl)
• 3.8 - 4.1 BY
– Oldest Known Rocks, Chemical Fossils
• 3.5 BY
– Algae - Stromatolites
• 1.8 - 2.0 BY
– Oxygen "Crisis"
• 1.5 - 1.7 BY
– Sex
Stromatolites,
South Africa
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Geologic Time
• 600-650 MY (MY = million years)
– First Complex Animals
Hallucigenia sparsa
Anomalocaris canadensis
Big Changes
• ~551 MY
– First Hard-Shelled "Critters" Trilobites fossils common
• ~500 MY
– all phylum of higher animals exist,
including chordates (vertebrates)
551-245 MY
• Paleozoic (=“Old Animals”) Era
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245 - 65 MY Mesozoic Era
Age of
Reptiles
Allosaurus fragilis, a large Jurassic theropod, one step
away from capturing a Dryosaurus altus.
www.dinosaursinart.com/allosaurus/ IMAG0009.JPG
65 - 0 MY
• Age of Mammals
(Cenozoic Era)
Horses through Time
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The Geologic Time Scale
• See p. 195
–in Plummer et al, 9th edition
• Dates to Know :
• 65 MY, 245 MY, 551 MY, 4.5-4.6 BY
Question of Time
If you plotted all of
geologic time on a
time line as long as
the circumference
of the Earth (25,000
mi or 40,000 km),
what distance
would represent 1
year?
Earth Image Source:
www.ameritech.net/users/ paulcarlisle/earth.jpg
Question of Time
• If you plotted all of geologic time on a time
line as long as the circumference of the
Earth (25,000 mi or 40,000 km), what
distance would represent 1 year?
• What distance would represent 75 years?
• What distance would represent 1 semester
(4 months = 0.33 year)?
• What distance would represent this lecture?
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The Whole Earth Time Line
• Circumference of the Earth is 25,000 mi
(40,000 km)
• What distance would represent 1 year?
–Figure this out in small groups.
–Easiest if we use metric units
• Divide Distance by # of Years
(4,600,000,000 yr)
The Whole Earth Time Line
40,000 km / 4,600,000,000 yr =
distance represented by 1 year?
40,000,000 m / 4,600,000,000 yr =
4,000,000,000 cm / 4,600,000,000 yr =
0.87 cm / yr
(0.34 inch / year)
The Whole Earth Time Line
0.87 cm / yr
(0.34 inch / year)
X 75 yr = 65 cm
(Lifetime = 25.7 inches)
X 0.33 yr = 0.29 cm
(Semester = ~1/8 inch)
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Whole Earth Time Line
40,000,000,000,000 microns /
(4,600,000,000 yr X 365 day/yr X 24 hr/day)
= 0.99 micron / hour
This Lecture would be just over 1 micron
long on the Whole Earth Time Line.
The paper you are writing on is about 100
microns thick
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