GEOL 101
Lecture 10:
Introduction to Geological
History
Chapter 8
Metamorphic Rocks, Continued Contact Metamorphism
Geology 101 Section 002 Quiz #3 (Due
in class (7 oops) 9 February 2006)
• 1. What are minimum temperatures and
pressures required for metamorphism of
rock?
2. Fill out the following table.
Metamorphic Rock under
Original Rock Regional Metamorphism
Metam. Rock under
Contact Metamorphism
****
Hornfels
Granite
*
Granite: unless
heat = extreme
Sandstone
*
Quartzite
Limestone
*
Marble
Bituminous Coal
*
Anthracite
Mafic Igneous Rx
*
Hornfels
Dolomite (Same
as One of the Above)
?
?
Shale
1
3. 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?
Geologic Time Scale
Relative Dating (Logic) &
Absolute (Radiometric) Dating
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
2
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
GEOL 101 Lecture #
3
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)
Superposition
• Stratified Sediments or Rocks
• Younger “Rocks” - Added to Top
of a Sequence
• Oldest “Rocks” at the Bottom of
Sequence
4
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
Breached (Eroded) Folds & Age of
Rocks. Superposition: Youngest
on Top
Younger Rocks
Older Rocks
Where are older rocks exposed at the surface? Younger Rocks
5
•
•
•
•
•
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 .
Fault Parts
6
Cross-Cutting Relationships - Plutons
3. 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?
•
•
•
•
•
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)
7
Unconformity
• Buried Erosion Surface .
Unconformity, Ouray Colorado
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.
8
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)
Maturity
Immature Sediments
Mature Sediments
Facies
9
Tools to Reconstruct Relative
Ages in Earth History
• Correlation .
Correlation:
• Marker Beds
–Volcanic Ash
–Iridium Layer
•(Bolide Impact) .
Volcanic Ash Marker Beds
10
Iridium Layer
0
,0
00
0
,
65
0
ye
s
ar
d
ol
Great Relative Dating Tool: Fossils
Trilobite
Correlation with Fossils
11
Strata = Layers
Stratigraphy =
Study of Layered Rocks
Stratigraphic Units
• Groups
• Formations
•
Members
Formation
• Smallest Mappable Stratigraphic Unit
• (vs. Outcrop)
12
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
Local Stratigraphy
• Pottsville Group
• Connequenessing Formation
•
Upper Connequenessing Sandstone
Crops Out at Coopers Rock
13
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?
How Do we Know That?
Carbon-14 Dating
= Radiocarbon Dating
14
Carbon Isotopes
Common, Stable
Uncommon, Stable
Uncommon,
Radioactive
Cosmic Rays “Zap” a Nitrogen-14
Proton into a Neutron to Form C-14
Beta Capture
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
15
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
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....
16
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.
What Can C-14 Dating Be
Used For?
• Dating Dead Things
• <100,000 years
17
Bristlecone Pine
Tree-Ring
Calibration of
C14 Dating
http://www.sonic.net/bristlecone/Images2.html
Dendrochronology
Another Absolute Dating Method
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
18
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
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
19
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
20