Physical Geology Review
the expanse of
material is vast!
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Internal Structure of the Earth
granitic/basaltic
crust = 0-40 km; mantle = 40-2900 km;
outer core (liquid) = 2900-5200 km; inner core = 5200-6400 km
from: www.usgs.gov
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Guidelines for Structural Analysis
• reference frame
– law of original horizontality (rocks deposited
on nominally horizontal surfaces)
– law of superposition (deposition of new layers
are on top of old layers)
– stratigraphic continuity (deposits tend to be
continuous laterally, or discontinuous strata that
lie in the same plane are equivalent in time)
• nature of contacts
– stratigraphic (bedding or unconformities)
– intrusive (igneous or sedimentary)
– tectonic (fractures/faults)
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Basic Principles
• principle of superposition
– younger rocks are deposited on top of older
rocks
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Basic Principles
• law of original horizontality
– all sediments originally laid down in horizontal
layers
Grand Canyon, Arizona
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Basic Principles
• law of original horizontality
– non-horizontal bedding implies some sort of
deformation
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The Alps of Sisteron, France
Interpreting Field Data
• strike and dip measurements
• topography following geologic structure
• geologic cross-sections
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“Undeformed”, horizontal rock
Cedar Point, UT
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Horizontal Bedding
in Sedimentary Rocks
younger
older
•
•
•
original horizontality
lateral continuity
superposition
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Strike and Dip
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Looking in strike direction,
dip is angle from horizontal
arrow pointing down dip
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Interpreting Field Data
• strike and dip = attitude of planes (beds, fractures, ...)
– strike is intersection of plane with horizontal
– dip is angle between plane and horizontal
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Vertical Beds
Miocene Monterey Fm., California coast
Laminated bedding with selective dolomitization (yellow beds)
1 meter
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Orientation of Planes (strike)
• 3d block diagram
Strike is
intersection
between plane
and horizontal
Horizontal is
reference frame
North is
reference
direction
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Orientation of Planes (dip)
• vertical cross-section view
strike
dip angle
Dip is inclination of plane from
horizontal, measured in vertical
plane perpendicular to strike
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Folds and Faults
- folds are evidence of
ductile rock deformation
- faults represent brittle
rock deformation
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Monocline on the San Juan River, Utah
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Lateral Continuity (or lack thereof!)
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Faults
• vertical cross-section view
dip angle
Hanging Wall
Footwall
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Faults
• 3d block diagram
Fault
Hanging Wall
Footwall
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Dip-Slip (Normal)
Hanging Wall
Footwall
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Dip-Slip (Normal)
Hanging Wall
Footwall
horizontal stretching = extension
vertical thinning
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Dip-Slip (Reverse)
Hanging Wall
Footwall
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Dip-Slip (Reverse)
Hanging Wall
Footwall
horizontal shortening = contraction,
vertical thickening
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Strike-Slip
Hanging Wall
Footwall
strike-slip faults are typically vertical,
but they can be dipping like this one
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Strike-Slip (right-lateral)
Hanging Wall
Footwall
as you step across the fault,
the block you are stepping onto moves to the right26
Strike-Slip (left-lateral)
Hanging Wall
Footwall
as you step across the fault,
the block you are stepping onto moves to the left 27
Oblique-Slip
Hanging Wall
Footwall
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Oblique-Slip
Hanging Wall
Footwall
both strike-slip and dip-slip
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Normal Fault with Basin Shaped
Fold in Hanging Wall
shear sense = normal
Foot
Wall
Hanging Wall
Slip vector
Fault
Plane
Bedding
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Fault drag & shale smear
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Strike-slip fault
and Drag folding
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Slickensides = shear
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Reverse Fault in Core
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Geologic Cross-section
Map
view
B
A
A
Crosssection
view
B
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Fracture Types
• shear failure – faults
– relative displacement is parallel to fracture face
= shearing mode
• tensile failure – joints, veins, dikes
– a fracture with relative displacement
perpendicular to fracture face = opening mode
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Joint are “just” cracks….
(looking at dip slopes)
orientation changes
from bed to bed
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Joints
(looking at bed scarps)
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A Joint filled with minerals = vein
Bristol Channel, UK
Ouachita Mtns., Arkansas
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A “Joint” filled with magma = dike
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Mt. Moran, Grand Tetons, WY
Famous Fractures in the Movies
The Middle East?
hardly!
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from Transformers
How about New
Mexico?
volcanic
source
propagating dike
curving path to line up with earth stresses
Volcanic Neck
and Igneous
Dike:
Shiprock dike
volcanic
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source
Igneous Rock Features
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from Press and Siever, Understanding Earth
Other Geomorphic Features
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Glacial Moraine & U-shaped Valley
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Glacial Moraine
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Yellowstone River
Immature River – Steep, Vshaped Canyons
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Mature, meandering river (Yellowstone River)
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High energy rivers – heavy sediment load
channel cut & fill,
graded bedding
braided stream
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Alluvial fan
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Unconformities
• represent non-deposition or erosion
• often indicate tectonic events or global sea level
change, laterally very continuous
• types
– disconformity (parallel sed layers above and
below, missing some time)
– nonconformity (sed rocks on top of
igneous/metamorphic basement)
– angular unconformity (rocks above and below
have different orientation)
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Angular
Unconformity
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Disconformity
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Nonconformity
Cambrian Flathead ss on top of
Precambrian granite
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