Crustal Deformation
Take-Away Points
1. Structures in the earth have practical
implications
2. Basic terms for earth structures
3. Small structures provide clues to much
larger structures
4. The earth’s crust is floating on the denser
mantle beneath
5. Gentle uplifts and downwarps affect the
interiors of continents
Economic Consequences of
Geologic Structures
• Tracing Coal Seams, Aquifers, etc.
• Ore Deposits are often localized along
faults and folds
• Petroleum Traps
1. Structures in the earth have practical implications
The Big Picture
1. Structures in the earth have practical implications
Anticlines
and
Synclines
2. Basic terms for earth structures
Monoclines
and
Homoclines
2. Basic terms for earth structures
Axial Plane
2. Basic terms for earth structures
How Fast Can Folds Form?
Siesta Valley
Which Way was Up?
Sedimentary Structures are governed by:
• Gravity
• Exposure to the Surface
They all have a “right way up”
3. Small structures provide clues to much larger structures
Foliation
Foliation is a
sheetlike
structure
that forms
when rocks
are
deformed.
3. Small structures provide clues to
much larger structures
Foliation
a. Flattening
b. Grain
Rotation
c. Solution
d. New
Minerals
e. Partial
Melting
f. Shearing
3. Small structures provide clues to
much larger structures
Foliation
In every case, the
foliation is:
• In the direction of
least resistance
• at right angles to
the direction of
greatest
compression.
3. Small structures provide clues to
much larger structures
Folds and Foliation
• On a small scale (microscopic to
centimeters), foliation forms by a variety of
mechanisms, but always at right angles to
the direction of greatest compression
• On a large scale (centimeters to
kilometers), rocks fold. The axial plane of
the fold is also at right angles to the
direction of greatest compression
3. Small structures provide clues to much larger structures
Folds and Foliation
3. Small structures provide clues to much larger structures
The Importance of Minor Folds
3. Small structures provide clues to much larger structures
How Geologists Use These Clues
• Here's an outcrop that might be seen in
the field.
3. Small structures provide clues to much larger structures
How Geologists Use These Clues
• Picture the axial
plane of the fold
as parallel to
the foliation.
• The other side
of the fold is
roughly a mirror
image of the
side we can see.
3. Small structures provide clues to much larger structures
How Geologists Use These Clues
• We can mentally
fill out the sketch
to get an idea of
the shape of the
fold.
• Note that we still
have no idea how
big the fold is,
only what kind it
is.
3. Small structures provide clues to much larger structures
What if we did
it the other
way?
3. Small structures provide clues to much larger structures
Isostasy
4. The earth’s crust is floating on the denser mantle beneath
The Airy Model
4. The earth’s crust is floating on the denser mantle beneath
The Pratt Model
4. The earth’s crust is floating on the denser mantle beneath
Both Ideas are Correct at Times
4. The earth’s crust is floating on the denser mantle beneath
How Isostasy Works
4. The earth’s crust is floating on the denser mantle beneath
Isostatic
Rebound in
Canada
4. The earth’s crust is floating on the denser mantle beneath
Isostatic
Rebound in
Scandinavia
4. The earth’s crust is floating on the
denser mantle beneath
Domes and Basins
5. Gentle uplifts and downwarps affect the interiors of continents
Domes and Basins
5. Gentle uplifts and downwarps affect the interiors of continents
Domes and Basins
5. Gentle uplifts and downwarps affect the interiors of continents
Take-Away Points
1. Structures in the earth have practical
implications
2. Basic terms for earth structures
3. Small structures provide clues to much
larger structures
4. The earth’s crust is floating on the denser
mantle beneath
5. Gentle uplifts and downwarps affect the
interiors of continents