Crustal Deformation

Take-Away Points1. Structures in the earth have practical

implications2. Basic terms for earth structures3. Small structures provide clues to much

larger structures4. The earth’s crust is floating on the denser

mantle beneath5. 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?

Which Way was Up?

Sedimentary Structures are governed by:• Gravity• Exposure to the SurfaceThey all have a “right way up”

3. Small structures provide clues to much larger structures

FoliationFoliation is a

sheetlike structure that forms when rocks are deformed.

3. Small structures provide clues to much larger structures

Foliationa. Flatteningb. Grain

Rotationc. Solutiond. New

Mineralse. Partial

Meltingf. Shearing

3. Small structures provide clues to much larger structures

FoliationIn 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 Points1. Structures in the earth have practical

implications2. Basic terms for earth structures3. Small structures provide clues to much

larger structures4. The earth’s crust is floating on the denser

mantle beneath5. Gentle uplifts and downwarps affect the

interiors of continents