Structural Geology Lab

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Last Modified: 08/20/09

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Structural Geology is perhaps one of the hardest subjects for beginning geology students to learn. It deals with geologic structures (faults and folds) that form when stresses (tension, compression, shear) act upon a body of rock. The difficulty is that it requires that the student think in 3-D -- which is a difficult skill to learn. However, there is hope! Knowing a few simple rules (and a lot of terminology) can allow even the beginning geologist to handle some rather complex structural problems. Let's get to it!

Click on the topic below to jump to that section:

Terms to Know
Broken Rocks
Folded Rocks
Map Symbols
Important Principles to Know
Block Diagrams

Terms to know:

Unconformity	Stress and Strain	Compression
Tension	Shear	Strike
Dip	Joint	Fault
Slickensides	Normal Fault	Reverse Fault
Thrust Fault	Axial Plane	Fold Axis
Limb	Fault Plane	Horst/Graben

* Up to Structural Geology Topic List *

BROKEN ROCKS

Joint: A break in a rock (crack) in which there is no relative movement of either side across the break.

Fault: A break in the Earth in which the rocks on either side of the break have been displaced (vertically and/or horizontally) relative to each other (see pictures below).

Strike-Slip fault: Movement of the Foot Wall and the Hanging Wall blocks is parallel to the strike of the fault plane. These faults form from shear stresses.

[Left and Right-Lateral Strike-Slip faults]

How do you name the strike-slip fault?

Just imagine you are standing on one side of the fault. How did the other side move relative to the side you're standing on? If it appeared to move left, it is a left-lateral strike-slip fault. If it appeared to move right, it is a right-lateral strike-slip fault.

Dip-Slip fault: Movement of the Footwall and the Hanging Wall blocks is parallel to the dip direction of the fault plane. These faults form from tensional stresses or compressional stresses.

[(Reverse) Fault Anatomy]

The above picture displays the "anatomy" of a Dip-Slip fault. Let's go over it in more detail...

Notice the terms 'Footwall Block' and 'Hanging wall Block'

These are old mining terms for the two sides of the fault (where the rocks have moved relative to each other)
The side a miner could walk down (put his feet on) was called the 'Footwall Block'
The side a miner could hang his lantern on was called the 'Hanging Wall Block'

The Footwall and Hanging Wall Blocks are separated by the Fault Plane

The Fault Plane can be thought of as the plane in which the rocks slide past one another in opposite directions
Movement arrows are placed around the fault plane in order to indicate relative movement of the fault blocks

There are 2 kinds of dip-slip faults: Normal and Reverse Dip-Slip Faults...

In Normal Faults, the Footwall Block moves UP relative to the Hanging Wall Block

An easy acronym is F.U.N. - Footwall Up = Normal
These form from tensional ("pulling apart") stress

In Reverse Faults, the Footwall Block moves DOWN relative to the Hanging Wall Block

An easy acronym is F.D.R. - Footwall Down = Reverse
These form from compressional ("pushing together") stress

Another important observation regarding Dip-Slip faults is that they move older rocks next to younger rocks.

* Up to Structural Geology Topic List *

FOLDED ROCKS

Hard, brittle rock may fold and not break if the stress is applied slowly and continuously over a very long period of time. Folds form purely from compressional stresses. Let's look at the anatomy of a fold...

[Anatomy of a fold]

Fold Types:

Syncline: Limbs dip towards the axial plane (shaped like a Smile - side view)
Anticline: Limbs dip away from the axial plane (shaped like an 'A' - side view)

[Syncline] [Anticline]

Anticlines and Synclines may be:

Symmetrical (the above pictures; axial plane is vertical; limbs dip at the same angle)
Asymmetrical (picture below (side view; Asymmetrical Anticline); note how axial plane is tilted; limbs dip at different angles)

[Asymmetrical Anticline]

* Up to Structural Geology Topic List *

Map Symbols:

Dip and Strike:

Horizontal Beds:

Vertical beds:

Normal Fault: (Note that the arrow points in the direction the fault plane is dipping; it also (therefore) always points to the hanging wall block!)

[Normal Fault symbol]

Reverse Fault (Thrust): (Note that the 'teeth' point in the direction the fault plane is dipping; it also (therefore) points to the hanging wall block!)

[Symbol for Thrust Fault]