Last Modified: 08/20/09 |
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!
Terms to Know | |
Broken Rocks | |
Folded Rocks | |
Map Symbols | |
Important Principles to Know | |
Block Diagrams |
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 |
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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.
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.
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...
Another important observation regarding Dip-Slip faults is that they move older rocks next to younger rocks.
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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...
Fold Types:
Anticlines and Synclines may be:
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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!)
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!)
Synclines: (Dark line marks fold axis; arrows denote direction limbs are dipping)
Anticlines: (Dark line marks fold axis; arrows denote direction limbs are dipping)
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Information Given by the Block Diagram:
1) The top of the block is a HORIZONTAL PLANE
- A dipping bed that intersects it forms a STRIKE LINE
2) The sides of the block are VERTICAL walls
- Will show the DIP DIRECTION of the beds
- Will show the DIP angle of the beds
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Older beds dip toward younger beds (unless overturned).
The lower the number the older the bed (1=Oldest)
How to draw block diagrams for synclines, dipping beds, etc.
How to recognize sequences (ex. 1234321 - a syncline)
How to determine the type of fault you're looking at (i.e., a Reverse Fault, Normal Fault, Right Lateral Strike Slip, etc.)
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