Created By Robert Almanza, May 3, 1998
*Premised Upon "Introduction to Earth Science Lab, Chapter 7"
TABLE OF CONTENTS
Introduction
The law of superposition provides that in a sequence of rock layers, each layer is older than the next layer above. This fundamental concept allows geologists to interpret the geologic history of sedimentary rocks. However, an exception to this rule occurs when tectonic activity within the earth's crust turns the layers upside down. Such tectonic acitivity, or deformation, is evidenced in the following geologic structures: Folds and Faults.
A fold is caused by differential stress - that is stress that is not equal in all directions. Three kinds of differential stress occur:
*compressive stress - squeezes and shortens bodies
*tensional stress - stretches and pulls apart bodies
*shear stress - results in slippage and translation.
Such stress results in a bend or flexure in stratified rocks. This bend, or "fold," can occur in a variety of ways . . .
1) Anticline - When layers of rock bend upward, the resulting fold is called an anticline.
Anticline
2) Syncline - When layers of rock bend downward, the resulting fold is called a syncline.
Syncline
3) Dome - a circular or elliptical fold in which the layers of rock fall away from a central point.
4) Basin - a circular or elliptical fold in which the layers of rock rise from a central point.
5) Monocline - a fold resulting in a local steepening in the dip of the strata. Note, there is only direction of dip in a monocline.
The two most common types are anticlines and synclines. They can each range in size from small wrinkles to great arches and troughs whose dimensions must be measured in miles. Interestingly, when traced in their long dimensions, folds ultimately disappear or die out, similar to wrinkles in a rug.
Anticlines and Synclines can further be divided:
Symmetrical Fold - a fold with the sides showing a mirror image with respect to the axial plane.
Asymmetrical Fold - a fold without a mirror image in respect to the axial plane.
Overturned Fold - a fold in which the axial plane is tilted and beds may dip the same direction on both sides of the axial plane.
Note: "axial plane" refers to the surface that divides the fold in half, while "fold axis" is the intersection of the bedding surface and the axial plane.
*folds may be plunging or non-plunging. If a fold is inclined to the horizontal, the dip of the axis is called the "plunge."
In contrast, in non-plunging folds contact lines separating formations shown in the surface view are parallel and straight. The contact lines in one of the two parallel views are horizontal and parallel. The contacts in the other profile view are arched up and down.
*A fault is a fractured surface or zone in rock in which one wall of the fracture is displaced relative to the other wall.
*Faults are produced by stress in the earth's crust that either compress or extend rocks. Before we can begin a thorough discussion of faults, a brief review of terminology must be undertaken.
FAULT TERMINILOGY
1) "Hanging Wall" - The surface that overlies the plane of the fault.
2) "Footwall" - The surface that lies below the plane of the fault.
3) "Strike" - The direction of the line of intersection of the fault plane with a horizontal plane. In other words, the direction in which the fault runs.
4) "Dip" - imaginary line down slope on a fault. The dip direction is perpendicular to the strike direction.
With that in mind, we are ready to discuss the different kinds of faults.
KINDS OF FAULTS
*Normal Faults
In a normal fault, the hanging wall moves down with respect to the footwall.
*Reverse Faults
In a reverse fault, the hanging wall moves up with respect to the footwall.
*Nature has provided its own illustrations
A normal fault
Reverse Fault
*Strike-Slip Faults
In this kind of fault, the two blocks of crust move either to the left or to the right relative to one another.
An example of this type of fault
is the San Andreas
Fault , found in California (taken from:
Schulz, S. S. & Wallace, R. E. (1997). The San Andreas Fault.[WWW document].
URL http://pubs.usgs.gov/gip/earthq3/ .
*Thrust Faults
This type of fault occurs as a result of an upper block of crust being driven over a lower block along a fault plane of very low inclination. The fault is produced by a compression of crustal rocks.
An example of this type of fault can be found in the Alps. For more info on thrust faults, click here
Temperature
As a rock is compressed, it becomes very hot. The hotter a rock is, the more likely it will soften and fold. Cooler rocks are more brittle, and thus, more likely to fracture.
Pressure
The greater pressure a rock is under, the more likely it is to fold. Note, pressure is produced by crustal rocks of the continents pressing down on the rocks underneath.
Rock Types
Certainly the composite of rocks influence what changes will take place under stress. For example, Sandstone is very brittle and would be more likely to fracture than say Rock Salt.
Conclusion:
While the actual bending of rock strata boggles the mind, nature has provided proof time and time again. When stress is applied to strata deep within the earth's crust, it no longer is brittle and folding occurs. Certainly, the beautifal structures created by this process are amazing. However, folds also harbor oil and natural gas and provide a geological history of a region. Such benefits must not be overlooked.
Faults, or fractures, are surfaces along which the rocks on one side are displaced with respect to those on the other. The four types of faults discussed include normal faults, reverse faults, strike-slip faults, and thrust faults. Whether a rock fractures can be influenced by a number of factors including temperature, pressure, and rock type.
For more information on
folds, faults and related topics... 
And for some geology humor
. . . 
Thanks for visiting my page. Let me know how you like it. You can e-mail me at: almanzar@esuvm.emporia.edu
References:
Bates, Robert L. Geology An Introduction. D.C. Heath and Company: Boston, Massachusetts; 1966.
Stokes, William. Introduction to Geology. Prentice-Hall, Inc.: Englewood Cliffs, New Jersey; 1978.
Schulz, S. S. & Wallace, R. E. (1997). The San Andreas Fault [WWW document]. URL http://pubs.usgs.gov/gip/earthq3/.Brantley & Topinka (1984) Description: Ground-Crack and Thrust-Fault Measurements using Steel Tape and Rebar. Volcanic Studies at the U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory, Vancouver, Washington: Earthquake Information Bulletin, 16(2), p 79-84. [WWW document]. URL http://vulcan.wr.usgs.gov/ Monitoring/Descriptions//description_crack_thrust_meas.html
Bret (1998). Geology Link Page [WWW document]. http://www.realtime.net/~revenant/geo.html
Drew, J. (1998). Joe's Vertiable "Rock" Page [WWW document]. URL http://www.umr.edu/~jdrew/geo.html
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