Rock and Roll: The New Madrid Fault System Part I: A Geology Primer – By Stephanie Osborn
Rock and Roll: The New Madrid Fault System Part I: A Geology Primer
Excerpted from Rock and Roll: The New Madrid Fault System, ©2017
By Stephanie Osborn
Images in this article are public domain.
This whole collection of writings started off with an email exchange, months ago. Our illustrious hostess was part of the discussion, and expressed interest in my converting the info to one or more blog articles. Then, when the LibertyCon programmers heard about it, they asked me to give a presentation on same, which I did.
The presentation was a full house, and at the end, there was a request for me to convert it to blogs and/or an ebook. I asked how many would like to see an ebook of the material; virtually every hand in the lecture hall went up.
A little over a month later, with additional research under my belt and factored into the manuscript, the book has gone live. And as promised, I am providing Sarah a series of blog articles on the subject. This series of blog articles is only a small fraction of the material contained in the ebook; it may be considered in the nature of a series of informative abstracts of the information contained therein. For additional information, may I recommend that you check out Rock and Roll: The New Madrid Fault System.
Part I: A Geology Primer
The New Madrid Fault Zone is the core of a large area in the central USA consisting of a couple of main faults and numerous secondary faults, as well as other significant geological structures and features. It produced an historic series of severe quakes in the winter of 1811-12 timeframe, and it continues active to this day.
The New Madrid Fault Zone is located in the central United States, roughly along the medial segment of the Mississippi River, from about Memphis, TN running north to at least the confluence with the Ohio River. There are strong indications that the fault zone extends considerably farther in each direction, though exactly how far remains in doubt.
Now let’s define some basic terms, so everyone can understand what’s being said when I dive into the meat of the discussion.
A fault is a crack in the rock around which the rocks actually MOVE. Stress fractures can occur in rocks that are experiencing non-moving stresses, such as temperature extremes, but these are NOT faults. Only if one or both of the stone blocks on either side of the fault experience movement is the fracture considered a fault.
The surface direction or trend of the fault is known as the strike. The vertical orientation of the fault is known as the dip. The importance of a fault’s dip lies in the fact that the actual source of the quake may not be directly under the part of the fault that shows at the surface.
A quake is generated if the fault has been unmoving for a prolonged period of time, with the cessation usually due to friction between the crustal blocks, and stress has built up sufficient to either exceed the “coefficient of friction” (a physics term characterizing the amount and type of friction between surfaces) between the fault blocks, or to actually shatter the rocks that are hung up on each other. At this point, the temporarily-stationary blocks “slip” past each other, releasing potentially tremendous amounts of energy in an earthquake.
The point where the slip begins is the hypocenter or focus, and it will usually be below the surface of the ground. The point ON the surface, directly above the hypocenter, is known as the epicenter. So the pertinent data to identify any given quake are: the magnitude, the latitude/longitude of the epicenter, and the depth of the hypocenter.
Not all faults, of any sort, will generate quakes, let alone major quakes. Some faults, or segments of faults, tend to experience slow, small, continuous movement called creep. This creep can cause slow deformation of structures on the surface, but will not cause widespread damage.
There are three principal kinds of faults, though there can be combinations. These basic types depend on the direction of movement of the fault blocks on either side of the actual fault.
- The normal fault.
In a normal fault, the main block aka the “hanging wall”—the block above the fault—moves down relative to the secondary block or “footwall” which is below the fault. Such faults are created under tension forces, and movement is produced as a result of continued tension. If the tension is removed, the fault ceases to be active.
- The reverse, or thrust, fault.
In a reverse fault, the main block or “hanging wall” moves upward and over relative to the secondary block or “footwall”, which may remain still or move downward and/or under. Such faults are created under compressive forces, and movement is produced as a result of continued compression. If the compression is removed, the fault ceases to be active.
- The strike-slip fault.
In a strike-slip fault, the blocks move past each other in a horizontal motion. Such faults are under slight compression, mostly in a transverse fashion, meaning the forces are across and at an angle relative to the fault.
- The oblique fault.
An oblique fault combines horizontal and vertical motion in various fashions; it is basically a strike-slip fault with either normal or reverse faulting added in, depending upon whether the overall forces combine to produce tension or compression.
Grabens and horsts
Put in everyday language, grabens and horsts are fault-block hills or mountains (the horsts) and valleys (the grabens). They are formed when several parallel-trending or nearly-parallel normal faults alternate dips (downward angle of the fault), which will in turn result in alternating uplifted and/or down-dropped blocks of stone. Usually the sequence of fault blocks is under tension (extension) in a direction perpendicular to the strike trend, causing them to spread apart, and enabling alternate blocks to slip downward.
Types of molten rock
Molten rock is generically termed “melt.” If the melt is below the surface, it is magma; when it has extruded onto the surface, it is lava. Any rock formed from cooled, solidified melt, regardless of location, is igneous rock.
A pluton is a magma intrusion into existing rock. It may intrude into existing rock in any of a number of ways: between different rock strata (layers), where the change in rock layer produces a localized weakness; into cracks in the rock, whether a joint or a fault; into areas of collapse, such as a caldera or sinkhole; or by melting/eroding away the rock and incorporating it into the melt.
Types of plutons include:
- Volcanic magma chambers
- Batholiths—These are, for all intents and purposes, “failed” volcanic chambers where a vent never reached the surface. Some were originally buried quite deep. They are essentially giant “bubbles” of magma that cooled and solidified over eons.
- Laccoliths—These are large “bubbles” of magma that, unlike the more vertical batholiths, are usually inserted horizontally, between existing rock strata; they tend to be lenticular, or lens-shaped. Over time, they, too, cooled and hardened.
- Sills—These are narrow horizontal intrusions, usually following a horizontal fault, joint, or other crack. Being relatively thin, they cooled and hardened fairly quickly on geologic scales.
- Dikes—These are narrow (but often long) vertical intrusions, typically following a vertical crack or fault. Other than spatial orientation, they are similar to sills.
To obtain a copy of Rock and Roll: The New Madrid Fault System by Stephanie Osborn, go to: