Monday, December 3, 2007
Find out What's happening to the Ground Surface below your feet in Washington County, UT
A Pleistocene Volcano, named Crater Hill, is one that has "Popped Up" within the last one-third million years
The Ground is moving near Hurricane, Utah!
The surface of the Earth is moving noticeably in Washington County, and if one hikes regularly on the trails, it is detectable.
Upon arriving in the towns of Hurricane and St. George, the remaining basalt flows from ancient volcanoes are the first visible clues that geological processes have happened fairly recently. Several of these flows are in populated areas, and form the barriers to traffic across the towns. These basalt-capped mesas have had molten rock flow similarly to streams over the surface of the ground in old creek beds, in times less than one million years ago (sometimes, at times only 100,000 years past). These mesas are now covered with rock that is very resistant to erosion (black basalt), and the paths of the stream beds which they occupied in ancient time can now be followed to determine the direction of flowing creeks in that long-ago period. Looking on a map for these mesas, it is apparent that creeks moved mainly southward from the 21 million year Pine Valley Mountains PVM- which is the direction of the trend of the old basalt-covered mesas. Nowadays, the streams have carried away the soft surrounding sandstones, so that new streams move in other directions- sometimes similarly to the Virgin, which moves southwest or west in Hurricane. So, erosion is a prime factor in determining the movement of the Earth’s Crust in Washington County. But there are other independent movements, which may be subtle, but are important in understanding what’s happening in this rapidly-changing region, Geologically. These are listed below, so that one can focus on them while hiking the region:
1. The Colorado Plateau, CP, is uplifting east of Hurricane, relative to the Basin and Range, B&R, to the west, and this may be sub-divided into several components:
a. The Plateau is uplifting far from the Hurricane fault, and this can be seen with the following link: http://www.ngs.noaa.gov/cgi-cors/corsage.prl?site=fred This shows a small upward movement, on the order of a centimeter, for the period of a few years of measurement. This may be due to the loss of mass removed by the Virgin and Colorado rivers, as sediment is carried away annually, and to the resulting reduction of weight pushing downward. This is called Rebound.
b. The CP is heated by several processes taking place now, and these result in thermal expansion upward (similar to an iron bar which expands when heated in a fire). This can be noticed when drilling wells, which are measured for abnormal bottom-hole temperatures. The usual suspects rounded up for explanation for this present heating include radioactive decay, heat given off by increase of density with forced deep burial of rock (similar to the heating of air in your tires, whenever you increase the pressure), and friction of one rock surface sliding over another.
c. Finally, there are places in CP which move much faster than others nearby, and these are under investigation. An example is the rising mountainous area east of Cedar City. It appears that the north-trending Hurricane fault is acting like the blades of scissors which move increasingly upward to the point of the scissors as one looks north of Laverkin.
2. The ground surface near previously hot volcanism reacts as the underground rock cools- reverse to that of thermal expansion- and the land above the volume with greatest cooling sinks. This is noticeable along the Hurricane fault, Hf, so that when the maximum cooling occurs at the fault exposure, there is tilting as the land above the cooling magma chamber sinks the most at the open fault surface, Hf. The western edge of the sinking mass remains relatively high in the form of a sharp edge of rock, known as a Hogback (since this mass incurred less heating in the first place). In this case there is not only vertical shrinking but also tilting locally- as much as one mile distant from the volcanic cone or plug (example: the Laverkin Hogback near the confluence park north of Virgin River).
Laverkin Hogback is Photographed between the two Hurricane Craters which helped create it
3. The Earth is shrinking as it slows, particularly at regions where an equatorial bulge developed in early days when the spin of the Earth was greater (days were shorter). This happens for the earth’s crust where the bulge is more noticeable- in the latitudes less than 45 degrees, This should be a continuing shrinkage and is thought to develop north to south, N-S, fractures in the crust. Accompanying these are perpendicular cracks, or those at 90 degrees, E-W, to the principal N-S ones (example: the N-S Hurricane fault).
4. Whenever masses of the earth’s crust collide with each other, particularly at small angles to each other, shear develops as the separate rock masses slide past. This causes shear laterally, of the rock masses, and this is noticed to occur mainly along northwest to southeast, NW-SE, orientations (and orthogonals- perpendicular- to these). Consequently, faulting occurs at 8 divisions of the circle of directions, or of the 360 degree compass rose- N-S, NE-SW, E-W, and NW-SE- 45 degrees apart. The diagonal directions should exhibit shear, while the others should exhibit normal faulting or simple downdrop. Normal faulting is by far the most common, while shear in the diagonal directions is the most difficult to observe- since it moves laterally along the earth’s surface with little noticeable displacement of geological formations vertically (example: the line of Wet Sandy and Santa Clara Creeks eroded into Pine Valley Mountains, PVM).
5. Finally, the earth reacts at irregular intervals, as adjustments to all of the above movements occur. This produces earthquakes, and these accompanying slipping at depth- above which is a place on the ground called the epicenter- in addition to violent movements at the earth’s surface. These locations tend to be found on lines of stress direction, frequently NW-SE orientations- called linears or lineaments, or open cracks in the ground surface. Springs and gases may issue along these linears, and chemical changes may be noticed at the surface of the ground (example: Pah Tempe hot spring and H²S gas near Sullivan’s Knoll).
Follow the sequences outlined above, so that you can see the result of movements occurring in the earth where you hike. This Washington County, Utah area has almost all types of Earth processes and movements known to Earth Scientists. Watching for these movements can help you in determining what to expect in the future for your Real Estate, your landscape, and your hiking trail.