The Arizona Geological Survey has produced a short video showing the time and place of earthquakes in Arizona from 1852-2011.  According to the survey, “The older quakes are culled from historical records, and thus are limited to only those large enough to be felt or caused damage. In the past few decades, locations are from seismometers in the region.”  An artifact of the data makes it appear that there are more recent earthquakes.

It is very interesting to see where the earthquake hotspots are.  Can you guess before seeing the video? (Note, it starts a bit slowly.)  A running calendar appears in the lower right side of the video.

This information and video are taken from “Arizona Geology” the blog of Arizona State Geologist Lee Allison.  See original post here.

See also:

Precariously Balanced Rocks and earthquakes

Earthquake hazard near Flagstaff assessed, Video

Where the Next Big American Earthquake and Tsunami Might Occur

Arizona Geologic History: Chapter 1, Precambrian Time When Arizona was at the South Pole

Arizona Geological History: Chapter 2, Cambrian and Ordovician Time

Arizona Geological History: Chapter 3: Devonian to Permian Time

Arizona Geological History Chapter 4: Triassic Period

Arizona Geological History Chapter 5: Jurassic Time

Arizona Geological History 6, The Cretaceous Period

Arizona Geological History 7: The Cenozoic Era

Precariously balanced rocks such as spires, hoodoos, and stacked rocks make interesting scenery.  They also may provide a valuable tool for assessing the seismic stability of an area according to a study by geologists from Arizona State University. (Report referenced below.) They studied balanced rocks in the Granite Dells near Prescott and devised a method of calculating how much ground shaking it would take to destabilize the rocks.   In this post, we will take a look at several types of precariously balanced rocks and see how they form.

The photo above was taken at the Texas Canyon rest stop on I-10 between Benson and Willcox, Arizona.  It shows the weathering pattern and pedestal rocks in the Texas Canyon quartz monzonite (a granite-like rock).  The development of this geomorphology begins underground with chemical and physical weathering along joints in the rock and removal of material to make the joints wider.  Erosion eventually exhumes the rocks.  The photo below from the Chiricahua Mountains shows the results of this process  in somewhat softer volcanic rocks.

Hoodoos, such as these in the Chiricahua Mountains contain a hard capstone over softer material.  The capstone prevents complete erosion of the underlying material.

Perhaps the most spectacular rock spires in Arizona are those in Monument Valley, seen in the photo below on a misty day.  Here, hard sandstone occurs between softer siltstone and shale layers.  The spires are remnants of differential erosion by wind and water.

The Arizona State University study goes into great detail on methodology and technology about proposed analysis of precarious rocks for usefulness is accessing the seismic stability of a region.  One wonders, however, if these rock formations are really as precarious as they appear because the hoodoos and balanced rocks in Texas Canyon and in the Chiricahua Mountains survived the 1887 Sonoran earthquake.

According to the Arizona Geological Survey (Fieldnotes, summer 1987): “On May 3, 1887 Arizona and the Southwest experienced a major earthquake that had an estimated magnitude of 7.2 on the Richter scale.  The epicenter was in Sonora, Mexico approximately 40 miles south of Douglas, Arizona.  The earthquake caused several dozen deaths, damaged buildings as far away as Phoenix, generated rock falls and fires triggered by rock falls in the mountains, and caused panic among the population.”

Reference:

Haddad, D.E., and Arrowsmith, J.R., 2011, Geologic and geomorphic characterization of precariously balanced rocks, Arizona Geological Survey Contributed Report CR-11-B.

See also:

Earthquake hazard near Flagstaff assessed, Video

Where the Next Big American Earthquake and Tsunami Might Occur

Spanish Scientists Find Technique to Predict Earthquakes Claiming 80% Accuracy

The Measure of an Earthquake

Local atmospheric changes may foretell large earthquakes

On January 9, 2012, the Obama administration announced a 20-year ban on new mining claims on public land near Grand Canyon National Park. The ban will not affect about 3,000 existing claims in the area. A principal stated reason for the ban was to forestall possible contamination of Colorado River water resulting from a mining accident. But uranium contamination from an accident is extremely unlikely according to a study by the Arizona Geological Survey:

To examine one potential impact of uranium mining in the Grand Canyon region on uranium levels in Colorado River water, Dr. Jon Spencer (AZGS Senior Geologist) and Dr. Karen Wenrich (Consulting Geologist) posed a hypothetical , worst-case, scenario involving an accidental spill of the entire contents of an ore truck hauling 30 metric tons (66,000 pounds) of uranium ore containing one percent uranium (ore grades in northern Arizona are typically somewhat lower), followed by flash-flood transport and dissolution of all spilled uranium into the Colorado River. In this scenario, the ore is pulverized and dissolved within a single year, releasing 300 kg of uranium directly into river waters.

The result: uranium concentration of Colorado River waters would increase from 4.00 to 4.02 ppb (parts per billion by mass); an increase of just one half of one percent that would be masked by natural uranium-concentration variations as determined by measurements reported in a recent U.S. Geological Survey study. Furthermore, the uranium content of Colorado River waters would remain well below the 30 ppb Maximum Contaminant Level set by the Environmental Protection Agency (EPA) for safe drinking water.

The small change in dissolved uranium content of Colorado River waters as a result of this hypothetical accident is due to the very large annual volume of river water that passes through the Grand Canyon and the approximately 60 metric tons of dissolved uranium that is naturally carried by the river each year.

The study is: Spencer, J.E. and Wenrich, K, 2011, Breccia-pipe uranium mining in the Grand Canyon region and implications for uranium levels in Colorado River water. Arizona Geological Survey OFR-11-04, 13 p. It may be downloaded here.

A U.S. Geological Survey report issued in 2010, provided data showing that the river carries an average of 120,000 lbs (a range of 40-80 tons) of uranium down the Grand Canyon every year. The uranium is apparently eroded from normal crustal concentrations over the large drainage area of the Colorado River basin.

Ref: Hydrological, Geological, and Biological Site Characterization of Breccia Pipe Uranium Deposits in Northern Arizona, Edited by Andrea E. Alpine, USGS SIR 2010-5025.

Besides eliminating hundreds of potential jobs, this decision denies the U.S. resources to produce electricity. This decision seems to be part of Obama’s plan to make energy costs “skyrocket” to make green energy appear more attractive.

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