Ask a Geologist Common Q&A
What types of rocks and minerals are common in Williams, AZ?
AZGS Answer
Williams, Arizona, is on the western-most edge of the San Francisco volcanic field. The rocks exposed near Williams are predominantly basalts with small phenocrysts (minerals) of olivine, pyroxene and plagioclase. You won't find the large olivine (peridot) crystals that are found in the San Carlos volcanic field of central Arizona.
The Bill Williams Mountain volcanic complex comprises a suite of coalescing andesitic to dacitic lava flows and volcanic domes emplaced about 3.5 million years ago on the southwestern edge of the San Francisco volcanic field, east of Flagstaff, Arizona. I don't have in front of me a description of Bill Williams Mtn. rocks, but andesites and dacites typically include: plagioclase feldspar, pyroxene, hornblende, biotite, and quartz.
Photo by Ted Grussing. The town of Williams is left of BWM. On the horizon, from right to left, are four younger volcanic vents: Mount Elden, the San Francisco Peaks (including Humphreys Peak the highest point in Arizona at 12,637’), Kendrick Peak and Sitgreaves Mountain.
Thanks for your question.
Mike
Good day,
I would like to see a map that shows the aquifer (aquifers) in Marana, Arizona (Twin Peaks Road and I-10 area). I was told by a geologist that there is a geological barrier that prevents water coming from Marana (I think the Santa Cruz River) to enter this aquifer. It is only rain water that is replenishing it. Please identify this barrier on the map you send me. Marana now has three wells taking water from this area. Several of our residents( Blue Bonnet Road/Oasis Road area) have had to deepen their wells. Two Marana city wells are on either side of the I-10 Freeway at Twin Peaks Road and the third well is on West Tangerine (I was told this well is not giving much water). Marana officials are telling me that CAP water is able to replenish the water removed by their wells. In addition to the map that shows the barrier; would you please confirm to me in writing that water can or cannot enter this aquifer from Marana. Please explain in detail so I can understand thi s better. New Arizona laws require that any new housing subdivisions have access to 100 years of renewable water (Marana has at least six or seven approved new subdivisions along the Twin Peaks corridor). I need to know that the current residents on Blue Bonnet Road/Oasis Road will continue to have water or are the city wells removing water from this area without replenishing it. Your help in understanding this situation is greatly appreciated. Gayle
AZGS Answer
Gayle,
All fine questions, but a better source would be the Arizona Dept. of Water Resources (ADWR). Your area of interest is encompassed by their "Tucson Active Management Area", which includes Marana and extends NW to Picacho Peak and Picacho Mountains.
ADWR has more than 800 monitoring wells in Arizona that provide information on water quality and groundwater levels. They use satellite InSAR imagery to track ground subsidence in areas where groundwater withdrawal is particularly high.
Regarding CAP water and recharge of existing aquifers, you’ll need to either address that with ADWR or CAP folks
Sorry I can’t be more help. We do engage in basin analysis studies, but more from the basin geometry and structural side than groundwater side of the equation.
This image illustrated the groundwater conditions in the Tucson AMA. You can download the entire 71p report on Tucson AMA at http://www.azwater.gov/azdwr/StatewidePlanning/WaterAtlas/ActiveManagementAreas/documents/Volume_8_TUC_final.pdf
Mike Conway, Arizona Geological Survey
Good afternoon,
I am a writer working on novel (Sedona is my hometown), and I'm doing some research on Oak Creek Canyon. I've been reading your very helpful "A Guide to the Geology of the Sedona & Oak Creek Canyon Area, Arizona"; thank you for this resource. However, I'm trying to determine what type of rocks/boulders lie in the bed of Oak Creek. I'm thinking in particular of the rocks in the Indian Gardens area--the large rounded grey stones scattered along the creek bed. Is this basalt talus as described in the guide or from a different source? Thank you very much for your time and any help you can offer.
Sincerely, Bryn C.
AZGS Answer
Hi Bryn, Nice pics. Because the boulders are worn and sculpted by running water, it’s a little hard to be certain.
I would not be surprised if the majority of boulders here were derived from basalt lava flows situated on the Mogollon Rim.
Mike Conway
Is geopathic stress, earth energies understood and accepted by geologists? Robin
AZGS Answer
Hi Robin,
You caught me out of my comfort zone. I had to look up geopathic stress. If I understand correctly, it is the study of earth energies (magnetic, gravitational?) and their effect on human well-being. I've never encountered that concept in the geologic literature. It does appear in some health literature, however.
Nonetheless, there seems to be no evidence that geopathic stress is a true physical phenomenon.
Thanks for a challenging question.
SO, a block of earth's crust between two parallel faults drops or subsides, becoming a basin, or graben, in the Basin and Range Province of the American West. For me, it's relatively easy to understand how tectonic forces, especially with the aid of gravity, can cause a block of crust to "sink," thereby becoming a basin. But what force is it that causes the adjacent block of crust to RISE, thereby becoming a mountain range, or horst? I pretty much understand the half-graben, which involves only ONE block of crust that is rotated, or tilted, with one side of it dropping down while the other side is, naturally, tilted up ... the "sunken" side becomes the basin, or valley, while the side that is tilted up becomes the mountain range. But my question concerns the scenario where TWO blocks of crust are involved with NO tilting -- in classic horst and graben topography, how does the horst (a complete block of crust) rise? What force is exerted on that single block of crust
AZGS Answer
Steve, Good question that goes directly to the heart of the mechanics of horst and graben formation.
Have a look at the illustration below. The red arrows at the end of the block represent the local stresses, which are extensional. So the crust is being pulled apart. The rocks of Earth's crust accommodates extension by breaking into discreet blocks along planes of weakness - faults.
With extension, alternating blocks (graben) physically subside while the intervening blocks remain in place, but are now relatively higher than the grabens. Arrows along the faults illustrate the 'relative' motion of the block, not the absolute motion. So the fault plane arrow shows the horst being uplifted 'relative' to the downdropped graben.
I hope this explains it.
Mike
Do you know of anyone or any recent study that has looked at impacts of climate change/drought on groundwater supply volumes in Arizona?
Thank you!
Kim
AZGS Answer
The answer is undoubtedly yes, although the effects of climate change on groundwater resources might be difficult to tease out of the data.
1) Groundwater recharge & climate change in the west - https://uanews.arizona.edu/story/groundwater-recharge-and-climate-change....
2) Modeling climate change impacts - and uncertainty - on the hydrology of a riparian system: San Pedro Basin (Arizona/Sonora) https://www.sciencedirect.com/science/article/pii/S0022169407004799.
For decades, farmers in central and southern Arizona have drawn on groundwater to irrigate their crops. As a result, groundwater levels of some aquifers in southeastern Arizona have dropped hundreds of feet. This rate of groundwater level fall far outstrips the effects of climate change or drought on groundwater recharge.
By way of example, drawdown of the Ogallala aquifer, situated east of the Front Range of the Rocky Mountains, has reduced groundwater volume such that hydrologists estimate that it would take as much as 15,000 years of 'average' annual rainfall to bring it back to levels observed in the early part of the 20th century. So while climate change and drought can undoubtedly impact groundwater levels, that signal is swamped by drawdown of the aquifer for farming and industry.
Check out the USGS National Water Information System (https://nwis.waterdata.usgs.gov/nwis) for vetted data on groundwater levels and conditions in Arizona. The Arizona Dept of Water Resources (https://gisweb.azwater.gov/waterresourcedata/) would be another good resource for you.
I hope this helps.
Mike
Hi. I live in Upstate NY near a small creek. The creek is mostly shale. As the shale erodes other rocks become visible that are round and curvy. They range in size from the size of a basketball to larger. Some are dome shaped with a dimple on them, others are long and curvy like they were mud or cement mix poured onto the shale as it was forming. I've never seen rocks like this before. If needed I do have pictures I could email you. I was wondering what these were called and how they were formed. Can you help me with this? Thanks, Clint
AZGS Answer
Hi Clint, They look very much like concretions. I did a quick internet search and ran across a manuscript describing the Devonian Hamilton Formation of NY, PA, OH, VA. The Hamilton Formation outcrops in the Cayuga Creek area. Apparently, concretions – of all shapes and sizes – are common in the Hamilton. At one time, some concretions were mistaken for ‘petrified tortoises’.
Concretions form as fine-grained calcium carbonate sediments coagulate around some object; a fossil fragment, rock …
I hope this helps.
Mike
, 
Hi, Can you tell what this is why it appears so odd?
AZGS Answer
Hello Harness,
This specimen looks very much like silica froth that accompanies the eruption of silica-rich magma. For example, obsidian. The obsidian dome in Long Valley, California, is coated with this type of material.
Compositionally, it is comparable to rhyolite - a volcanic rock, but textually, it is a volcanic glass (pumice) that forms as gas - mostly H2O and CO2 escape from the depressurized magma.
Cool stuff. Where did you say you found it?
Mike
This is not a question about geology, but about State Geological Survey markers. To whom do 1/4 section markers belong ? I ask because the one which used to be at one corner of our property has been removed by someone and I am wondering who to report this to and if it can be replaced. Perhaps you can refer me to the proper agency? Betty
AZGS Answer
Hello Betty,
I visited the Public Land Survey System page and they note the existence of monuments at quarter-section corners. I guess if you were reporting a missing or stolen monument, the Bureau of Land Management would be the place to start.
The Arizona BLM office in Phoenix, phone # 602.417.9200.
Good luck
What is the oldest exposed rock in Arizona? How old is it? And what geologic formation is it in? John
AZGS Answer
John,
The oldest rocks in Arizona are the Vishnu Basement Rocks exposed in the Upper Granite Gorge, Grand Canyon. The Brahma Schist, part of these basement rocks, are about 1.75 billion years old. Locally the Brahma is interlayered with Rama Schist Xr and Vishnu Schist (Xv).
There are some similarly aged rocks in the Prescott area.
Brahma Schist consists of amphibolite, hornblende-biotite-plagioclase schist, biotiteplagioclase schist, orthoamphibole-bearing schist and gneiss, and metamorphosed sulfide deposits.
Mike Conway