Colorado River

USGS claims that mercury and selenium are accumulating in the Colorado River

A study conducted by the U.S. Geological Survey (USGS) claims to have found “relatively high -compared with other large rivers” concentrations of mercury (Hg) and selenium (Se) in the food web along the Colorado River between Glen Canyon Dam and the Grand Canyon, The study was done in the summer of 2008, but curiously, results were just published in the journal Environmental Toxicology and Chemistry in August 2015. Perhaps they were taking advantage of publicity associated with the toxic spill from the Gold King mine in Colorado earlier this month.

USGS Hg Se study map

Some excerpts from the press release:

“The study, led by the U.S. Geological Survey, found that concentrations of mercury and selenium in Colorado River food webs of the Grand Canyon National Park, regularly exceeded risk thresholds for fish and wildlife. These risk thresholds indicate the concentrations of toxins in food that could be harmful if eaten by fish, wildlife and humans. These findings add to a growing body of research demonstrating that remote ecosystems are vulnerable to long-range transport and bioaccumulation of contaminants.”

“The study examined food webs at six sites along nearly 250 miles of the Colorado River downstream from Glen Canyon Dam within Glen Canyon National Recreation Area and Grand Canyon National Park in the summer of 2008. The researchers found that mercury and selenium concentrations in minnows and invertebrates exceeded dietary fish and wildlife toxicity thresholds.”

“Although the number of samples was relatively low, mercury levels in rainbow trout, the most common species harvested by anglers in the study area, were below the EPA threshold that would trigger advisories for human consumption.”

See full paper:

From the paper:

“Sampling occurred from 12 to 28 June 2008. At each site, we collected representative basal resources (organic matter and primary producers), macroinvertebrates, and fishes. Basal resources included fine benthic organic matter, seston (suspended organic matter), epilithon (benthic biofilm), attached algae (Cladophora sp.), and epiphyton (diatoms attached to Cladophora). We collected fine benthic organic matter from sandy depositional habitats using a Ponar dredge (0.052 m2 ) deployed from a boat.”

As far as I can determine, the study analyzed fewer than 25 samples of each group along 250 miles of river. That is indeed a very low number upon which to form conclusions.

“In the present study we found no significant differences in Hg and Se accumulation among sites throughout the Grand Canyon.”

“There is a well-documented antagonistic interaction between Se and Hg, whereby Se protects animals from Hg toxicity when Hg:Se molar ratios are approximately 1 or less. The Hg:Se molar ratios were typically much lower than 1 in the present study, ranging from 0.04 (rainbow trout) to 0.38 (fathead minnow) among fish species. Assuming that Se and Hg in prey are equally transferred to consumers, this large excess of Se in this system suggests that the risks of Hg toxicity could be considerably lower than the Hg wildlife risk values alone would indicate.”

From the press release:

“The good news is that concentrations of mercury in rainbow trout were very low in the popular Glen Canyon sport fishery, and all of the large rainbow trout analyzed from the Grand Canyon were also well below the risk thresholds for humans,” said one of the study authors.

“We also found some surprising patterns of mercury in rainbow trout in the Grand Canyon. Biomagnification usually leads to large fish having higher concentrations of mercury than small fish. But we found the opposite pattern, where small, 3-inch rainbow trout in the Grand Canyon had higher concentrations than the larger rainbow trout that anglers target.”

Regarding mercury: “Airborne transport and deposition — with much of it coming from outside the country — is most commonly identified as the mechanism for contaminant introduction to remote ecosystems, and this is a potential pathway for mercury entering the Grand Canyon food web.” Selenium is derived from “irrigation of selenium-rich soils in the upper Colorado River basin contributes much of the selenium that is present in the Colorado River in Grand Canyon.”

The paper abstract notes that “consistent longitudinal patterns in Hg or Se concentrations relative to the dam were lacking.” That would seem to cast in doubt the proposed source of selenium from upstream irrigation of agricultural land. The “relatively high” concentrations they were talking about in fish are 0.17–1.59 ppm Hg and 1.35–2.65 ppm Se.



The Redwall Limestone of the Grand Canyon


Redwall 2

The 350-million year old Redwall Limestone is one of the most prominent features of the Grand Canyon. Its features:

• Diverse and long history over the last 350 million years.
• Magnificent cliffs and red walls.
• Composed of ~99.5% pure limestone, ~95% of which is biologically formed in the presence of organisms.
• Forms very chemically resistant cliffs, yet it is a very soft rock (slightly harder than a
finger nail).
• Has 1,000’s of miles of interconnected caverns spread out over the Colorado Plateau.
• Has many caverns, some with ancient and modern speleothems.
• Is the source of carbonate for the growth of abundant travertine deposits.
• Provides precious minerals, trace metals and uranium in breccia pipes
• Is a major source of high quality groundwater to numerous and voluminous springs in
the canyon and region, consumed by most visitors to the canyon.
• “Living in the Past” implies the past of the Redwall Limestone is living with us today.

Brian Gootee of the Arizona Geological Survey has designed a 27-slide history of the Redwall Limestone intended for guides and educators. It can be downloaded here:

It has great graphics and an interesting story.

See also:

Origin of the Grand Canyon
Origin of the Lower Colorado River – a geological detective story



Origin of the Lower Colorado River – a geological detective story

The origin of the Lower Colorado River has been a controversial topic within the geological community.  The latest hypothesis was put forth by Philip Pearthree of the Arizona Geological Survey, Kyle House (now with USGS), and Michael Perkins (Univ. of Utah).  Their paper, “Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona,” Geological Society of America Special Paper 439, has just been awarded GSA’s prestigious Kirk Bryan Award for Research Excellence for 2013.  (The paper is pay-walled at GSA but you can download the full paper here, 26.8 Mb.)

To set the stage for the research in this paper, here is some background taken from my post Origin of the Grand Canyon.  The Colorado Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction into Utah and Colorado. Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest.  Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago.

Now we take up the story as told by House, Pearthree, and Perkins in their paper.  Their story is based upon detailed stratigraphic mapping of the Bouse Formation and associated sediments in the Lower Colorado River valley, and upon tephrochronology, the precise dating of two layers of volcanic ash interspersed in the sediments.

In a nutshell, these authors propose that as the ancestral Colorado River cut through the Colorado Plateau and began to flow south, the water filled a series of basins from north to south, one at a time.  When the northernmost basin was filled, it breached the natural dam on its south side, and filled the next basin to the south.  This process was repeated until the river reached the Gulf of California.  The paper goes into great stratigraphic detail in support of this hypothesis which is illustrated in the graphic below.


An earlier competing hypothesis proposes that an estuarine river cut northward from the Gulf of California, to form the Lower Colorado River.  This hypothesis was initially supported by the presence of limited marine fossils in the lowest member of the sediments along the river.  House, Pearthree, and Perkins counter this by proposing that:

“The lowermost Bouse lake would have encompassed an immense area extending from Parker Valley to the Chocolate Mountains and westward into a series of low-lying basins in the Mohave Desert. Recent hydrologic modeling suggests that it may have taken tens of thousands of years to fill this extensive lake to overflowing due to likely high rates of evaporation; consequently, the lake water could have become quite saline prior to spilling into the Yuma area. This might have allowed salt-water fauna to survive in the lake, but the mechanism for transportation of marine fauna into such a lake, if it existed, is disputed.”

In addition, they found that “strontium isotope ratios in Bouse carbonates throughout the extent of the deposit are more similar to modern Colorado River water than to seawater.” Also, the northward cutting river hypothesis requires over 1,800 feet of uplift along the river after the lake sediments were deposited.  Case closed  — for now.

The paper is well-illustrated with maps, cross-sections, and photographs.

See also:

Origin of the Grand Canyon

Grand Canyon age controversy heats up

Grand Canyon age controversy heats up

The age of the Grand Canyon of Arizona has always been controversial and more fuel has been added to the controversy with the publication in Science on November 29 of a new study by researchers Rebecca Flowers and Kenneth Farley who say they have evidence that the Grand Canyon “was largely carved out by about 70 million years ago.” Their full paper is behind a pay wall but you can read the press release here. The contentious problem with that age is that the current Colorado River has been flowing along its present course and direction for only about 6 million years. For that story, see my post, written in 2011: “Origin of the Grand Canyon.”

The Arizona Geological Survey has weighed in on this controversy in their new Fall-Winter 2012 issue of Arizona Geology magazine with an article by Wayne Ranney, a geologist who has long studied the canyon and has written a book about it. What follows are excerpts from Ranney’s article.

“The new theory involves two very complex and complicated laboratory techniques that can reveal when the canyon’s rocks were brought close to the surface. Using tiny apatite crystals collected from the basement rocks in the canyon (Vishnu Schist or Zoroaster Granite), the information yielded two different stories, one for the history of the western Grand Canyon and the other for the eastern canyon, where most tourists see the gorge. The results said that western Grand Canyon (downstream from Lava Falls) was cut to within a few hundred meters (about 1,000 feet) of its present depth by 70 Ma [million years ago]. The second story reported that the eastern area was the site of a canyon of similar proportions to the modern canyon by 55 Ma, and cut in Mesozoic rocks now completely eroded away. Incredibly, the western canyon was cut by a river that flowed exactly opposite to the modern Colorado River and the researchers call this the California River.”

Reread the paragraph above. It says that in the eastern canyon area, a canyon equal to the current one was formed, then disappeared.

Ranney continues:

 “When the Cal Tech group began their study they assumed that the apatite samples would reveal that Grand Canyon’s rocks were buried in unequal amounts of overlying rock – unequal because the canyon today has 5,000 feet of relief and the lower samples should have been buried under more material than those collected from near the top.”

That concept is shown in figure 1. The red dots show the relative position where Flowers and Farley collected their samples.

GC expected

“After running the laboratory technique the samples produced surprising results to the researchers. They showed that no matter from what depth the samples were collected, they all appeared to have been buried under equal amounts of overlying rock [figure 2]. When the tops of the blue arrows are connected here, they reveal a canyon-like topography in eastern Grand Canyon about 70 Ma. Below is a diagram [figure 3] that shows their interpretation of the data – a gorge of similar proportions was cut into the Mesozoic rocks that are now stripped back to the modern Echo and Vermilion Cliffs.”


Ranney opines that the laboratory technique used by Flowers and Farley “is not as evolved as one might hope for. Some assumptions are made that could result in different outcomes.”

GC 2-3

Ranney also notes: “The evolutionary history of the Colorado River shows that its exact course through the canyon to the Gulf of California was accomplished in only the last 6 million years.” He emphasizes, however, that the age of the Colorado River is not necessarily the same as the canyon, “the age of its [the river’s] ancestors or some early incarnation of the canyon need not be so strictly confined.”

Read Ranney’s entire article here.

Check out other stories in Arizona Geology Magazine here.

For more geology stories, see my Article Index page.

Desert Museum will open its new aquarium January 5

The Arizona-Sonora Desert Museum has just announced that it will open its new Warden Aquarium “Rivers to the sea” on January 5, 2013. The aquarium will feature and interpret aquatic life from the Colorado River and from the Sea of Cortez (Gulf of California). I’m told that at certain times visitors will be able to touch some slimy sea creatures. The aquarium is located adjacent to the gift shop just inside the museum entrance.

Because visitor capacity in the aquarium is limited, visitors will be assigned a viewing time when they enter the museum on a first come, first served basis. Alternatively, for an extra $5 fee you can get a confirmed viewing time reservation.

To learn the details of the exhibit go here:

Lake Mead has series of small earthquakes

The earth, for all its faults, adjusts to changing conditions. One of those changing conditions is more water entering Lake Mead. Over the past two months or so, there has been a series of small earthquakes, magnitude 2.1-2.5, in the Lake Mead area.

State geologist Lee Allison opines that these quakes are “due to the load on the rocks under the reservoir as the late, and large, snow pack runoff in the Rockies is filling the lake.” (See also)

According to the Las Vegas Review-Journal, meltwater from heavy snowfall last winter is filling the reservoirs:

The river system that fills Lake Mead and supplies 90 percent of Las Vegas’ drinking water is on track for its third wettest year since Lake Powell was filled for the first time in 1963.

The surface of Lake Powell has risen to its highest level in a decade…

The surface of Lake Mead is now 20 feet higher than it was a year ago, and current projections — ones now likely to be adjusted upward — call for it to rise another 33 feet by Aug. 1, 2012.

Last month’s inflow ranked as the second largest Lake Powell has ever seen in July. The 4.35 million acre-feet of water that poured into the reservoir on the Utah-Arizona border that month was almost three times the July average, and the flow in June was even greater — 5.4 million acre-feet, or almost 24 times the amount of water used in the Las Vegas Valley all of last year.


In other news:

The Arizona Geological Survey is currently featuring a video about the 7.4 magnitude Sonoran earthquake of 1887 which also shook southern Arizona.


For more information on earthquakes, see:

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

Earth Fissures in Arizona

A home buyer’s guide to geologic hazards

For a brief history of Arizona geology, see my seven-part series:

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




Uranium mining and its potential impact on Colorado River water

There has been much concern and controversy about mining uranium near the Grand Canyon and its possible impact on water in the Colorado River, water that is used to supply drinking water in much of Arizona, California, and parts of Nevada. There are over 1,300 known or suspected breccia pipes in the region, many of which contain uranium oxide as well as sulfides of copper, zinc, silver, and other metals. According to the Arizona Geological Survey, “Total breccia-pipe uranium production as of Dec. 31, 2010, has been more than 10,700 metric tons (23.5 million pounds) from nine underground mines, eight of which are north of Grand Canyon near Kanab Creek.”

A new paper from the Arizona Geological Survey examines those concerns:

A new study by the Arizona Geological Survey (AZGS) shows that potential accidental release of uranium to the Colorado River due to a mining-related accident in the Grand Canyon region would cause little change to the large annual flux of dissolved uranium that is carried naturally by the river.

In July 2009, U.S. Secretary of the Interior Ken Salazar called for a two-year withdrawal of nearly one-million Federal acres from exploration and new mining claims in the Grand Canyon region in response to concerns about the potential environmental impact of uranium mining. As part of the withdrawal process, the U.S. Bureau of Land Management, working in cooperation with federal, state, county and tribal agencies, including the AZGS, released on 17 February 2011, a Draft Environmental Impact Statement (DEIS). The DEIS identified increases in uranium concentration in water due to mining-related activity and subsequent impact on downstream water quality as a “relevant issue for detailed analysis”.

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.

UPDATE: 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

The map below shows the location of the breccia pipes.

Breccia pipe locations AZ

 The graphic below shows a cross-section of a typical breccia pipe.

Breccia pipe secton

Origin of the Grand Canyon

The Grand Canyon of the Colorado River in Arizona is one of the world’s most awe-inspiring places. The canyon is 277 miles (446 km) long, 6,000 feet (1829 m ) deep from rim to river, and 18 miles (29 km) wide at its widest point.

The canyon cuts through the Colorado Plateau, a highland that has existed since the Laramide orogeny which built the Rocky Mountains beginning about 70 million years ago. The canyon exposes rocks which range in age from almost 2 billion years to 200 million years. The earlier sediments represent deposits in warm, shallow seas, while the younger layers represent desert sand dunes.


The origin of the canyon has been the subject of geological debate. The following story, pieced together from several sources, most notably, the Arizona Geological Survey, is based on the most recent research and age dating.

The Colorado Plateau was uplifted to an elevation of about 2 miles (3.2km) above sea level. One theoretical study suggests multiple stages of subsidence and uplift related to plate tectonic movement.

The Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction. Deposition of evaporites and the Green River formation in Utah and Colorado, indicate that these northeast flowing rivers emptied into a series of lakes which were mountain bound, similar to the Great Salt Lake in Utah. (See paleomaps here.) (Some researchers suggest that there was an outlet to the Gulf of Mexico.) Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest.

Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago. The evidence and details of the story are continued now by the Arizona Geological Survey “Arizona Geology, Winter 2005.”

The Colorado River [now] leaves the Colorado Plateau at the Grand Wash Cliffs where it flows into Lake Mead. A long, north-south trending valley at the foot of the Grand Wash Cliffs, known as Grand Wash trough, contains lake sediments that were deposited between about 11 and 6 million years ago. While the lake sediments were being deposited, alluvial fans derived from highlands on both sides of the valley were deposited on the flanks of the valley. There is no evidence in these sedimentary rocks of a large river like the modern Colorado entering the valley. Indeed, if the modern Colorado had entered the valley, it would have quickly filled the lake with sand and gravel. A volcanic ash bed near the stratigraphic top of the limestone is 6 million years old, so the Colorado River must have arrived in this area after this time.


South of Lake Mead the Colorado River traverses 400 km (250 mi) of low desert before arriving at th head of the Gulf of California south of Yuma. Deposit of the Bouse Formation record a brief but deep inundation of basins along the course of the river. The Bouse Formation consists of a basal layer of silt limestone that is typically one to several meters thick and is commonly overlain by up to hundreds of meters of siltstone and minor sandstone. These sediments were thought to have been deposited in an estuary of the developing Gulf of California after a U. S. Geological Survey (USGS) paleontologist first reported the presence of marine invertebrates in 1960. Estuaries generally have variable salinity conditions because, during times of high river flow, fresh water dilutes the salt water while sea water is dominant in times when river input is low. This leads to a mix of marine, brackish and fresh water organisms like those represented by the fossils in the Bouse Formation.

The Bouse Formation is exposed at elevations of up to 536 m (1760 ft), with elevations generally increasing from south to north. If in fact the Bouse Formation was deposited in an arm of the sea, then the lower Colorado River trough must have been uplifted by up to 536 m in the past 5 million years. USGS geologist Ivo Lucchitta proposed in 1979 that not only was the lower Colorado River trough uplifted in the past 5 million years, but that this uplift was part of a more regional tectonic uplift that carried the Colorado Plateau up to its current high elevation. Three discoveries in the past 10 years have cast doubt on the interpretation that the Bouse Formation was deposited in an estuary. In a paper published in 1997, Professor P. Jonathan Patchett of the University of Arizona and Jon Spencer of the AZGS showed that the strontium isotope composition of Bouse Formation limestone was similar to that of Colorado River water, and quite different from sea water. They proposed that the Bouse Formation was deposited by the Colorado River in a series of lakes that filled with river water and spilled over, eventually linking the river with the Gulf of California.

In 2002, during the course of a field mapping project in the Bullhead City–Laughlin area (Mohave Valley), Kyle House of the Nevada Bureau of Geology and Mines and Phil Pearthree of the AZGS discovered evidence that initial inundation of the northern part of the Colorado River trough was marked by southward-transported flood deposits. Such flood deposits would not be expected for initial inundation from sea water derived from far to the south, but they are consistent with an upstream lake spillover and initial influx of Colorado River water derived from the north in the Lake Mead area. These flood deposits are directly overlain by the basal limestone of the Bouse Formation. On the flank of the valley, they found a volcanic ash layer just below the Bouse Formation that was determined by Mike Perkins of the University of Utah to be 5.5 million years old. Bouse deposition was succeeded by a period of erosion, which was followed by deposition of at least 250 m (800 ft) of sand and gravel that is clearly associated with the Colorado River. This period of massive river aggradation ended about 4 million years ago, as another volcanic ash was found near the top of the river deposits.

Yet another volcanic ash indicates that the Colorado River had downcut at least 60 m (200 ft) below the level of maximum aggradation by 3.3 million years ago. Most recently, Rebecca Dorsey of the University of Oregon said that Colorado River sands first arrived in the Salton Trough south of Yuma 5.3 million years ago. Such sands would have been deposited before reaching the Salton Trough if any large body of standing water such as a lake or an estuary existed along the course of the Colorado River. The sand deposits thus reveal the existence of a through-going Colorado River by 5.3 million years ago and mark the beginning of an enormous influx of river sediment that has filled the Salton Trough since that time.

All of these recent studies are consistent with the concept that the Bouse Formation was deposited in a geologically short-lived chain of lakes that were created by initial influx of Colorado River water into previously closed basins. At the mouth of the Grand Canyon, the Colorado River first arrived less than 6 million years ago. Along the course of the lower Colorado River, it appears that all of the Bouse Formation was deposited between 5.5 and 5.3 million years ago and Colorado River sands arrived in the Salton Trough less than two hundred thousand years after deep flooding of Mohave Valley. The influx of Colorado River water and sediment at this time marks the inception of the modern Colorado River.

Tens to possibly hundreds of meters of river sand and gravel were deposited in Mohave Valley and elsewhere in the lower Colorado River trough in the subsequent 1- to 1.5 million years, as a tremendous volume of river sediment accumulated in the Salton trough. This massive aggradation probably resulted from rapid erosion in the Grand Canyon as the Colorado River downcut through that region.

The studies mentioned above have created a new problem for geologists. What are we supposed to make of the marine and estuarine organisms in the Bouse Formation? These organisms require salty water, but have been found only in sediments of the southernmost of the basins in which the Bouse Formation was deposited (the large Blythe sub-basin). Recent calculations show that evaporation in the southernmost lake as it was filling with river water could have elevated salinity to sea-water levels. The organisms, however, would have to be carried from the sea to the lake and delivered in sufficient numbers to provide a reproducing population. This seems like an impossible task, but long distance transport of aquatic organisms by birds has been documented in a number of places.

In conclusion, the abrupt arrival of the Colorado River to the low western desert region as a series of lakes roughly coincides with the beginning of incision of the Grand Canyon. Before this time,

water that flowed off of the west slope of the Rocky Mountains and into the interior of the Colorado Plateau must have terminated in a lake on the Plateau or exited the Plateau along an unknown route. We think it most likely that cutting of the modern Grand Canyon began with spillover of a very large lake in northeastern Arizona and rapid incision of the lake outflow point about 5.5 million years ago. Details of the spillover and development of the Colorado River through Grand Canyon are not known, but we suspect that it involved catastrophic flooding and rapid erosion. As the Colorado River propagated downstream through the Basin and Range province, it sequentially filled, spilled over, and drained a series of formerly closed basins, eventually linking with the Gulf of California by 5.3 million years ago.

But, the Colorado River has not always been free flowing to the Gulf of California. Volcanic flows have dammed the river several times in the last 1 million years.

For a broader view of Arizona geological history, see my seven-part series (links in Article Index).

Young Volcanic Fields of Arizona

The volcanic history of Arizona spans more than one billion years. There are seven volcanic fields which have erupted within the last four million years. One of them entombs pottery of local inhabitants of the time. One dammed the Colorado River in the Grand Canyon several times. Another produces world-class gemstones, and another is associated with the 1887 earthquake which shook Southern Arizona and Northern Sonora.

The map below, from the Arizona Geological Survey shows the location of these recent and older volcanic fields. This article discusses those shown in red. For more information on rock names, see the Igneous rock naming page to the left.


Uinkaret Volcanic Field (UI on the map)

The Uinkaret volcanic field lies on the north rim of the Grand Canyon in northwestern Arizona. Four major eruptions of basalt from this field flowed into the Grand Canyon and dammed the Colorado River between 725,000 and 475,000 years ago, between 400,000 and 275,000 years ago, between 225,000 and 150,000 years ago, and between150,000 and 75,000 years ago. (Source).

Dating was done by radiometric methods using the ratio Argon-40 to Argon-39.

Some of the eruptions flowed down the canyon as much as 75 miles. Some of the dams reached more than 700 feet high. When the river eventually over-topped and broke the dams there were great floods. There is no consensus on how long the dams lasted. Some think they may have lasted up to 20,000 years and formed large reservoirs.

According to the Smithsonian Institution, “One lava flow, from Little Springs, south of Pliocene Mount Trumbull, has a cosmogenic helium age of 1300 +/- 500 years BP. Pottery sherds dated at between 1050 and 1200 AD were found within the Little Springs lava flow, which occurred about the same time as the Sunset Crater eruption in the San Francisco volcanic field to the SE.”

San Francisco Volcanic Field (FL near Flagstaff)

SFpeakThe San Francisco volcanic field near Flagstaff has been active for about 6 million years. The oldest eruptions occurred near the town of Williams. Sunset Crater, a cinder cone east of Flagstaff, is less than 1,000 years old.

spThe US Geological Survey says, “It is likely that eruptions will occur again in the San Francisco Volcanic Field. With an average interval of several thousand years between past periods of volcanic activity, it is impossible to forecast when the next eruption will occur. U.S. Geological Survey (USGS) scientists believe that the most probable sites of future eruptions are in the eastern part of the field and that the eruptions are likely to be small. These future eruptions may provide spectacular volcanic displays but should pose little hazard because of their small size and the relative remoteness of the area.”

The San Francisco mountains, which include Humphreys Peak, Arizona’s highest mountain at 12,633 feet, is a stratovolcano which erupted between 1 million and 400,000 years ago. This volcanic mountain consists of interspersed layers of andesitic lava, cinders, ash, and volcanic mudflows.

The younger volcanic cones and their flows are basaltic, such as SP crater (71,000 years old) and Sunset Crater. According to the USGS,

“Most of the more than 600 volcanoes in the San Francisco Volcanic Field are basalt cinder cones. Basalt has the lowest viscosity of all common magmas. Cinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity. SP Crater, 25 miles north of Flagstaff, is an excellent example of a cinder cone and its associated lava flow.”

SunsetCraterThe volcanic field also contains lava domes. These are dome-shaped, steep-sided piles of viscous dacite and rhyolite lava. These can expand like balloons when lava wells up inside the dome, or the dome can break and develop by buildup of successive layers.

A side story, the cinder cone gold scam: Among my duties as an exploration geologist was examining properties submitted to our company by third parties. One submitted property was a gold prospect in one of the basaltic cinder cones. It is extremely unlikely that gold occurs in cinder cones, but a friend of one of my company’s directors was interested, so I looked.

Upon arriving at the property and meeting the owner, I asked where he thought the gold was. I collected samples at those places, and others. The property owner just happened to have an assay lab on the property and offered to analyze the samples I collected. Just to see what would happen, I gave him a few handfuls of some of the samples. Miraculously, his lab found gold in the samples. When I got back to Tucson, I had the remaining material assayed by a reputable laboratory which failed to find any gold. Now, that result is a truly amazing “nugget effect.” Conclusion: the assayer was either incompetent or crooked.

Springerville Volcanic Field (SP)

Most of the basaltic flows in the Springerville volcanic field are between 2.1 million and 300,000 years old. Some older flows, 6- to 8 million year old flows occur to the south. There are no stratovolcanoes here, only about 400 cinder cones and their associated flows.

San Carlos Volcanic Field (SC)

The San Carlos volcanic field is on the San Carlos Indian Reservation south of Springerville. The basalt and peridotite lavas erupt between 7 million and 500,000 years ago. Peridotite, from whence the term peridot comes, is a coarse igneous rock consisting mainly of the minerals pyroxene and olivine. Peridot is gem-quality olivine, a magnesium-iron silicate. It is claimed that 90% of the world’s peridot comes from Peridot Mesa in this volcanic field. Peridot Mesa is a diatreme (a breccia-filled vent formed by gaseous explosion) and was followed by lava flows.

San Bernardino Volcanic Field (SB)

The San Bernardino volcanic field in the southeast corner of Arizona contains about 130 vents and cinder cones, maars, and flows of olivine basalt. These were erupted between 1 million and 27,000 years ago. An earthquake in 1887 was centered just south of the border in the San Bernardino Valley.

Maars are craters created by a steam blast. Paramore Crater is the largest of these, about 1 km by 1.5km and about 60 m deep.

In the northern extension of the San Bernardino Valley, the San Simon Valley, there are still many hot springs.

Sentinel Volcanic Field (SE)

The Sentinel volcanic field, west of Gila Bend, AZ, contains basalt flows 2- to 6 million years old. This field may be related to the larger Pinacate field to the south.

Pinacate Volcanic Field (PI)

Most of the Pinacate volcanic field is in Sonora, just south of the Arizona border. Pinacate contains eleven giant-maar craters and hundreds of cinder cones. (See photos here and here.) The field has been active for 2 or 3 million years and eruptions have occurred as recently as 13,000 years ago. (Source.)


A map showing young volcanic fields in Arizona and New Mexico may be found here. This is a PDF file. It shows a remarkable straight line of volcanic fields from San Carlos in Arizona through Taos, NM. It also gives ages of the volcanic units. (You have to enlarge the view.) The map was prepared as part of an assessment for geothermal energy.

Constructing a Landmark, Hoover Dam Bypass Bridge

BridgeThe Hoover Dam Bypass Bridge is the longest concrete-arch bridge in North America. The 300-foot precast/post-tensioned concrete columns supporting the roadway at either side of the arch are the tallest in the world, it’s the second-tallest bridge in North America and it’s one of the most technically challenging bridges ever constructed.

The director of engineering for the project was Dave Zanetell, a graduate of the Colorado School of Mines (my alma mater). He had to “knit two state governments, four federal agencies, five general contractors, and dozens of consultants into an effective unit.”

“Strung nearly 1,000 feet above the Colorado River, the 1,900- foot arch-bridge solves problems that stymied engineers for more than three decades.”

Read the full story of this project from the Colorado School of Mines Magazine.