Hiking Mount Humphreys of the San Francisco Peaks in Northern Arizona: Part I - Geologic History

In winter, snow-blanketed summits of the San Francisco Peaks embrace a cloud-shrouded Inner Basin. Both features are remnants of a massive stratovolcanic that met a catastrophic demise. That event anointed Mount Humphreys the highest point in Arizona and its only alpine mountain, standing reign on the crater's northwest rim.

Mount Humphreys in late afternoon from the west

Photo courtesy of Ted Grussing. Please visit Ted and his photos here.

The San Francisco Peaks take second stage to the Grand Canyon in notoriety and magnitude but is far from lacking it in grandeur and visibility. Called San Francisco Mountain geologically or simply “the Peaks” by the locals, it dominates the skyline on the southwestern Colorado Plateau in northern Arizona for nearly a hundred miles in any direction. The edifice is both revered and held sacred by no fewer than thirteen Native American tribes. The Hopi call it "Place of the High Snows" and the Navajo, "Shining on Top."

HAPPY LANDINGS

Looking north, this was my majestic view on the short flight from Phoenix (within the Basin and Range Province) to Flagstaff (on the Colorado Plateau), almost a 6,000 foot difference in elevation. Providing a scenic backdrop to Flagstaff, Kendrick Peak is in the haze at the far left and Mount Elden is on the far right. On center stage, Mount Humphreys hides in the clouds with its sister peaks. Rising abruptly above the surrounding plateau, the Peaks makes its own weather locally.

IN THE WORD’S OF MALLORY “BECAUSE IT’S THERE”

Mount Humphreys (35°20′46.83″ N, 111°40′40.60″ W) lies around 10 miles north of Flagstaff where I was to join my good friend Wayne Ranney on a geological tour of the western Colorado Rockies in mid-July. The idea of climbing Humphreys became a plan when he emailed back that “It’s doable!” That meant I had to make my  ascent the morning after my flight from Boston to Flagstaff via Phoenix. Translation: Sea level to 12,633 feet within 18 hours of my arrival and a guaranteed high altitude-headache for days. 

Humphreys Trailhead is adjacent to the Arizona Snowbowl ski area’s parking lot at an elevation of 9,281 feet. The trail (red line on the topo map) first crosses a flat meadow and then switchbacks its way up Humphreys’ western flank to the Agassiz Saddle. Turning north, it follows the ridgeline to Humphreys’ treeless summit with an elevation gain of 3,652 feet.

Notice the moderately steep, gullied-outer flanks of the mountain and its steeply-eroded inner flanks that lead down to an Inner Basin and Interior Valley with an open outlet to the northeast. These time-worn vestiges are testimony to the majestic ancestral stratovolcano that towered over the site long ago. The geological remnants are important clues to geologists who have attempted to reconstruct the stratovolcano's original geomorphology, the time-events that led to its demise and its erosive history.  

 (From LocalHikes.com)

A LONG, STEEP, ELECTRICALLY-CHARGED ASCENT

Guidebooks categorize the climb to Humphreys’ summit as “strenuous.” It’s an almost five mile, steep ascent with loose cinders near the top for a little added punishment. According to the stats, one out of three hikers turns back. Humphreys’ angular elevation profile is thought to closely mimic that of the original stratovolcano.

(Modified from LocalHikes.com)

Wayne did email back one noteworthy caution. “Be off the summit by 11 AM to avoid the lightning!” It seems that the Colorado Plateau and the Peaks in particular are assaulted by intense summer thunderstorms called “monsoons”, the Southwest’s electrical version of high winds and heavy rain. Geology books even direct you to a rock-type that forms from the numerous lightning strikes at the top. We’ll hunt for them on our climb in my post Part II.

WORD TO THE WEATHERWISE

Personally, I think of Asia and the Indian Ocean when monsoons are mentioned, but there's actually a North American version! The word is Arabic for “season” that is best interpreted as “seasonal shifts” in the wind. Moist rivers of tropical, summer air from the Mexican Sierra Madre’s and the Gulfs of Mexico and California are subjected to intense, daytime heating that rises and condenses over the Desert Southwest. Voila. Meteorological fireworks! This is what it looks like on the weather channel.

Green arrows indicate moisture sources for the North American Monsoon.

(Modified from southwestweather.com/wx/wxmonsoon.php)

The backpacking pro’s at Peace Surplus in Flagstaff put it this way, “Watch the sky for thunderheads, dry lightning, fierce winds and hail. Whatever you do, don’t get caught above the treeline on Humphreys. It’s a lightning rod!” My second stern admonition.

Sufficiently reinforced by virtually everyone including my smartphone (“SEVERE THUNDERSTORMS!”), I decided to be at the Humphreys trailhead well before dawn in order to reach its treeless summit before the heat cooked the atmosphere into a monsoon. That left me totally un-acclimatized and severely sleep deprived, but there was no way I wasn’t going up!

A FIERY AND EXPLOSIVE BIRTH

Mount Humphreys is one of six summits between 11,000 and 13,000 feet that are connected by a ragged, ridge-line with shallow intervening saddles. Collectively, they form the rim of the Peaks that began as a long-lived, explosive stratovolcano some 2.78 million years ago. Today, San Francisco Mountain (SFM hereafter) is a collapsed, eroded remnant of its former self, albeit a massive one. A cartooned-version of the events might have progressed something like this, although many aspects of its cone-building and erosive history are conjecture.

 (Modified with my colors from tulane.edu/~sanelson/geol204/volclandforms.htm )

ANATOMY OF A STRATOVOLCANO

Stratovolcanoes are typically tall (1000’s of feet), wide (many miles), with steep-sides (30º to 35º), long-lived (tens to hundreds of thousands of years) and formed from multiple eruptions. Hence, they are larger and more structurally diverse than other volcanic edifices.

Layer upon layer of alternating outpourings of lava, pyroclastic debris (cinders and ash) and lahars (mudflows) accumulate as the volcano gradually assumes a vertically-stratified and conical shape called a stratocone. Stratovolcanoes are alternately referred to as “composite” cones or stratocones reflecting their layered components that are deposited both effusively and explosively.

A typical “stratified” stratovolcano

(Modified Pearson Prentice Hall, Inc., 2006 from oak.ucc.nau.edu/wittke/GLG101/5.pdf)

Stratocones are found globally especially at convergent tectonic plate margins. In fact, subduction zones are characterized by them, and most historical eruptions are represented by them (i.e. Mount St. Helens in Washington, Fuji in Japan, Krakatoa in Indonesia and Vesuvius in Italy). SFM, as we shall see, is unique in that it is located far from any plate margins and is thus described as an example of intraplate volcanism.  

A POSSIBLE TWO-CONE EDIFICE

The precise geomorphic evolution of the SFM stratocone is a subject of ongoing debate. This reconstruction of the Peaks paleovolcano shows a theorized two-coned paleo-structure. The cones and their summit vents are thought to have been adjacent but not coeval that may have formed in two eruptive stages with as many as four in total. The two-cone determination was based on the dating of cone-building andesites (categorized as Younger and Older), defining remnant, triangular flanks called planèzes (formed by the intersection of two master gullies), and the fact that two resistant, cone ridges reside within the Inner Basin. The present day outer, lower slopes of the volcano have not been modified on the depiction below.

(From Karatson et al, 2010)

A CATACLYSMIC DEATH

The paleovolcano catastrophically lost its northeast flank between 250,000 and 400,000 years ago. Whether the cataclysmic event caused the explosive extravasation of the bowels of the volcano outward, upward or a collapse inward, it transformed the stratocone into the horseshoe-shaped ring of mountains we see today. Within the volcano’s core, a caldera formed, a central depression resulting from the withdrawal of magma from the underlying reservoir. Today, within the extinct stratocone's epicenter, the caldera is known as the Inner Basin, and its breach is at Lockett Meadow. Sugarloaf Mountain stands guard at the Inner Basin's northeast portal and is the youngest product of the stratovolcano's evolution.

The San Francisco Peaks showing its many summits and Inner Basin components

(Created from Google Earth)

An incredible 1,000 times greater in magnitude than the 1980 eruption of Mount St. Helens in Washington State, SFM likely had a similar profile both pre- and post-cataclysm. Viewed from a distance, we can appreciate the enormous mass of material lost when the summit failed, estimated at 80 km³.

The explosion of Mount St. Helens caused many geologists to rethink their ideas about volcanoes with some suspecting its scooped-out shape to be the result of a sideways rather than a vertical blast. Originally thought to have achieved a height of 15,500 to 16,000 feet, the explosion would have shaved 3,000 to 4,000 feet from its summit. Putting its pre-demise stature into perspective, that’s 800 feet taller than Mount Whitney, the highest mountain in the lower 48 states!

With Sunset Crater behind me to the east, this view of the Peaks looking west

across Bonito Park outlines the contour of a hypothetical paleo-stratocone.

THE CONTEMPORARY INNER BASIN TAKES SHAPE

Subsequent to cone-building activity and caldera formation, the 5 x 3 km elliptical Inner Basin of the Peaks began to assume its contemporary form possibly with an immediate flank collapse. Multiple onslaughts of Pleistocene alpine glaciers sculpted the volcano’s inner flanks into cirqued walls, exposing the stratocone’s internal architecture and plumbing, while mantling the valley-floor with glacial till, outwash and moraines. During Ice Ages and interglacial periods, the volcano's high altitude has generally promoted glacial rather than fluvial erosive-processes. During the Holocene, the enlarged Inner Basin received veneers of alluvium (river and stream deposits), colluvium (gravity-slope deposits), and unsorted debris-avalanche deposits and lahars (mud flows) from its gravitationally unstable flanks.

Taken in May from about 10 miles east of the snow-covered Peaks, the open-caldera to the northeast is very evident. The mountain’s outer flanks are thought to preserve some contours of the original exterior of the stratocone, although somewhat eroded and draped with a cloak of colluvium. We’re on the eastern flank of the San Francisco Volcanic Field (SFVF hereafter) in the vicinity of Sunset Crater. Characteristic of the field, notice the many cinder cones and dark, basaltic tephra that showered the now-vegetated landscape. That's snowcapped, lofty Mount Humphreys standing reign over the Peaks' northwest rim.

Under overcast but non-electrical dry-skies, I'm standing on the summit of Mount Humphreys (Post II forthcoming) on a bed of andesite rubble at 11,633 feet. Over my right shoulder is the subdued, glacially-cirqued ridgeline of the stratocone’s north rim, and over my left is the tail-end of the south rim. Within their embrace the lush Inner Basin slopes toward its outlet to the northeast through the Interior Valley and Lockett Meadow. Beyond the Peaks numerous cinder cones and lava flows pepper the east flank of the SFVF, where the above photo was taken. I'm above Humphreys' treeline, where wind-contorted, stalwart bristlecone pines have transitioned to the domain of tundra vegetation in sparse pockets, the only flora that can survive the harsh conditions at the summit.  

TRANQUIL LOCKETT MEADOW OF THE INNER BASIN

This panorama, photographed under intensely blue autumnal skies in 2009, faces the Inner Basin and the crater's curved rim. We’re in most-serene Lockett Meadow within the caldera looking west. In fact, in the center-distance you can see the Agassiz Saddle (where I'm standing in the above photo) with Mount Agassiz to its left, followed by Fremont and Doyle. To the right of the saddle, Humphreys is blocked from view by the stratocone’s north rim. Directly behind me, Sugarloaf Mountain’s rhyolitic dome formed much later (91 ka) and is considered to represent the end of SFM's volcanic activity.

Mixed conifers and aspens are luxuriating in the clear mountain air. This heavenly valley belies the intense geological upheaval that once engulfed the Inner Basin, the very center of the paleovolcano. Only a geological irony such as this can produce such peaceful perfection!

A FIELD OF VOLCANIFORMS

SFM is the geological centerpiece and largest eruptive center of the Late Miocene to Holocene SFVF in north-central Arizona. It is approximately a 4,800 square kilometer system (100 km east-west and 70 km north-south) of over 600 cinder cones, 8 silicic centers in addition to lava flows, lava domes and vents that began erupting about 6 million years ago. It’s located on the southwest margin of the Colorado Plateau (a curious locale) and shares a similar relationship with several other late Cenozoic-age, intracontinental, primarily basaltic fields (important point) near the boundary of the Transition Zone of the Basin and Range Province (make note of that too). These fields were formed during the latest uplift of the Colorado Plateau (more notes please).

San Francisco Volcanic Field (red) and other Late Cenozoic volcanic fields younger than 5 Ma (black) and 5 to 16 Ma (outlined) show their relationship to the province-boundaries. Note that the Colorado Plateau is surrounded essentially on three sides by the Basin and Range Province.

(Modified from Tanaka et al, 1986)

The SFVF’s eruptive products range from dominantly basalt to rhyolite (keep taking notes) and are largely monogenetic (having formed from a single eruption episode). The field overlies erosionally-stripped Early Mesozoic through Paleozoic sedimentary sequences down to a deep Precambrian metamorphic foundation, the basic stratigraphic structure of the Colorado Plateau.

The following shaded-relief map of the SFVF depicts landforms over 100 feet in elevation. SFM and specifically Mount Humphreys (red arrow) are near the center of the field north of Flagstaff. Cinder cones pepper the field, some with lobate lava flows emanating from their vents that follow the notheast dip of the plateau. Faults such as Mesa Butte on the west and Doney on the east are associated with volcanics. Not only young by geological standards but with progressively younger volcanics to the east (two more items of interest), the field extends from the town of Williams to the Little Colorado River, 30 miles or so east of Flagstaff. We’ll attempt to unify all our noteworthy observations momentarily. 

The SFVF roughly extends from Bill Williams (BWM), Sitgreaves (SM) and Kendrick Mountains (KM) on the west of the field to beyond O’Leary Peak (OP) and Sunset Crater on the east end of the field. Curiously, the eruptive dates of the volcaniforms on the field grow progressively younger to the east.

(Modified from geopubs.wr.usgs.gov/fact-sheet/fs017-01/fs017-01.pdf)

Just outside Flagstaff, this photo captures the spectacular SFM looking west. Our perspective encompasses the entire sixty-mile, east-to-west breadth of the SFVF. Barely visible on the far left is the silicic lava dome of Bill Williams Mountain along Mesa Butte Fault on the western flank of the field. Nearer to view is elongate, dacitic lava dome of Mount Elden presiding over the city of Flagstaff. To its right is the collection of peaks that comprise SFM including the diminutive rhyolitic dome of Sugarloaf Mountain to the far right. In the foreground are numerous cinder cones that mark the field’s eastern flank.

MAGMA VISCOSITY DICTATES ARCHITECTURE AND BEHAVIOR

Silicon dioxide or just “silica” (along with temperature and pressurized-gases) increases magma’s viscosity making it thick, sticky and less-fluid. Resistance to flow determines a volcano’s architecture and behavior. Thus, silica-rich magma tends to construct tall, layered stratovolcanoes such as the Peaks with explosive eruptions. On the other hand, silica-poor magma flows readily with effusive eruptions, such as on the volcanic field. Its volcaniforms are largely “lowly” cinder cones and sheet-like lava flows. Compare magma composition, rock type and viscosity on the igneous mineralogy chart.

Mineralogy of Igneous Rocks

(Modified from oak.ucc.nau.edu/wittke/GLG101/4.pdf of Pierson Education 2011)



The Peaks’ intermediate rocks are largely andesitic and dacitic in keeping with the stratocone's verticality; whereas, the field’s rocks are basaltic, consistent with its subdued profile. Lava domes within the field are roughly circular and mound-shaped. Their steep-sided, bulbous architecture results from the slow extrusion of viscous, silica-rich lava of dacite (Mount Elden at Flagstaff’s eastern outskirts) and rhyolite (Sugarloaf Mountain). Lava domes form endogenically from interior expansion to accommodate new lava and exogenically by the external piling up of lava.   

   

FRACTIONAL CRYSTALLIZATION

As we’ve seen, our stratovolcano within the field is both an exception on the landscape architecturally, compositionally and behaviorally! What might account for the stratocone’s silica-rich composition within a volcanic field that’s largely silica-poor?



Melting of the mantle produces basalt which rises buoyantly. As basalt cools, it evolves chemically. Minerals start and stop crystallizing fractionally in an order based on their melting points which also selectively removes various elements. The result is that the parent magma differentiates into new melts of more “highly-evolved” magmas with different compositions. It all happens in an orderly and predictable sequence called the Bowen Reaction Series. The various minerals derived fractionally are also on the chart above.

The bottom line is that the resultant magmas, be they silica-rich or poor, dictate the architecture and behavior of volcaniforms on the Earth’s surface. But what causes a basalt melt to begin with, and what is the origin of volcanism within the SFVF?

LAND-BASED VERSION OF THE HAWAIIAN ISLANDS

The origin of volcanism within the SFVF remains unclear. It has been compared to the Hawaiian Islands where the oldest volcanoes are on one side of the complex, and the most recent are on the other. Although the San Francisco field is land-based (continental) and the Hawaiian chain is water-based (oceanic), both systems are basaltic in composition and exist within intra-plate locales, far from inter-plate boundaries where volcanic activity typically occurs.

Inter-plate convergence is responsible for the “Ring of Fire” of volcanoes and seismic activity that surround the Pacific Ocean. By the way, the Atlantic Ocean is surrounded by a “Ring of Passivity” (my terminology) coinciding with its passive margins devoid of volcanic activity.

 (From crystalinks.com/rof.html)

A MANTLE PLUME EXPLANATION FOR INTRAPLATE VOLCANISM

How can occurrences of intra-plate volcanics be explained? It's a question that's plagued geologists for decades. One popular theory states that the fields lie above a “hotspot,” a stationary or fixed zone within the mantle (or core-mantle boundary) where a fountain of magma called a mantle plume buoyantly convects upward from great depth (lava lamps are a good visual metaphor) and partially melts the overlying crust.

As the overlying plate (continental-North American Plate in the case of the SFVF and the oceanic-Pacific with the Hawaiian Islands) migrates over the fixed-hotspot, the locus of volcanic activity follows on the surface. Thus, a chronological chain of Hawaiian volcanoes erupts through oceanic crust. On land such as the SFVF, continental crust partially melts which is underlain by pooling, buoyant basaltic magma. Voila!

Mantle Plumes Beneath Oceanic and Continental Crust

(Modified from faculty.weber.edu/bdattilo/shknbk/notes/htsptplm.htm)

Intraplate magmas are derived anorogenically rather than orogenically, without a mountain-building process and plate collision. Anorogenic magmas are produced from varying amount of partial melting of an “oceanic-island, basalt-like mantle source” from lower crustal material. Orogenic processes, the more often thought of mode of mountain-building and crust-generation, occurs during interplate collisions at subduction zones such as the Pacific Ring of Fire.

AGE PROGESSION AND A GEOLOGICAL FORECAST

This explains the oldest volcaniforms on the west side of the SFVF and the youngest on the east. The progression of volcanic activity coincides with the direction and rate of North American plate migration over the hotspot, a half inch per year (the rate at which our fingernails grow)! It also provides somewhat of a geological forecast of where and when on the field future eruptions are most likely to occur.

Given the trend (“younging” from west to east), we can anticipate that the next eruption will be somewhere in the east of the field. Given the frequency of over 600 eruptions in 6 million years, the “average” time between eruptions is 10,000 years, although magma production has decreased in the last 250,000 years. Now you know how to plan ahead, if you live near Flagstaff.

DO DEEP-SEATED MANTLE PLUMES REALLY EXIST?

Plate tectonic theory provides an elegant explanation for Earth’s geological features, and in particular, for Earth’s two types of basaltic volcanism, mid-ocean ridge and island-arc, both of which occur at plate boundaries (transform and convergent, respectively). The theory has failed to provide for an adequate explanation for volcanic activity independent of plate motions that occurs far from plate boundaries such as the SFVF’s intraplate volcanism. Developing in the wake of "tectonic plate" theory, "mantle plume" theory has become a popular concept that filled the intraplate-volcanism geological-void.

In recent years, however, the notion of hotspots and deep-seated mantle plumes has been widely criticized for being too ad hoc and readily amendable, too convenient or too vague, too flexible, too simple and yet too elegant an explanation for a process that is both physically and geochemically undetectable and untestable.

How then, did the plume model come to dominate geodynamics? "Maintenance of the status quo is often the hallmark of scientific endeavor, and the more effort that goes into expounding an idea, the more the belief increases that new observations will only refine details to the model, which belies other reasons as to why concepts have changed so little.” (A.D. Smith et C. Lewis, 1999).

Alternative “plume-less” hypotheses look to the upper mantle, and even back to plate tectonics and subducting slabs to generate intraplate melting anomalies. How might this concept be applied to the SFVF?

COMPRESSION GIVES RISE TO EXTENSION

Beginning in the latest Jurassic, the Farallon Plate initiated its subduction journey beneath the west coast of the North American Plate. Ultimately, the Colorado Plateau was uplifted en masse with little relative deformation. With the Farallon’s consumption, compression reverted to extension by the Early Miocene. That gave birth to the Basin and Range Province which bounds the Colorado Plateau on three sides by extensional forces. The SFVF and other fields are positioned near the boundary of the Colorado Plateau and the Basin and Range’s Transition Zone. In fact, the growth of SFM and the SFVF was dominated by regional extension with NE-SW orientation of the principal tectonic stress axis. 

A NON-PLUME EXPLANATION FOR THE SFVF

The fields were formed as a consequence of the latest uplift of the Colorado Plateau possibly via melting induced by pressure reduction as crustal extension and normal faulting of the Basin and Range Province advanced eastward. Perhaps cracks or rents in the tectonic plate induced by lithospheric extension might allow magma to flood through a gap in the “skin” resulting in a surface expression of volcanism without a plume. It’s also conceivable that the location of the volcanic fields on the plateau may also be controlled by major lineaments within the lithosphere, deep-seated Precambrian zones of structural weakness within the basement of the plateau.



Hypothetical Intraplate Volcanics from (A) Plume-derived Deep Mantle Source

and from (B) Plumeless Shallow Mantle Source

The SFVF is positioned along the boundary of the Colorado Plateau’s thicker crust and the Basin and Range’s thinner crust. The abrupt change in crustal thickness may have perturbed mantle flow sufficiently to create eddies in the mantle close to melting temperatures, ultimately producing numerous discrete basaltic melting events consistent with an “oceanic island basalt-like” mantle source. These are a few of the many plumeless scenarios for intraplate magmatism that focus on a plate tectonic explanation but still evoke a mass of buoyant rising magma from a shallower source within the mantle. 

THE COLORADO PLATEAU’S “RING OF FIRE”

We can now envision the SFVF (red) and the other Late Cenozoic fields (gray) lying on a Colorado Plateau's “Ring of Fire” and their possibly originating from an ascending mantle plume or plumelessly from crustal extension, normal faulting and a thinning lithosphere as basin and range extension gradually encroaches into the plateau on three sides. The thinned-lithosphere would theoretically facilitate the rise of buoyant magma, while fractional crystallization would further modify these melts. This may explain why Arizona has so many geologically young volcanoes and the reason why the SFVF is in close proximity to the province-boundaries.



Cenozoic igneous rocks (orange) form a “Ring of Fire” around the periphery of the Colorado Plateau.

SFVF indicated with arrow.

(Modified from The Earth Through Time from www.higheredbcs.wiley.com)



AN OPEN INVITATION

Please join me on my upcoming post Part II and get as high as you can get (legally) in Arizona as we climb the geology of Mount Humphreys of the San Francisco Peaks.

 

Spectacular view of the Inner Basin looking due east on the final push to the summit on Mount Humphreys.