Hiking Mount Humphreys of the San Francisco Peaks in Northern Arizona: Part II – My Geologic Ascent

Just 10 miles north of Flagstaff resides a spectacular edifice known as San Francisco Mountain in geological circles or simply “the Peaks” by the locals. It is the centerpiece of the San Francisco Volcanic Field and towers over the surrounding Colorado Plateau with its six-summits rising to an elevation between 11,000 and 13,000 feet. In fact, it’s the only alpine mountain in Arizona and the tallest in the state with the peak of Mount Humphreys rising to 12,633 feet. My plan was to climb Humphreys and take notes on the geology along the way.

This telephoto shot of the Peaks’ southern flank taken from my hotel in Flagstaff belies its massivity.
The angular summit on the left is Mount Agassiz which blocks our view of Mount Humphreys.

A Google Earth big picture
Seen from the west, San Francisco Mountain’s peaks and interconnecting ridgeline form a horseshoe-shaped ring. Within the Peak’s embrace lies a deep, elliptical central depression called the Inner Basin, the caldera of an ancient volcano. It leads to Lockett Meadow, and ultimately to the northeast breach where diminutive Sugarloaf Mountain stands guard, the last breath of the volcano to erupt. These landforms are the eroded remnants of a massive stratovolcano that erupted 2.78 million years ago. With typically steep flanks, a conical shape and a multi-layered architecture, it catastrophically met its demise between 250,000 and 400,000 years ago.

The Humphreys trailhead is located at the Snowbowl ski area’s car park. Notice the six peaks
that comprise San Francisco Mountain and its caldera, the Inner Basin.  

The final events are still debated by geologists, be they a vertical or sideways blast (alla Mount St. Helens) or a cataclysmic collapse into its own structural plumbing. Most investigators agree that a mechanism of collapse, subsidence or engulfment due to the withdrawal of magma from its magma chamber is responsible for the volcano's contemporary presentation rather than an evacuation outward. Either way, the volcano was reduced to a geothermally-extinct shell, exposed to the ravages of time and erosion. A multitude of Pleistocene alpine glaciers, Holocene gravitational flank collapse and debris flows left their marks on the ravaged stratocone.

Please visit my previous post Part I where I discuss the Peaks’ geo-morphology and geo-genesis in greater detail. 

“Big Picture” Stratigraphy

San Francisco Mountain (SFM hereafter) resides within the San Francisco Volcanic Field (SFVF hereafter) along with a plethora of volcaniforms. The entire field is situated near the southwestern boundary of the geomorphic province of the Colorado Plateau with the Basin and Range’s Transition Zone. Before we initiate our geological ascent, let’s review the volcano’s stratigraphy from the top down.

San Francisco Mountain stratigraphy

The conical shape and vertical stratification of SFM is attributable to the alternate layering of effusive and explosive eruptive materials of lava, pyroclastic debris and lahars (mudflows). SFM is considered to be an andesitic-dacitic stratovolcano built mostly by effusive activity that produced andesites (85%), dacites (12%) and rhyolites (1%). The andesites extruded from central vents fed from a magma reservoir; whereas, silicic lava tended to erupt from the volcano’s base and flanks. Magmas are generally plagioclase-dominated with products exhibiting magma-mixing.

A succession of older and younger andesites and dacites are thought to represent eruptive stages, four in all. “Older Andesite” lava flows constitute mainly the western part of the volcano (including Humphreys summit), while “Younger Andesites” are present on all flanks. Dacites are found on all slopes of the volcano but principally on the lower flanks. Both andesite and dacite are of intermediate mineralogical composition and are silica-rich which affects the volcano’s architecture and behavior. 

Simplified Geologic Map of San Francisco Mountain

Red outline marks the stratovolcano’s geologic boundary

(Karatson et al, 2010)

This geologic cross-section is through Humphreys and Fremont Peaks, two of SFM’s six peaks, and transects the caldera of the volcano. In the region of Humphreys, notice the layered Older and Younger Andesites (Qao and Qay) and Dacites (Qd and Qdo) that constitute the flanks of SFM (specifically an upper pyroxene andesite, a hornblende biotite dacite and a lower hypersthene dacite). On the floor of the Inner Basin are two parallel, resistant ridges called Core Ridge and Secondary Core Ridge and their dikes. They are remnants of the central conduits that fed the volcanic edifice. Radial dikes also fed flank eruptions. We’ll observe many of these structures on our climb of Humphreys.

Cross-section of the SFM through the Inner Basin from NNW to SSE
(Modified USGS map of SFM, Coconino County, Arizona by Richard F. Holm, 1988)

Here’s a link to a strat map of the SFM complex: http://ngmdb.usgs.gov/Prodesc/proddesc_9878.htm.

The San Francisco Volcanic Field

The SFVF (below) is a 4,800 square kilometer region decorated with over 600 Late Miocene to Holocene volcaniforms. It includes the Peaks and monogenetic (single eruption) cinder cones, lava domes, vents, dikes, and associated lava and pyroclastic flows. Volcanism both evolved and migrated on the field in an increasingly easterly direction with greater acceleration, increased magma production and eruption frequency. The field dips northeast at 1/2°-2° coincident with the planar surface of the Colorado Plateau. The field is predominantly basaltic; whereas, composition ranges from basalt to andesite to dacite and rhyolite.

Much can be said and remains to be learned about the field's enigmatic intraplate locale, its tectonic implications, its relationship to other late Cenozoic volcanism in this sector of the Colorado Plateau, and to the advancement of basin and range extension. Again, please visit my previous post Part I for elucidation.

Colorado Plateau stratigraphy

SFM rests on a bed of “older” basaltic flows from 10 to 4 Ma. The volcanic field overlies a mile-thick sequence of sedimentary Paleozoic (Cambrian through Middle Permian Kaibab Limestone) and Mesozoic (Early Triassic Moenkopi Formation) rocks of the Colorado Plateau. The Phanerozoic strata, in turn, unconformably overlie a Proterozoic crystalline basement complex. These layers are best seen within the Grand Canyon, only 45 miles away. Many geologists suspect the Grand Canyon to have formed within the last 6 million years, the time frame of the genesis of the SFVF. What a juxtaposition of geological activity!

Concurrent with volcanism on the plateau’s southern margin, normal faults that formed the Basin and Range Province in southern Arizona have encroached upon the plateau. In association with thinning of the crust, magma has found its way to the surface not only on the Grand Canyon’s North Rim but into the SFVF. Many geologists view the presence of faulting and volcanism as a clear indication that someday the Colorado Plateau will become an extension of the Basin and Range, regions that have already succumbed to extension.   

Schematic Cross-section beneath San Francisco Mountain
(From Morgan et al, 2004)

Mount Humphreys Trailhead

With temps in the upper 50’s, gray overcast skies, and concerns about lightning and lack of visibility at the summit, I was anxious to initiate my climb very early. I arrived at the mountain before sunrise after a short drive from Flagstaff on US 180. I suspect that many flatland-easterners such as myself think of Arizona as having mostly deserts, but there are half-dozen or so ski areas within the state, and actually 25 peaks over 10,000 feet!

Humphreys trailhead (black dots) is at the Arizona Snowbowl’s parking lot at the base of Agassiz’s western flank (35°19′52.61″ N, 111°42′41.73″ W). It crosses a ski trail and abruptly plunges into the Kachina Peaks Wilderness of the Coconino National Forest. After switchbacking its way to the Agassiz Saddle, it heads north to Humphreys across the cols that connect a few false peaks. The journey, considered strenuous by most accounts, is 4.8 miles with an elevation gain of 3,652 feet.

(Modified from Arizona Snowbowl’s Trail Map)



By the way, Mount Humphreys’ namesake was Andrew A. Humphreys, a profane and no-nonsense, war-loving Union Army Brigadier General and Chief of the U.S. Army Corps of Engineers that surveyed the region. SFM was named earlier in the 17^th century by Franciscan priests living at a nearby Hopi mission.


Mesa Butte Fault and its lava domes

Viewed at dawn from the car park, we see the closely-spaced lava domes of Bill Williams Mountain (far left, dated 4.2 to 3.6 Ma), Sitgreaves Mountain (left of center, dated 2.9 to 1.9 Ma), and Kendrick Peak (far right, dated 2.7 to 1.4 Ma). Further northeast along the fault lies Slate Mountain (1.5 Ma). Their silicic to intermediate rocks are viscous, silica-rich dacites, andesites and rhyolites. These volcaniforms mark the western and youngest section of the SFVF between 10 and 30 miles to the west.

The domes are aligned (see strat map below) on a northeast trend of the 150 km long Mesa Butte Fault, likely longer within the subsurface. This high-angle, normal fault resulted from extensional forces that concentrated volcanic vents along its course, the path of least resistance for the episodic ascent of rising magma. These fault systems of late Cenozoic age are related to ancient fracture systems at depth that transect a Proterozoic crystalline basement. They are viewed as indicative of the encroachment of extension on the plateau. Vent alignments along or parallel to these deep-seated crustal structural trends are common on the volcanic field and are often associated with basaltic cinder cones, dike injections and even silicic volcanoes.

Merriam's Life Zones
Ascending a mountain is analogous to traveling into increasingly northern latitudes as harsher and less tolerant growth-conditions for both flora and fauna are encountered. The idea that climatic gradients determine vegetative communities neither began nor ended with the biologist C. Hart Merriam in 1889. However, his concept of “life zones” that succeed each other with elevation was a milestone in the newly developing science of ecology. His research took him to the depths of the Grand Canyon and to the heights of the San Francisco Peaks which contain four of his six zones.

Merriam’s Life Zones (right) and their modern names (left) are labeled on the profile of the Peaks below. The elevation of the zones varies, since the north-facing slope is cooler and wetter than the south-facing slope. These zones can be extended to cover all the world's vegetation types with the addition of the tropical zone, and fluctuate over time in response to the dynamic nature of Earth’s climate.

 (Modified from cpluhna.nau.edu/Biota/elevational_range.htm)

Plunging in to a mixed conifer forest

After leaving the parking area, the trail skirts the base of a grass-covered ski trail before plunging into a tall, aromatic mixed-forest of Aspen, Ponderosa pine and Douglas fir at 9,375 feet, dark in the subdued morning light. We just entered the Mixed Conifer Forest of Merriam’s Canadian Zone. The lofty peaks of SFM generate their own weather with elevated precipitation and cooler temperatures compared to the surrounding semi-arid Colorado Plateau with its pinyon and juniper of Merriam’s Upper Sonoran Zone. We’re walking on a large, gravity-dispersed colluvial apron originating from the flanks of SFM.   

Switchbacks gauge one’s ascent
Once in the forest, the trail enters a series of five or six switchbacks that traverse Humphreys' steep slopes and large gullies, and serve as milestones to gauge one’s ascent. Only one visible outcrop was seen, but scattered within the forest numerous dark to medium-gray andesite boulders have weathered from outcrops undoubtedly from above. In addition, the outer slopes are an amalgamation of alluvium in all drainages, colluvium of silt, sand, pebbles and boulders, talus on the higher and steeper slopes, glacial till and outwash (larger outer gullies), and coarse, unsorted deposits of both avalanche debris and lahars. Blanket all of the above with a mixed forest and dense understory.

A stream of boulders

The first and third switchbacks abruptly reverse directions at a massive boulder stream, typical of glacial environments, that is rather difficult to negotiate. You can spot it on the Google Earth map above. The rock slide consists of unconsolidated boulders of andesite that have cascaded down the mountain’s flank likely facilitated by the movement of ice and a millennia of freeze-thaw cycles. Looking downslope to the west from out on the stream, a lone cinder cone on the volcanic field can be seen in the distance.

The third switchback

Upon gaining some elevation by the third switchback, I again ventured out onto the stream and was rewarded with a picture-perfect view of Sitgreaves (left) and Kendrick Mountains (right) to the west, similar to the perspective at the trailhead. Kendrick is the second highest volcano in the field at 10,418 feet. Much of its plant cover was burned in a devastating forest fire in the summer of 2000. One can only imagine the immense sound generated by this catastrophic avalanche of rock. Notice how Engleman Spruce is beginning to invade the stream from its periphery.

Mount Agassiz

After the final switchback, the trail headed upslope through more open timber with views of Mount Agassiz and its ski trails across Snowbowl Canyon, and the Agassiz Saddle high on the ridge. A five-minute hail storm had me concerned about the weather, but I pressed on and it abruptly abated. Above the treeline, notorious summer monsoons punish the peaks with lightning, fierce winds and rain. The temps can drop 40 degrees in minutes with snow possible even in summer. Climbers beware! At nearly 11,000 feet, this is the Spruce-Fir Forest region that Merriam called the Hudsonian Zone. Humphreys’ treeline is about 11,400 feet.

Agassiz is second in height to Humphreys at 12,356 feet. Named after the celebrated Swiss geologist, paleontologist and educator (1807-1873), one of his many areas of study was ice ages and glaciers that coincidentally sculpted the Peaks.

The Agassiz Saddle

We’ve reached the barren and exposed, wind-whipped Agassiz Saddle at 11,800 feet that connects the summits of Humphreys to the north (left) and Agassiz to the south (right). From here, the Weatherford Trail heads south to the summits of Fremont and Doyle. This is the jagged rim of the stratovolcano comprised of dark gray Older Andesites and some dacites. Standing atop the saddle, you can really appreciate the caldera’s massivity, peering into its depth and surveying the perimeter of the rim.

Although skiing is allowed on Agassiz, it is forbidden to hike above the treeline year round due to the federally-listed and ecologically-threatened, flowering groundsel Packera franciscana (also Senecio franciscana). Besides the talus slopes of Agassiz, Humphreys and the saddle, it is found nowhere else in the world. For all you botanists out there, this plant is a ragwort and a member of the sunflower family. Its future is uncertain in light of climate change predictions since there is little habitat available for the plant to migrate upward in a climate-warming scenario. We are about to enter the protected Arctic-Alpine Zone where hiking off trail is prohibited.

 

The Inner Basin

In the photo below, the 5 X 3 km caldera is 3,280 feet below the saddle. Its deep Inner Basin is bounded on three sides by the steep walls of the volcano's eroded inner flanks with its outlet blocked by the rhyolitic dome of Sugarloaf Mountain (SL) that erupted about 220,000 years ago, the youngest product subsequent to the stratovolcano’s andesitic-dacitic evolution. The central cavity is a subject of debate in regards to its formation during the active phase of volcanics and its subsequent erosion. 

The basin’s evergreen and aspen-carpeted floor has glaciated features such as cirqued-walls, a U-shaped valley, unsorted deposits of till and outwash, and moraines. It’s blanketed with unconsolidated, poorly-sorted volcaniclastic debris shed from the inner flanks via a combination of glacial erosion and mass wasting that coalesces toward the mouth of the Interior Valley. Fluvial contribution appears minimal save intermittent drainages. Springs and wells within the porous and permeable glacial deposits of the Inner Basin are important sources of water for the nearby city of Flagstaff located just south of the Peaks. 

The purplish-red color of the slope on the right is from the high concentration of scoria coming downslope from a parasitic cone that was once active of the flank of the main volcano. Both scoria and basalt are extrusive rocks and that take vesiculation to the extreme. Vesicles are a result of trapped gas within the melt at the time of solidification.





Core Ridge                                                                                                                                               Dominantly-andesitic Core Ridge (CR in the above photo) and its andesitic-dacitic dikes are remnants of the volcano’s conduit system and amongst the oldest rocks of the central complex. A linear Core Ridge divides the Inner Basin into two embayments and may have exerted control over glacial erosion after its exhumation, since two cirques and moraines are found north and south of the ridge. The ridge has experienced topographic inversion whereby it stands out in relief attributable to its differential resistance to erosion, largely glacial. It is said to be erosionally emergent. Some geologists have observed a coincidence of vent alignment and a linear, east-notheast-trend between Core Ridge, the Interior Valley) formed after the construction of the stratovolcano and before Sugarloaf), the Sugarloaf dome, O'Leary Peak and Strawberry Crater. That suggests that they formed under the influence of a common structural control and that the magmas may be closely related in genesis.  

  

Geologic Map of Humphreys Peak and the Inner Basin in the vicinity of Core Ridge

(Qao), Older Andesites; (Qay), Younger Andesites; (Qd) Older Dacites; (Qdo) Older dacites;
(Qs), Surficial Deposits; (Qcc), Andesites of Core Ridge; (Qdi, Qai), Dikes of Core Ridge.

The eastern flank of the San Francisco Volcanic Field

Beyond SFM in the haze (above photo) lies the eastern side of the geologically-recent SFVF. It contains numerous cinder cones and lava flows including the dacite-porphyry domes (240,000 and 170,000 years) of double-topped O’Leary Peak (OL) on the left and the scoria dome (SC) of Sunset Crater (about 1,000 years ago). The tan, unforested area of Bonito Park (BP) is an inter-conal basin consisting of lavas and cinders overlying outwash from SFM glaciation.

Mount Humphreys’ inner flank

In the photo below, looking north from the saddle, Mount Humphreys’ summit at 12,633 feet is about a mile away on the corner of the northwest rim. Notice its inner flanks cut in cross-section that possess layered lava flows, dozens in all, extrusive deposits of andesite, dacite, tuff and pumice. The eruptive deposits moved upward, outward and then downslope from the volcano’s former central vent, now-vanished with the explosion that evacuated the core.

One might assume that the evolution of the volcano’s conical shape is simple in that it forms via the successive layering of eruptive products. But in reality, many stratovolcanos are complex with convoluted histories that are challenging to unravel. This is the case with SFM with its cone-collapse, rebuilding, and even multiple vent locations. Its conical profile is the result of aggradation (eruption and emplacement of volcanic materials) and degradation (destructive processes of erosion, glaciation, gravity-driven avalanching and post-eruption mass wasting). Long-term erosion is climate-driven. Traditionally, volcanic cones are better preserved in arid, cool climates rather than humid, equatorial ones.

The inner flank stratigraphy

This close-up (below) of Humphrey’s glacially-cirqued, inner flank reveals lens-shaped cross-sections of dacite and andesite lava flows. Stratovolcanoes are also called “composite" volcanoes from the alternate layering of effusive and explosive deposits.  The internal structure and plumbing of the edifice was initially revealed when the volcano met its demise and later sculpted by three major Pleistocene glaciations that ended about 10,000 years ago and followed by extensive Holocene gravitational collapse.

I wasn’t the only creature enjoying the view!

Bristlecone pine of the Krummholz

In the upper reaches of Merriam’s Hudsonian Zone wind-twisted, climate-stunted Bristlecone pines reach an age far greater than any other single-living organism known, up to nearly 5,000 years. They grow so slowly that their small stature belies their true age. This region is also referred to as the Krummholz or “crooked-wood” zone, the transition zone to the alpine tundra. Bristlecones are well suited to the harsh conditions of cold, wind, low precipitation and short growing season at the treeline. They are under protection at many National Parks, where their existence is threatened by human trampling, fungal disease and pine beetles.

An igneous sampling from Humphreys

Just below Humphreys’ summit, I made this impromptu grouping of igneous rocks based on color, texture and grain size. Clockwise from the top, we have medium gray dacite, reddish-brown andesite, vesicular basalt, rhyolite, vesicular pumice and pumice again. Do you agree with my identification?

The alpine tundra of the Peaks

Merriam’s Timberline or Sub-Alpine Zone begins at about 12,000 feet. Above that, the only alpine tundra environment in Arizona is located on the Peaks within Merriam’s Arctic-Alpine Life Zone. The defining characteristic of a tundra is its lack of trees, a Finnish word meaning “treeless heights.” At first glance, the exposed summit of the tundra appears depauperate and barren, but it’s far from that. Though treeless, bitter cold, swept by incessant desiccating and abrading high winds, and bombarded by ultraviolet radiation, it sustains a stalwart population of low (prostrate) shrubs, mosses, grass-like sedges and lichens that are genetically adapted to the extremely harsh growing conditions.

The arctic tundra of high latitudes is ecologically synonymous with the alpine tundra of mountain tops. Plant survival adaptations include ground-hugging, waxy and hairy leaves, low nutritional requirements (the cold, thin soil slows decomposition and nutrient-cycling), and adventitious roots (allowing severed rhizomes in the unstable talus to regenerate a new plant rather than reproducing vegetatively).

Seen below, fragile and slow-growing tundra vegetation clings to life in isolated pockets amongst lichen and moss-encrusted rocky crevices and depressions of andesite cobbles and boulders near Humphreys’ summit.

A sign warns hikers to “STAY ON THE TRAIL” to prevent irreparable damage to the fragile tundra. Although protected, the plants may be threatened due to climate change, the inescapable challenge that we must all face.



Mount Humphreys summit

I reached the summit of Humphreys at 9:30 AM, four hours and 15 minutes from the trailhead with ample stops for photos along the way. The trail on the ridgeline crossed a few false peaks as a tease and at times was both difficult to find and negotiate in the loose cinders. I lost it a few times and had to backtrack, but with the summit in view, the destination was obvious. The top was slightly cool, perhaps about 50º F with only a slight wind and overcast skies. There was a brief interlude when the clouds parted allowing the sun to shine directly on top. I spent almost an hour checking out the spectacular view and the amazing geology.

Seen from its north side, this is the rubble-strewn peak of Humphreys capped with an Older Andesite flow with a K-Ar age of 0.43 ± 0.83 Ma.

Southwest view of a false summit
This view to the southwest looks back on a false summit. The trail follows the ridgeline.

South view of Mount Agassiz and its Saddle

The west flank of Fremont Peak on the south rim is on the top left. Its ridge leads to Agassiz, the angular summit to the right. The oxidized iron of the scoria-stained slope is the west end of the tail of Core Ridge which unites with the Agassiz Saddle on the west ridge. Notice the linear growth-pattern of the trees in the basin that follow drainages and talus slopes that have developed.

Mount Agassiz’s glaciated summit is in the background. In the foreground, Older Andesites on the summit of Humphreys are harbingers of protection from the elements for the hardy vegetation of the tundra.

The San Francisco Volcanic Field to the west

Looking west through the haze, we can see the three lava domes of Bill Williams, Sitgreaves and Kendrick Mountains on strike with the Mesa Butte fault on the west side of the volcanic field. Notice the loose, volcanic rubble scattered about. 

The volcano’s outer flanks

In this wide-angle photo looking downslope, the outer flanks of the volcano have eroded into valleys and gullies that lead to poorly sorted debris fans of cobbles and boulders. These fans are heavily vegetated and splay outward radially from every direction beyond the volcano's visible base. The provenance of the clasts within the fans is located in the lavas and pyroclastic deposits above the fans. This can be seen on the bedrock map above. Studies of the debris fans called planezes that surround the Peaks have led some geologists to theorize a dual-cone volcano. In addition, portions of the outer slopes bear the signature of glaciation in the form of till, outwash and moraines. The boulder stream is not the same one that we encountered on our ascent. From the summit to the planar surface of the plateau well beyond the base of the volcano it’s a 5,000 foot difference in elevation!

View to the east

Facing east along the summit-line of Aubineau and Reese on the crater’s north rim, the forested Inner Basin is off to the right (south). Directly beyond the peaks of the north rim the dual-topped cinder cone of O’Leary Peak is directly in the line of sight. To the south (right) of O’Leary, an array of cinder cones pepper the landscape including Sunset Crater, all on the eastern side of the volcanic field.



View to the north

To the north in the haze lies elongate Gray Mountain 35 miles away entering from the left (west), the monoclinal east limb of the Kaibab Upwarp. It is the surface manifestation of a Precambrian fault at depth that was reactivated during Laramide time into a massive domal uplift. Barely visible at the far left is the mist-shrouded North Rim of the Grand Canyon, 65 miles away. In the foreground are many cinder cones that delineate the north side of the SFVF including SP Crater with its barely visible lava flow.

In summation

Three liters of water and 7 ¾ hours later with ample time for photos and reflection on the summit, I arrived back at the trailhead. At the bottom, I stopped at the register where I first signed in. Forty-four names had been added to the list since my start at sunrise. A busy day for all on the Humphreys Trail.

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.