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.



Flight Plan: Part III - The Henry Mountains Laccolithic Complex on the Colorado Plateau

This is the third post on my recent aerial investigation of the geology of south-central Utah. For the earlier portion of the flight, please visit my first two posts entitled “Part I – Geology of the San Rafael Swell” and “Part II – Geology of the Circle Cliffs Uplift at Capitol Reef.”



Photo Above: The Henry Mountains

Framed by the Henry Mountains, Factory Butte's badlands are formed in the Blue Gate Shale Member of the Cretaceous Mancos Shale, and its summit at 6,321 feet is in the resistant Muley Canyon Member. Twenty miles to the south, Mt. Ellen at 11,506 feet of the Henrys is clad in late May snow. A faint image of Table Mountain lies directly in front of Ellen, while the snowless peak to the left (east) is Bull Mountain. From their isolated and remote position within the Henry Basin, the Henry Mountains

appear to be “springing abruptly from the desert.” (G.K. Gilbert, 1880)

TAKING TO THE SKIES TO STUDY THE GEOLOGY ON THE GROUND

While traveling through Utah’s backcountry in May, my good friend, geologist and author Wayne Ranney (WayneRanney.com and http://earthly-musings.blogspot.com/) suggested that we take to the air to investigate the geology. From the ground, it can be challenging to fully appreciate the scale and geological relationships of the Colorado Plateau’s massive landforms. From the air, the landscape takes on an unparalleled, big picture-perspective and provides some beautiful photos as well.

OUR FLIGHT PLAN

Taking off from Price, Utah, we flew south over the crests of the San Rafael Swell and the Circle Cliffs Uplift, better known as Capitol Reef, paying special attention to the geology of their monoclines, the San Rafael Reef and the Waterpocket Fold, respectively. On our return to Price, we circled the Henry Mountains just north of Lake Powell. We mapped out a roughly 500-mile, ellipse and lifted off early in the morning to catch the best light on the terrain.

THE HENRY MOUNTAINS

Where are they?

The Henry Mountains are located on the Colorado Plateau in south-central Utah (38°06'36.04" N, 110°49'21.97" W). They project a good 6,000 feet above the contiguous terrane of the blue and red rock desert of the plateau making them a highly recognizable landmark from considerable distances. The range is surrounded by Laramide-age uplifts, while the Henry Mountain complex intruded into the Henry Basin (also Henry Mountains Basin), a synclinal landform of the same age. The basin’s topography varies from steep, rugged terrain in the Henry Mountains in the east to a series of dissected mesas and buttes, and eroded cuestas and hogback ridges along the western margin.

To the west of the basin lies the Circle Cliffs Uplift and its Waterpocket Fold, a portion of which has been set aside as the Capitol Reef National Park. The northern boundary is the badlands and slopes within the Henry Mountains Syncline, and beyond, the uplift of the San Rafael Swell and its monoclinal Reef. To the east and south is the Colorado River, and below it, the Monument Upwarp, Lake Powell and the Glen Canyon National Recreational Area.



The Henry Mountains are located at the far left of center highlighted in yellow. Other regional laccolithic complexes of the Colorado Plateau are highlighted as well. The Laramide-age uplifts, basins and monoclines, the Paradox Basin and the Colorado River system are also labelled. 

(Modified from Tectonics of the Region of the Paradox Basin, Guidebook, Kelley 1958a, 1958b)

Who was Henry?

Beginning with the renown geologist John Wesley Powell in 1869, the remote and unexplored wilderness of southern Utah and northern Arizona along the Colorado River became a source of great scientific and exploratory fascination. Seeing the Henrys, Powell called them the “Unknown Mountains,” and rightly so. They were the last mountain range in the lower 48 states to be explored. Upon his return in 1871, he officially named them the Henry Mountains, after Joseph Henry, a close friend, supporter and secretary of the Smithsonian Institution.

Two groups of high peaks
The Henry Mountains are a 56 mile-long and 19 mile-wide, isolated string of five rugged, high peaks. From north to south, the range is clustered into two main groups, each dome being 6-10 miles in diameter. The larger northern group consists of Mt. Ellen (11,506 feet), Mt. Pennell (11,371 feet) and Mt. Hillers (10,723 feet). The southern group, also called the Little Rockies, includes Mt. Holmes (7,930 feet) and Mt. Ellsworth (8,235 feet). On our flight, we flew between the northern and southern group. That gave us a great view of Mount Hillers.



The two groups of the Henrys lying in the Henry Basin are visible in this NASA image.

They lie on strike with the Waterpocket Fold, ten miles to the west.

The Colorado River can be seen in the south snaking its way to Lake Powell.

(Image Science & Analysis Laboratory, NASA)



BASIC GEOLOGY OF THE HENRY MOUNTAINS COMPLEX AND BASIN

The Henry Mountain Basin
The Laramide Orogeny, a continuation of Cretaceous mountain-building, provided compression on the Colorado Plateau that resulted in numerous high-relief uplifts separated by small intervening basins. The uplifts and monoclines that we flew over on the earlier portion of our flight (my Posts I and II) demonstrated these landforms. One such basin is the Henry Mountains Basin that received localized intrusions of magma into shallow crustal levels, and that "pushed up" the Henry Mountains. More on that later. 

Having been stripped of its Tertiary deposits, the synclinal basin’s surficial bedrock is composed largely of the multi-membered Mancos Shale (which has experienced numerous revisions). These marine mudstone, siltstone, shale and sandstone deposits were deposited during the initial transgression of the Western Interior Seaway during the Early Cretaceous. The sedimentary section in the Henry Mountains is dominated by sandstones and shales ranging in age from Permian to Cretaceous.

In western regions of the basin are found the mesa-capping, fluvial sandstones of the Tarantula Mesa Sandstone that border the Waterpocket Fold. As we shall see, exposures of strata underlying the Cretaceous deposits down to the Permian are dramatically revealed by the formation of the Henry Mountains.

Stratigraphic column for the Henry Mountains region at the time of emplacement (~25 Ma). The approximate structural levels of selected igneous intrusions are indicated in the margin to the right. The Emery Sandstone is now referred to as the Muley Canyon Member.

(From Jackson and Pollard, 1988)

 

How did the Henry’s form?

Powell assigned the geologist G.K. Gilbert the task of studying the Henrys, which he accomplished in two field studies in 1875 and 1876. Gilbert’s 1877 report became the first thorough and classic investigation of the Henry Mountains. In the 1950’s, the geologist Charles B. Hunt further studied them and offered his own interpretation of their formation.

Gilbert reported that the the mountains

“mark a limited system of disturbances, which interrupt a region of geologic calm, and structurally, as well as topographically, stand by themselves.” He was referring to the fact that the peaks that share a common geological genesis. They formed when large igneous bodies intruded the flat-lying stratigraphy of the Colorado Plateau. The emplacement domed the overlying strata into a mushroom-shape which eventually eroded from the summits of what we know as the Henry Mountains of today. Each peak is an intrusive complex consisting of a large central, concordant (forming parallel rather than cutting-across existing strata layers)  "floored" laccolithic intrusion and many smaller satellite intrusions in a manner similar to lava flows emanating from its parent volcano.

In his insightful analysis on the Henrys, Gilbert coined the term “laccolite” (from the Greek word for “cistern” or “pool”) for the igneous structure resulting from the emplacement process, an intrusive (rather than extrusive) phenomenon. It was a "ground-breaking" thought at the time (excuse the pun); however, his hypothesis has been challenged.

Early sketch from the field notebook of G.K. Gilbert in 1875 of his conceptual model of a laccolith

(From Hunt, 1988).

Gilbert eventually devised this “half-stereogram” of a laccolith that intruded between flat-lying rock layers and domed the over-lying strata. Notice the flat-floor of the intrusion. The rear panel shows how erosion has unroofed the sedimentary cover of the dome, thereby exposing its igneous core.

(Report on the Geology of the Henry Mountains, G.K. Gilbert, Department of the Interior,

USGS of the Rocky Mountain Region, 1877)

The geology and geometry of the Henrys

The core of each of five intrusive centers is a separate diorite-porphyry structure that, at the summit, is bordered by an irregular and enigmatic zone of shattered sedimentary rock, appropriately called the “shatter zone.” Rocks within this zone are a complex intermingling of sedimentary and igneous rock. Laccoliths typically arise from relatively viscous magmas such as the diorite found at the Henrys which is texturally designated as largely a sodium-rich plagioclase and hornblende porphyry. Diorite is a gray to dark gray intermediate intrusive igneous rock and results from partial melting of a parent mafic (high iron and magnesium) rock. As we shall see later, that's a significant point of interest regarding the mountain's intra-plate locale on the Colorado Plateau.  

Many of the intrusive centers are surrounded on their periphery by clusters of smaller laccoliths, bysmaliths, dikes and sills. Their partially exposed, eroded pieces and remnants were visible from the air, as we shall see. In varying stages of exposure and surrounding the base of the centers are the basin's exposed sedimentary rocks, ranging in age from Late Permian to Late Cretaceous, which have been uplifted and deflected by the igneous intrusions that have arched their overburden skyward. Erosion has unroofed the cover from the summits of the intrusive centers and differentially exposed the verticalized rocks around their bases.

Emplacement ages for the Henry Mountains intrusions are from about 31 to 23 Ma, which have been radically revised from earlier calculations. A clear pattern in terms of spatial migration of emplacement ages amongst the various intrusive centers does not appear to exist. The entire complex appears to have been assembled in less than one million years.

What is the most likely emplacment scenario?

The Henry's formation appears to have begun with tongue-shaped sills and thin laccoliths fed by vertical dikes  emplaced in successive stacks. With the thickening of a major laccolith, the faulting of bedding planes was induced which began to tilt the overlying sills. Peripheral dikes and faults formed as lateral growth of the laccolith ceased. Formed from multiple intrusions, the major laccolith began to thicken vertically. Vertical growth or domal uplift of the overlying Mesozoic host rocks provided the accommodation space, the space-making mechanism, for the intrusions. The overlying strata, responding to the vertical displacement, developed numerous faults cut by vertical dikes. Subsequent erosion has removed as much as 3-4 km of sedimentary overburden that bared the crystalline cores of the intrusive centers.

Mt. Hillers possesses the best exposures of intrusions and sedimentary rock contacts. Note the diorite-core (red) at the summit and at various sills, satellite laccoliths and bysmaliths on the peak’s periphery (especially at noon to two o’clock). We’ll see these on our fly around. Also notice the large shatter zone (pink) surrounding the summit and the upturned Mesozoic rocks surrounding its base, especially the highly recognizable Navajo Sandstone (yellow).

(Modified from Emplacement and Assembly of Shallow Intrusions, Field Guide, Horsman et al, 2010)

Uplifted and reflected overburden
The following cross-sectional diagram of Mt. Hillers shows its central intrusion having uplifted and deformed the overburden of the basin during its emplacement. The slope of the uplifted layers increases with proximity to the dome and has a “doubly-hinged shape” consisting of a “concave-upward lower hinge” and a “convex-downward upper hinge.” The hinges are connected by a central limb of almost constant dip. This is largely where (A) upturned strata are differentially exposed and eroded, remniscent of the monoclinal erosion we saw earlier in our flight. This is also the zone of peripheral volcanic intrusions (sills that were emplaced horizontally and later tilted) and networks of bedding plane-faults (that have accommodated the strains of bending and stretching).

Are the intrusive centers of the Henry Mountains laccoliths or stocks?

The architecture of the Henry's subterranean volcanic structure underlying the domes is not without controversy. Although they are represented in geology textbooks as classic laccolithic mountains (Gilbert’s concept), contemporary analyses have suggested that they are larger more complex intrusions called stocks (Hunt’s concept).

Structurally, laccoliths may be low in height and range from circular to tongue-shaped in form. Stocks have greater height and are cylindrical in shape. Laccoliths are fed by a dike or stock; whereas, stocks do not have a feeder since they are continuous at depth. Laccoliths grow from a thin sill that thickens, thus are floored and concordant (parallel to rock layers). Stocks grow upward through zone-melting or diapirically, thus are not floored and discordant (cross-cutting rock layers).

Gilbert hypothesized that sill intrusion preceded the inflation of an underlying laccolith. Hunt believed the central intrusions are cylindrical stocks that are sheathed with a zone of shattered sedimentary rocks and that laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. Recent findings have confirmed the presence of a floored laccolithic intrusion but have not ruled out a stock at depth.

Gilbert’s concept of laccoliths in Mt. Hillers: A, Cross section with diorite in black; B, Subsurface structure;
C, Idealized laccolithic intrusion with a narrow feeder at its base.

(Original modified from Gilbert, 1877. From Processes of Laccolithic Emplacement, Jackson)

Hunt’s concept of relationships between the stocks and uplift of beds of Mt. Hillers

            (Modified from Hunt 1953. From Processes of Laccolithic Emplacement, Jackson)

LACCOLITHIC RANGES OF THE COLORADO PLATEAU

The Henrys are not alone

Between the Late Oligocene and Early Miocene on the Colorado Plateau, magmatism in the form of laccoliths occurred on the Colorado Plateau in seven laccolithic ranges, the largest of which are the Henrys (31 to 23 Ma). Their “magma-blister”, laccolithic-architecture is shared by other ranges such as the Abajo (29 to 23 Ma) and La Sals (28 to 25 Ma), and singular Navajo Mountain, about 65 miles due south of the Henrys. Navajo Mountain, still retaining its sedimentary rock cover and its crystalline core not yet exposed by erosion, is believed to represent an early stage in the intrusive process. All these laccolithic complexes possess a coincidence of timing (collectively 32.3 to 22.6 Ma), a style of intrusion, and the same or similar chemical signatures. That has given rise to the conclusion that they share a similar origin.

An enigmatic intra-plate locale

In viewing the aforementioned laccolithic centers as a group, one must also consider potential relationships to coeval igneous activity elsewhere on the Colorado Plateau, namely its east and west margins. The emplacement of the Marysvale volcanics (34 to 21 Ma) on the west and the San Juan volcanic field (32 to 23.1 Ma) on the east fall within the time frame of the laccoliths. All of the aforementioned volcanic centers are found in a rather incongruous location, when one considers that volcanic activity classically occurs at the boundaries of tectonic plates (excluding  intraplate activity at hotspots, not the case here). Furthermore, coeval igneous activity on the west and east margins of the plateau exists in sharp contrast to the locale of our intra-plateau laccolithic centers. I recall my exact thoughts when I saw these landforms for the first time. "What's the big picture? What’s going on tectonically? Is there a relationship of genesis?"

Looking below the surface

One explanation of Oligocene magmatism centers on the crust of the Colorado Plateau, thick (between 45 to 50 km) and largely undeformed, and on its mafic composition, stronger and more resistant to deformation than the more silicic crust to the west. This is in contrast to the thinner, fault-riddled crust of the Basin and Range Province to the west of the Colorado Plateau (~30 km). And to the east at the Rocky Mountains, the crust is thick but also highly faulted. It has been surmised that faults on both sides of the Plateau acted as conduits to facilitate the rise of magma to the surface. So how does that explain mantle-derived, intraplate magmatism at the laccolithic centers and their "sudden" timing during the Oligocene after such a long period of quiescence?

Modern geochronology and geochemistry to the rescue
Revised ages of the intrusions have made it clear that mid-Tertiary magmatism on the Colorado Plateau was part of voluminous regional magmatism in the North American Cordillera. The data suggests the existence of an essentially continuous, thousand-mile plus, intra-continental magmatic zone that extended from western Nevada through southern Utah to southwestern Colorado, and south to west Texas during the Oligocene to Miocene transition. As we shall see, the length of the zone and its perpendicular orientation to the trend of subduction along the western coast, help to explain Mid-Tertiary igneous activity. In addition, the isotopic geochemical signature of the Henry's rocks tells us that the magma was derived from partial melting of subducted oceanic crust in the mantle, the characteristic mark of a magmatic arc.

The Farallon Big Picture
The rapid subduction of the oceanic Farallon Plate beneath the continental North American Plate proceeded at a flatter trajectory in the region that drove the Laramide Orogeny. It was the presence of the Farallon Plate that provided the voluminous and widespread source for arc-related magmatism. But how? Plate convergence presumably slowed by 50 Ma, which drove the dense Farallon deeper into the mantle causing it to founder and break up. That allowed underlying buoyant, hot mantle to rise and heat the base of the crust resulting in its partial melting.

That's not all. As the less-dense melt ascended through the mantle, it pooled at the base of the silicic crust, owing to its greater mafic-density. That facilitated a silicic, crustal melt. The final result was the shifting of the original mafic composition of the mantle-derived melt to a melt with a more intermediate composition (our diorite!). That further retarded its journey of ascension, eventually stalling its rise into the shallow crust in a neutral-buoyancy state. Voila! That produced the magma that fed the Henry Mountains (and the other laccolithic-derived landforms of the Colorado Plateau), identifiable by its mafic, arc-like affinity. Amazing stuff. 

In summary, magmatism at the laccolithic centers is likely a consequence of the subduction of Farallon oceanic lithosphere. That exerted control over the composition, distribution and timing of magmatism after the Laramide Orogeny. The transport of relatively small volumes of magma within the laccoliths to shallow crustal environments indicates suppression by the unique physical properties of the high-strength lithosphere of the Colorado Plateau relative to contemporaneous magmatism in the Great Basin to the west and the San Juan Mountains to the east.


TAKING FLIGHT

Photo Below: The Waterpocket Fold and the Henry Mountains seen from the west

Having crossed the Circle Cliffs Uplift at Capitol Reef National Park from west to east, we emerged at its plunging monocline, the Waterpocket Fold. This curvaceous portion of the monocline, seen below, depicts numerous strike valleys and exposed ridges formed by the variably resistant and susceptible strata to the forces of erosion. About ten miles away looms a portion of the northern group of the Henry Mountains. Table Mountain is located furthest to the north (left), followed by Mt. Ellen and its lesser summits.

Photo Below: Mt. Pennell and the Henry Basin badlands

We then turned to the south and followed the monocline, eventually banking due east toward the Henry Mountains. In this view, we are over the badlands just east of the monocline at the western portion of the Henry Mountain Basin and facing the western flank of Mt. Pennell. Its slope gradually drapes into the foreground until plunging into badlands of the heavily eroded Blue Gate Member of the Mancos Shale and capped by mesas formed in the Muley Canyon Member (formerly the Emery Sandstone Member). Pennell's flanks are covered with Quaternary colluvial deposits consisting of slide material, slumps and talus.

Photo Below: Mt. Pennell from the southwest

This view of Pennell’s southwest slope shows well the gradual drape of the sedimentary rocks out over the basin from uplift of the igneous dome. The slope is comprised of members of the Cretaceous Mancos Shale draped over by Quaternary colluvium shed from the mountains. The rising flank of Mt. Hillers is almost seen to the right.

 

Photo Below: Mt. Hillers from the south

We headed between the northern and southern groups of the Henry Mountains. The photo faces north towards Mt. Hillers’ southern flank. The northern three mountains are more mature intrusive centers than the smaller southern group, each with more component intrusions in a wider range of sizes and geometries. Of the intrusive centers of the northern group, Hillers has the best exposures of intrusions and sedimentary rock contacts. The peak is considered a more mature, later stage in the emplacement process. 

Recall that as the laccolith evolved, it elevated the overlying strata which have since eroded from the dome and flanks of Mt. Hillers, leaving its denuded igneous core exposed. At the southern and southeastern base of Mt. Hillers, Cretaceous and Jurassic strata, and various dikes can be seen to have been uplifted and deflected in a manner analogous to a trap door opening skyward. The crest is composed of diorite, and below it, the shattered zone, not well displayed owing to the distance. The lower flanks are composed of Glen Canyon Group deposits, and closer to the base, the deposits of the San Rafael Group. The slopes surrounding Hillers and all the peaks are marked by radial drainage patterns. It is estimated that almost 5,000 feet of Cretaceous sedimentary rocks that once covered the intrusions and Canyonlands country have been stripped away by erosion, exposing the igneous rocks that cored the intrusions.

Having just left the Waterpocket Fold, its upturned strata bear sharp contrast to the geo-dynamics operating at the upturned strata at the Henrys. At the fold, the uplifted and subsequent exposure of the sedimentary strata is related to its monoclinal drape over a Precambrian fault at depth. On the other hand, the uplift and exposure of the sedimentary strata at the Henry Mountains is related to laccolithic doming. In both circumstances, the forces of erosion have acted upon the strata. The tectonic commonality that the two share is the subduction of the Farallon Plate beneath the North American Plate. In the case of the Waterpocket Fold, the mechanism was Laramide compression; whereas, in the case of the Henrys, the mechanism was post-Laramide magmatism and emplacement related to Farallon foundering.


Photo Below: Mount Hillers’ upturned dikes and sedimentary strata from the southeast

In this view, we have flown around the southern extent of Mt. Hillers to its southeastern flank. Seeing Hillers in profile, the dramatic upturned nature of the Mesozoic strata is readily discernible. Again, notice the sedimentary strata draping away from the base of Hillers. This indicates the areal extent of the intrusion at depth, far greater than what is seen at the surface. Also, note the sharply upturned Early Jurassic Navajo Sandstone. Strata of the San Rafael Group lie circumferentially outside of it, upturned as well, but bending into the subsurface as part of a long, sloping-limb that is buried by colluvium. Vertical dikes can be seen cutting through the buff-colored Navajo Sandstone and running towards the summit. Numerous bedding plane-faults exist in order to accommodate the flexure of the  bedrock. Hillers and its satellite intrusions are thought to have been assembled within no more than one million years.

The steeply-dipping, deflected beds of Jurassic Navajo Sandstone provide photogenic evidence of the primary space-making mechanism for the magma of Mt. Hillers’ central intrusion that of "roof-uplift." The oldest sedimentary unit that is exposed on the southern flank of Mt. Hillers is the Permian Cedar Mesa Member of the Cutler Formation. This provides one with a sense of the depth of the Hillers’ intrusive center!

Photo Below: Mt. Hillers' eastern flank

We’re now facing the eastern flank Mt. Hillers. Amongst the various other peaks of the Henrys, the igneous intrusions exhibit varying stages of development. For example, early centers possess a sedimentary cover that not only dominates the margins but can be traced almost to the summit. In addition, satellite intrusions around main intrusive centers exist in a highly varied spatial variation, but the main intrusive center is consistently a laccolith with numerous dikes and sills above a large central pluton.

 

 

PHOTO BELOW: The Black Mesa bysmalith and Maiden Creek sill of Mt. Hillers

From this vantage point on the eastern slope of Mt. Hillers (just out of view to the left), we again see the peaks of Mt. Pennell (left) and Mt. Ellen (right). As seen on the western flank of the mountains, a moderate inclination of the sedimentary layers continues for several kilometers away from the intrusive centers at each peak, a testimony to the doming that has occured at depth.

Dominating the left center of the photo are a few of Hillers' well studied satellite intrusions, the cliff-forming outcrops of the Black Mesa bysmalith (center), and at bottom center, the Maiden Creek sill. Immediately out of view to the right is the Trachyte Mesa laccolith. Each of these intrusions provides a snapshot of the growth history of a small pluton during its progressive assembly, thought to have occured in multiple pulses, as magma input increased.

The Maiden Creek sill provides evidence of the first episode, the Trachyte Mesa laccolith records the first two stages, and the Black Mesa bysmalith (an overinflated, cylindrical intrusion that cross-cuts adjacent strata) records all three. These satellite structures developed on the margin of the Mt. Hillers complex and are thought to have been emplaced from weeks to years.
             

SW-NE Cross-section of Mt. Hillers

Notice the drape of the sedimentary layers away from the central intrusion, and the dikes and sills bent upward along with the strata by the doming. Also note the depiction of the Black Mesa bysmalith and the Maiden Creek sill fed by a system of dikes and sills.



Schematic cross-sections through three satellite intrusions illustrating their emplacement mechanisms.

(Emplacement and Assembly… Field Guide, Horsman et al, 2010)

Photo Below: Mt. Ellen and Bull Mountain from the east

Having turned the corner on our flight around the Henry Mountains, we’re now heading north. The last of the Henry's peaks, Bull Mountain (9,187 feet), can be seen from the southeast. The intrusive center of Bull Mountain is a bysmalith similar to Black Mesa.

Photo Below: The northern section of the Henrys from the east

Here's our final glance at the northern section of the Henrys from the east while flying over the mesas and benches around the Dirty Devil River. That's Table Mountain, also a bysmalith, at the far right (north) with Mt. Ellen, the South Summit Ridge and finally snowless Ragged Mountain to the south. Notice the omni-present, photogenic clouds hovering over the range "making" their own weather in this arid region of Utah.

I'll continue with my upcoming and final post on our flight, as we head back to Price, Utah.

Highly Recommended Reading: 

Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney, 2008.

Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado by Robert Fillmore, 2011.