Desert Mountain—Utah's latest GeoSight

"now a quiet, remote monument to that violent geological time." (Smith 2025)

Desert Mountain's peaks and ridges lower right quarter of photo; white mark is high point (Google Earth).

Desert Mountain is small mountain—an isolated cluster of low peaks, ridges and knobs in Utah's West Desert. The first geologist to write about it thought "Desert Hills" more appropriate (Loughlin 1920). But its story is huge—complicated and filled with drama. And being geological, it's long.

We could start 300 million years ago, when collisions on both sides of a young North America were deforming it far inland, for example in today's western Utah. Or we could start 300 million years before that, when western Utah was covered in shallow water of the great Paleozoic Sea. Or we could go back yet another 300 million years to the creation of that sea, when the supercontinent Rodinia was coming apart. But we won't. Instead we'll start three months ago, on a hot spring day.

Shortly before I left home, the spring issue of Survey Notes showed up in my mailbox. Inside was a GeoSight—a new one, and in the general area of my travels. Of course I would go there! Of the many resources offered by the Utah Geological Survey (UGS), my favorite is GeoSights. I visited my first, the Honeycombs, in 2012. Awed by the rocks and their story, I've been geotripping to GeoSights ever since.

Sunset on the Honeycombs, 2012.

My visit to Desert Mountain was about 80 miles round trip from Delta. I took UT Hwy 6 north to the Jericho Callao Road, then drove west. Pavement soon gave way to gravel, a bit rough in places but generally good. The road crossed open juniper woodlands and sparse dry grasslands, with expansive playas to the south. After 22.5 miles, with Desert Mountain visible nearby, I stayed left at a junction and was soon at its base.

Approaching Desert Mountain from the north. Kelly Hewitt photo via Google Earth.

Rocks abound, trees not so much; pale band close to road is a fence covered in tumbleweeds.

The road continued along the base of a steep slope with granite outcrops, then climbed a short distance to Desert Mountain Pass where there was no shade to be had. I parked and reread Jackson Smith's GeoSight article inside the van, cooled by light breezes wafting through opened windows and doors.

Desert Mountain was born c. 30 to 40 million years ago, during the Great Ignimbrite Flareup which ravaged much of Nevada and western Utah. For 15 million years large volcanoes, supervolcanoes and complexes of supervolcanoes (1) produced on the order of 5.5 million km3 of volcanic material—great clouds of ash that blocked the sun, huge volumes of rock fragments hurled hundreds of miles, and searing pyroclastic flows that destroyed everything in their path. For comparison, the 1980 Mount St. Helens eruption produced only one km3 of material (source).

Geologic map shows rock units discussed here. Blue B's mark shoreline of glacial Lake Bonneville, black lines are faults, labeled arrows are mine (Smith 2025).

Today's rock outcrops suggest that the life of the Desert Mountain volcano had three stages. In the first, viscous rhyolite oozing from vents formed thick deposits of lava. Rhyolite outcrops are the remains of this relatively peaceful eruption (not part of my tour).

But while the lava oozed ever so slowly, trouble was brewing below. Gas was accumulating in the viscous magma, increasing in pressure until it literally exploded. Massive amounts of rock fragments and ash were sent flying. These pyroclastic deposits are said to be common east of the mountain, a project for a cooler day.

The eruption largely emptied the magma chamber, causing the roof of the volcano to collapse. The result was a caldera—a very large bowl-shaped depression. When the roof collapsed it broke up into a mishmash of preexisting rocks and erupted material, forming today's volcanic breccia. I may have seen it at Desert Mountain Pass, adjacent to the beautiful pale granite outcrops.

A close look at the volcanic breccia of Desert Mountain (UGS).

Desert Mountain Pass. Is that volcanic breccia behind the granite? I thought so at the time.

From the pass I drove south, using the geologic map to figure out what I was seeing. Occasionally I spotted large outcrops of much darker rock. This is granodiorite, an older intrusion predating the Desert Mountain volcano (age unknown).

Utah Juniper on pale granite; dark granodiorite in distance.

The star of the show, hands down, was the beautiful pale granite, the youngest and most extensive of Desert Mountain's rock outcrops. It was not present during the cataclysmic eruption, arriving later in the volcano's life. In the third stage, remaining magma rose but didn't reach the surface. Instead it cooled deep enough to form visibly crystalline rock—an exceptionally pale granite sometimes called leucogranite (2).

That may have been the final eruption but obviously there's more to the story, for the granite no longer is fully buried. Exhumation started about 17 million years ago, when the part of North America between the Wasatch Mountains and the Sierra Nevada (today's Basin and Range Province) began to stretch east to west. This extension deformed and fragmented older landscapes, including Desert Mountain. The caldera was uplifted, tilted and fractured, allowing erosion to slowly expose and sculpt the lovely granite.

Desert Mountain granite. Sonny Wilson photo, via Google Earth (cropped).

Granite on the west side of Desert Mountain. gjagiels photo, via Summit Post.

Spectacular outcrop at south end of Desert Mountain. Hmmm ... what are those black and white bands?

There's one more chapter in the Desert Mountain story. It's incomplete, difficult to properly place in the overall timeline, and has rock classification issues. But there's also fun to had.

Sometime after the granite was intruded—perhaps while the magma was cooling or later during extension (or both)—molten material filled fractures forming dikes. Whitish aplite dikes formed first, followed by dark andesitic dikes (3). How do we know the order? By their cross-cutting relationships! These are fun to find and worthy of attention for the story they tell. At Desert Mountain, the white dikes cross the pale granite and are therefore younger. The dark dikes cross both the white dikes and the granite and are therefore the youngest of the three.

 Wonderful display of cross-cutting relationships; white arrows mark less conspicuous aplite.

I considered camping at the base of this outcrop but it was much too hot for me. On the drive to Delta, I stopped and took one more photo of the beautiful pale granite. Then I continued south.

See the dike?

Notes

(1) Desert Mountain is part of the Thomas-Keg-Desert mountains caldera complex (DeCourten 2003). The Honeycombs, mentioned early in the post, may be related.

(2) The pale granite at Desert Mountain was called leucogranite early on (e.g., Kattelman 1968). Now "leucogranite" is increasingly used for a pale granite formed in collisional tectonic settings, for example in the Himalayas (E. H. Christiansen, personal communication). For more, see Miller's excellent Perspective (2024). He explains that though collisional is by far the most common tectonic setting for leucogranite formation, it can form in others, including extensional, if certain conditions are met (e.g., composition low in aluminum). In any case, "leucogranite" is used in Jackson's GeoSights article; Christiansen prefers "granite". [Suggestion to UGS: In GeoSights articles, cite a few sources for additional information.]

(3) The dark dikes also are controversial. According to Jackson, the rock "apparently" is very dark lamprophyre—a catch-all term for various peculiar ultramafic rocks not amenable to the usual classifications (source). Christiansen and colleagues prefer andesite.

Sources (in addition to links in post)

I'm grateful to Eric Christiansen, Professor Emeritus at Brigham Young University, for answering my questions about igneous rocks at Desert Mountain, and for his appreciation of cross-cutting relationships.

Brigham Young University. 2013. Supervolcano in Utah: massive ancient volcano discovered by BYU geologists. YouTube.

DeCourten, FL. 2003. The Broken Land; adventures in Great Basin geology. U. Utah Press.

Loughlin, G. F., 1920, Desert Mountain, in B. S. Butler, and others, Ore deposits of Utah: U. S. Geol. Survey Prof. Paper III, 444-445. PDF

Miller, CF. 2024. Granites, leucogranites, Himalayan leucogranites ... Elements 20(6):359–364. doi: https://doi.org/10.2138/gselements.20.6.359

Rees, DC, Erickson, MP, Whelan, JA. 1973. Geology and diatremes of Desert Mountain, Utah. Utah Geological & Mineralogical Survey Special Studies 42. PDF

Smith, J. 2025. Geosights: Desert Mountain, Juab County, Utah. Utah Geological Survey, Survey Notes.

Peculiar Eruptive Mountains on the Colorado Plateau

The La Sal Mountains rise 8000+ feet above the Colorado Plateau (source).

In 1875, two geologists employed by the US government were studying mountains in southeast Utah. They worked 90 miles apart, each one in an isolated cluster of peaks rising above the mostly horizontal Colorado Plateau. They found the same strange type of structure and the same kinds of igneous rocks, and in their reports published two years later, they reached the same conclusions.

Albert Charles Peale was an employee of the War Department, specifically the Geological and Geographical Survey of the Territories led by Ferdinand Vandeveer Hayden. His party—one geologist, two topographers, two packers, and a cook—was surveying the Grand River District in western Colorado and eastern Utah (1). They spent a week in the Sierra la Sal, "which afforded magnificent opportunities for work" and then headed south. But hostile locals ("Indian trouble") brought field work to a sudden end. In their hasty exit, "all [rock] specimens had to be abandoned."

Grove Karl Gilbert was an employee of the Department of the Interior, specifically the Geographical and Geological Survey of the Rocky Mountain Region led by John Wesley Powell. On his descents of the Colorado River, Powell had seen an unmapped cluster of peaks to the west, which he named the Henry Mountains (2). They looked volcanic—domed, with dark lava-like rock on the top. Volcanology was a young science then, and geologists were debating whether volcanos were elevated craters or built from accumulated lava. So Powell sent Gilbert to the Henrys to "determine the facts" (Hunt 1988). He and his party stayed two months, more than enough time to answer the volcano question.

Peale worked in the La Sals, Gilbert in the Henrys; map based on data from National Atlas, labels added.

That winter Peale wrote up his findings, but two years would pass before Geological Report on the Grand River District was published (3). He estimated they had surveyed 6000 square miles of which "the greater part ... is plateau in character, the Sierra la Sal being the only mountain group." It was an isolated cluster of about 30 peaks arranged in three "eruptive centers". Peale was emphatic about origins: "there can be no doubt of the eruptive character of the mountains... porphyritic trachyte has been pushed up through the sedimentary layers which now dip away from the mountains" (Peale 1877a).

Peale called the La Sals "eruptive mountains of a peculiar type ... igneous and yet non-volcanic" (1877b). They were non-volcanic because lava didn't reach the surface. But neither were they plutons emplaced deep underground. Instead, magma had stopped somewhere in between, deforming the overlying rocks. The intruded rock was exposed much later by erosion. There was no name for this type of structure, so he described it in detail, pointed out its peculiarity, and left it at that.

Sections across Sierra la Sal showing tilted sedimentary strata on intruded trachyte (Peale 1877a, cropped).

From the Sierra la Sal, Peale studied the Henry Mountains off to the west. He knew John Wesley Powell (Gilbert's boss) thought they were volcanic—"the summits of these mountains mark in reality the level of former valleys down which the volcanic material flowed" (Powell 1875, quoted by Peale). But even from ninety miles away Peale could see that was incorrect. "l am inclined to class the Henry Mountains with the Sierra la Sal and Abajo [Mountains], as their outline is similar ..."

From his vantage point in the heart of the Henrys, Gilbert "agreed" with Peale (unknowingly). The peaks were neither elevated craters nor accumulated lava nor even volcanic. In fact they were a novel type of structure, as he warned his readers:

"If the structure of the mountains be as novel to the reader as it was to the writer, and if it be as strongly opposed to his preconception of the manner in which igneous mountains are constituted, he may well question the conclusions in regard to it while they are unsustained by proof. I can only beg him to suspend his judgment until the whole case shall have been presented." (Gilbert 1877)

Gilbert gave the novel structure a name—laccolite—thereby making the Henry Mountains the type locality for laccoliths (today's term). He distinguished them from volcanic eruptions, where lava reaches the surface and accumulates. "The lava of the Henry Mountains behaved differently ... it stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds."

Gilbert's sections across the familiar Mountain of Eruption (volcano) and the novel Laccolite.

Like Peale, Gilbert had to wait two years for publication of his findings. He finished his monograph the winter after his second season in the Henrys. "It was at once put in type, and in anticipation of a speedy issue the current year [1877] was marked on the imprint..." But the many illustrations caused delays. Geology of the Henry Mountains was finally bound and distributed in 1879.

By that time a wealth of information about igneous mountains had accumulated, prompting Gilbert to prepare a second edition (1880). It differed from the first mainly in the addition of an Appendix: Recently Published Descriptions Of Intrusive Phenomena Comparable With Those Of The Henry Mountains. At the end of the section about Peale's findings in the La Sals, Gilbert concluded, "All of these features are paralleled in the Henry Mountains and they leave no reasonable doubt that the structures are identical."

I visited the Henry Mountains in 2012, accompanied by the spirit of Grove Karl Gilbert. I camped at Starr Springs as he had, and hiked to the spectacular south face of Mount Hillers, "revetted by walls of Vermilion and Gray Cliff sandstone" as he explained.

South face of Mount Hillers—steeply tilted sandstone on flanks, intruded trachyte on crest (Jack Share).

Vermillion sandstone "tilted almost to the vertical".

Since then, I've been keen to visit more of the peculiar eruptive mountains on the Colorado Plateau. Last September I finally did, spending a week in the Sierra la Sal.

La Sals upper right; snow highlights 3 clusters of peaks. Upheaval Dome upper left. (Google Earth)

Peale's three eruptive centers live on, though they're now called intrusive centers ("eruptive" means volcanic). But these are special intrusions—emplaced at depths intermediate between volcanos (surface) and plutons (deep). They now have their own descriptor—hypabyssal, aka subvolcanic (but still laccoliths).

Two hypabyssal intrusion-cored peaks: Castle Mountain (left) retains a cap of sedimentary rock; La Sal Peak (right) is trachyte (Ross 1998, cropped).

The three intrusive centers of the La Sal Mountains are conveniently named northern, middle, and southern (4). All normally are accessible via the paved Loop Road, but road construction kept me in the northern one. From a very nice small primitive campground, I hiked to see what I wanted to see—the distinctive features of these peculiar eruptive mountains.

It was a short walk to Castle Valley Overlook with views of the Colorado Plateau. The Plateau doesn't look horizontal, but the rock layers are. The spectacular landforms—towers, buttes, rims. deep winding canyons—are erosional. True uplifts like the La Sal Mountains are uncommon. Gilbert called them "disturbances in a region of geological calm."

Looking northwest near Castle Rock Overlook; in the valley bottom left of center is Round Mountain, a small intrusion perhaps connected to the La Sals.

Let's head on down and see what we can see.

Among Peale's important observations were tilted sedimentary strata that "now dip away from the mountains". He concluded they were pushed up and tilted by rising magma. The hike provided good views of steeply tilted sedimentary rocks.

Nearly vertical beds of reddish sedimentary rocks below trachyte slopes of Grand View Mountain (left); high peaks visible on horizon just right of center.

With part of the Loop Road closed, the high peaks weren't easy to access. So the next day we hiked up a rough dirt road to view trachyte. It's common above the flanking sedimentary rocks, forming steep slopes and discouraging travel as Peale noted. "The only difficulty met with in the study of this interesting region is the great amount of debris that has accumulated ..."

Some kind of outcrop (could be rhyolite) beyond steep slope of "debris".

Trachyte with a dark xenolith—country rock broken off and carried up by magma. 

Fall colors on trachyte.

Mount Peale, a large laccolith and high point of the La Sals (Suffusion of Yellow).

Peale was hesitant to identify the igneous rock of the La Sals, without specimens to give to petrologists for "critical examination". But it looked very much like rock he had seen in similar intrusions in Colorado. So he assigned it to a general category—porphyritic trachyte. It seems trachyte was the accepted name for shallow intrusive rocks low in silica in Peale's time (see Appendix in Gilbert 1880). Now it may be trachyte or diorite, depending in part on whom you ask (5). Being very much a 19th century naturalist at heart, I will stick with trachyte.

On the other hand, everyone agrees the rock is porphyry—visible crystals (phenocrysts) in a fine-grained matrix of trachyte. This is a very cool rock, with an interesting history. As the magma rose it gradually lost heat, eventually dropping to a temperature where hornblende and plagioclase formed crystals. This changed the composition of the remaining molten magma, and when it stopped c. 6–10 km below the surface, it rapidly crystallized to form the trachyte matrix (Ornduff et al. 2006; see also Fractional crystallization).

Porphyritic trachyte mementos from Henry Mountains (left) and La Sal Mountains.

As I drove away from the La Sals, I thought a lot about the pioneering geologists of the American West. Like me, they were inspired by geology and the beauty of the landscapes, but their geotripping was very different. Travel (route-finding required) and camping were much more challenging. And where specifically should they go? (no guidebook). However they had the promise of discovery, which surely made up for all the hardships!

AC Peale and two unidentified men, probably during the
Geological & Geographical Survey of the Territories (Smithsonian Archives).

Notes

(1) The Grand River was the section of the Colorado above the confluence with the Green. Its name was changed in 1921.

(2) Powell named the cluster of peaks for Joseph Henry of the Smithsonian Institution, who helped secure funding for Powell's exploration of the Colorado River.

(3) Publication of Peale's report was delayed through no fault of his own. As his boss, FV Hayden explained, it was caused by "the great increase of labor incident to the International Exposition at Philadelphia", labor that would have gone toward preparation of reports. In general, the regular Reports of the Geological and Geographical Survey of the Territories were inadequate for sharing discoveries. In 1874, a new publication—Bulletins— was created to "publish without delay ... new or specially interesting matter". Peale had an article in Bulletin No. 3, about the peculiar eruptive mountains of Colorado and adjacent Utah, including the La Sals (1877b).

(4) Some sources refer to the La Sal intrusive centers as composite plutons or coalesced intrusions.

(5) Ross (1998) reported that La Sal igneous rocks were 59–71% Si02 (silica), and called them trachyte based on "the Total Alkali-Silica classification of LeBas and others". Wilson and others (2016, based on reports from 1953, 1959, and 1992) reported that igneous rocks of the Henry Mountains were 58–63% SiO2, and called them diorite. (Thanks to Mike for taking a stab at trachyte vs. diorite.)

Sources

Bartlett, RA. 1962. Great Surveys of the American West. Norman, OK: University of Oklahoma Press.

Fillmore, R. 2011. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado. Univ. Utah Press.

Gilbert, GK. 1877. Report on the Geology of the Henry Mountains. GPO. BHL.

Gilbert, GK. 1880. Report on the Geology of the Henry Mountains. 2nd edition. GPO. Google Books PDF. Appendix p 153–161 contains added material about igneous mountains.

Gould, LM. 1927. Geology of the La Sal Mountains, Utah Papers of the Michigan Academy of Science, Arts and Letters Vol. 7: 55-106. HathiTrust

Hunt, CB. 1958. Structural and igneous geology of the La Sal Mountains, Utah. USGS Professional Paper 294-1. PDF

Ornduff, RL, Wieder, RW, Futey, DG. 2006. Geology Underfoot in Southern Utah. Mountain Press Publishing. (see Vignette 32, Intruders in a sedimentary domain)

Peale, AC. 1877a. Geological report on the Grand River District, in Hayden, FV. Ninth annual report of the United States Geological and Geographical Survey of the Territories (p. 31–101). BHL

Peale, AC. 1877b. On a peculiar type of eruptive mountains in Colorado. Art. XVIII in US Geological and Geographical Survey of the Territories Bulletin No. 3: 551–564. BHL

Powell, JW. 1875. Exploration of the Colorado River of the West. Geographical and Geological Survey of the Rocky Mountain Region (Henry Mts p. 200-203).

Ross, ML. 1998. Geology of the Tertiary intrusive centers of the La Sal Mountains, Utah; influence of preexisting structural features on emplacement and morphology in Laccolith complexes of southeastern Utah; time of emplacement and tectonic setting. USGS Bull. 2158: 61-83. PDF

Wilson et al. 2016. Deformation structures associated with the Trachyte Mesa intrusion, Henry Mountains, Utah, Implications for sill and laccolith emplacement mechanisms. J. Structural Geology 87: 30-46. free online

Scaling a Dome of Broken Glass

View south from top of Obsidian Dome; Glass Creek Dome mid photo, Sierra Nevada on skyline.

After touring the Mono Craters, accompanied by the spirit of pioneering geologist Israel Russell, I drove south on US 395 to see more of eastern California's volcanics. This time I was led by Robert P. Sharp and Allen F. Glazner, authors of Geology Underfoot in Death Valley and Owens Valley (1997, first edition).

I was intent on visiting a rhyolite dome or coulée (lava flow), landforms that had puzzled Russell during his fieldwork in the Mono Basin in 1883. "These outbursts of acidic lava are in strong contrast with the overflows of basic rock with which geologists are most familiar ... [which] are frequently quite liquid at first, flow rapidly, and reach a distance of many miles before congealing sufficiently to check their progress." Not so the Mono coulées. They barely reached beyond the foot of their cones.

Mono Craters, added arrows point to coulées—short thick lava flows with rugged surfaces and steep sides (from USGS 1971).

Russell rightly concluded the lava had been extremely viscous and therefore flowed very slowly. He considered these coulées unusual, perhaps unique. Surely geologists would be eager to study them. "When the valley in which these craters are situated becomes more familiar to tourists and geologists, they can not fail to be widely known as typical illustrations of mountains formed of acidic lavas." And so it came to pass.

Time has proven Russell wrong in thinking the Mono coulées unique; similar flows have been found around the world. But he was spot on about geological interest. Rhyolite volcanoes are relatively uncommon, so their abundance in eastern California makes the area attractive to volcanologists. Especially intriguing is their youth. Most erupted in the last 10,000 years, several in the last thousand. And they may be only sleeping.

Between Mammoth Mountain and Mono Lake "virtually every hill is a young volcano" (Sharp & Glazner 1997; NASA photo 2000).

Rhyolite domes and coulées are not easy to examine, as Russell explained. "The extreme ruggedness of the coulées is due to the fact that they hardened at the surface during the time they were still moving. The crust thus formed became broken and involved in the pasty material beneath in a most complicated manner. ... Even at the present day, after many blocks have fallen and the formation of a talus slope has commenced, the climber finds it extremely difficult to scale these rugged and broken escarpments of glassy fragments."

Flow front, North Coulée, Mono Craters.

However not far from Mono Craters is a dome that can be scaled without difficulty. On the west side of Obsidian Dome a road ascends the rugged broken escarpment to the top. A locked gate limits access to all but foot traffic.

The Obsidian Dome volcano erupted in 1350, just 633.5 years ago (associated tree mortality occurred in late summer of that year). Lava slowly oozed from the vent and flowed outward, forming what looks like a giant cowpie 1–2 km across and 50–100 m thick, its surface rough with jumbled blocks of volcanic rock. Aside from its shape, Obsidian Dome is similar to the Mono Crater coulées that Russell found so striking with their steep sides and rough surfaces. "Obsidian Coulée" is actually more appropriate.

Obsidian Dome, the cowpie coulée. Google Earth 2019.

Looking ± northeast at Obsidian Dome; blue line is gated access road. Gray area is an old pumice quarry, the reason for the road. Google Earth 2019.

Obsidian Dome and its road are featured in the geological vignette "Ominous Ooze" in the first edition of Sharp and Glazner's guide. I like the first edition very much. It provides more detail in descriptions and discussions, for example nine pages are devoted to Obsidian Dome vs. only three in the second edition. I found it especially helpful in appreciating the varied and beautiful rocks on display—all rhyolite and yet so different!

Formidable slope of broken glass, slightly worried field assistant for scale.

But the ascent turned out to be a stroll :)

Chunks of black glass were beautiful to the eye but challenging for the camera's light meter.

"Glass" and "glassy" often appear in descriptions of rhyolite volcanoes. Accustomed as we are to the transparent stuff, this can be confusing. Broadly speaking a glass is a non-crystalline solid that cooled so quickly from a liquid state that crystallization was impossible. A myth persists that glass is actually a super viscous liquid that flows at the scale of centuries (for example in old window panes). This has been discounted. But the transition of glass from liquid to solid remains an unsolved problem in physics.

Whatever the exact nature of glass, the volcanoes that extruded these domes and coulées were well equipped to produce it. The magma was >70% silica; at such a high concentration silica tetrahedra (molecules) bind tightly to each other, making super viscous lava. Not only did it barely flow, it was so viscous that other kinds of atoms couldn't move around and bond with their brethren to form crystals. When crystal-poor lava such as this cools rapidly, for example by being carried to the surface in a volcanic eruption, obsidian and other forms of volcanic glass result.

The tiny silica tetrahedron plays a big role in volcanoes (source).

Obsidian with scattered small crystals in a matrix of glass.

At the base of the coulée and along the road to the top I saw lots of obsidian with bands of pale pumice—also a glass but filled with bubbles. Perhaps it formed from frothy lava during a more explosive phase. These rocks were especially beautiful with their varied combinations of banding, curves, and swirls.

What happened here?!

Amid the shiny black broken glass, and sometimes bonded to it, was dull pinkish orange and gray rock that seemed out of place. It turned out to be one of the more interesting finds of the day—stony rhyolite, which has the same chemical composition as obsidian but is crystalline. Even more intriguing, given enough time obsidian will become stony rhyolite.

Those atoms that initially were stymied in their attempts at crystal formation don't give up! It may take a million years but eventually they find a way through the silica tetrahedra, meet their brethren, form crystals, and convert the non-crystalline obsidian to stony rhyolite.

Stony rhyolite can be as beautiful as obsidian.

But why is there stony rhyolite on Obsidian Dome, which erupted just 633.5 years ago? The explanation may be water. Water vapor can facilitate crystallization by breaking bonds in the silica tetrahedra, making the lava less viscous. Perhaps some of the magma contained enough water vapor to produce stony rhyolite right away (Sharp & Glazner 1997).

The hike wasn't long, but there was so much to see! Finally we reached the top. The landscapes were surreal.

Coarsely vesicular obsidian, part of a squeeze-up (read more here).

Sharp and Glazner end their vignette with the difficult but unavoidable question—why are these volcanoes here? Volcanologists have made some progress in answering it. They've even drilled deep into Obsidian Dome, thanks to the road to the old pumice quarry. But that's the subject of a future post, after our next visit.

The Inyo Dike, suspected source of Obsidian Dome and its neighbors (Reches & Fink 1988).

Sources

Fisher, RV, Heiken, G, Hulen, JB. 1997. Volcanoes: crucibles of change. Princeton U Press.

Reches, Z., and Fink, J.H., 1988, The mechanism of intrusion of the Inyo dike, Long Valley caldera, California: J. Geoph. Res. 93:4321–4334. https://doi.org/10.1029/JB093iB05p04321

Russell, IC. 1889. Quaternary History of Mono Valley, California in USGS 8th annual report (If the USGS PDF is slow to load and read online, try HathiTrust.) Russell's report was printed separately in 1984 by Artemisia Press, Lee Vining, CA (out of print).

Sharp, RP, and Glazner, AF. 1997 (4th printing 2003). Geology Underfoot in Death Valley and Owens Valley. Mountain Press. NOTE: A revised second edition was published in 2022 (Glazner, Sylvester, & Sharp). Based on the areas I've visited, it seems more science light. But of course maps and illustrations are far better. Perhaps buy both.

Vogel, TA, et al. 1989. Petrology and emplacement dynamics of intrusive and extrusive rhyolites of Obsidian Dome, Inyo craters volcanic chain, eastern California: J. Geoph. Res. 94:17,937–17,956. PDF (Open Access)