Strange Volcanic Sisters on California's Central Coast

A geo gift for Christmas!

One of my relatives has an eye for fun and fitting gifts. Most recently she gave me a tote bag featuring the Nine Sisters of northern San Luis Obispo County—rocky peaks all in a line, reaching from the town of San Luis Obispo to the sea. The northwesternmost, Morro Rock, is the one everybody knows.

Morro Rock, "the most striking scenic feature on the coast of California" (Fairbanks 1904).

The other Sisters are not nearly as famous, though thousands of people race past them daily. Nor is their number agreed upon. When I was a kid on the Central California Coast long ago, there were seven. Now the more commonly used number is nine. But as geologists know, there are many of these very strange peaks—all of the same rock, all very steep, and all neatly aligned. And they wonder: How did this happen? Why are they here?

The 23 Sisters or Morros (Spanish for "hills") are also known as the Morro-Islay Volcanic Complex.

The Nine Sisters are labeled, with others in between; from northwest (top) to southeast (bottom). See full panorama by SLOhiker.

In October of 1860 William H. Brewer, a recent hire with the brand new California Geological Survey, boarded a steamer in New York City. After a week at sea, he arrived in Panama, crossed the isthmus by railroad in a day, boarded a steamer headed north, and ten days later arrived in San Francisco, where he learned of Lincoln's election. The evening was spent observing celebrations—"fireworks, processions, etc."

After supplies had been secured, Brewer and three others traveled back south by steamer to San Pedro, the port of Los Angeles. In early February, they headed north by horse and mule-drawn wagon, prepared to survey as far as Monterey.

California Geological Survey field party, 1864 (not the 1861 crew); Brewer in chair (Brewer 1930).

As they worked their way north, Brewer wrote letters describing the surrounding country and their adventures in great detail. He mailed them to his brother (where postal service was available), who had been directed to share them with family and friends, and then hold for Brewer's return. Amazingly, all but two or three letters were delivered; they were published in 1930 (source of quotes here).

In early April they entered San Luis Obispo County on terrible roads—"no road in fact, but a mere trail, like a cow path, hardly marked by the track of wheels, and often very obscure." A bad wreck in the Arroyo Grande delayed them for a day as they reassembled and reloaded the wagon. They continued on the so-called 'better' road to San Luis Obispo, arriving a few days later.

Brewer found San Luis Obispo to be "a small miserable town" in a lovely setting:

"San Luis Obispo town lies in a beautiful, green, grassy valley, about nine miles from the sea. ... This valley is more like a plain, from four to six miles wide and fifteen or twenty long, running northwest to the ocean."

They camped near a butte that was "beautifully rounded, about eight or nine hundred feet high and perfectly green."

Brewer's party likely camped near today's Cerro San Luis (Leif Arne Storset photo).

Though beautiful, the butte was quite strange in the way it rose so suddenly from the plain. And there were many such buttes, all equally odd, all curiously aligned.

"These buttes are a peculiar feature, their sharp, rugged outlines standing so clear against the sky, their sides sloping from thirty to fifty degrees! ... A string of these buttes, more than twenty in number, some almost as sharp as a steeple, extend in a line northwest to the sea, about twenty miles distant, one standing in the sea, the Morro Rock, rising like a pyramid from the waters."

"Through this plain rise many sharp peaks or 'buttes'—rocky, conical, very steep hills" (hakkun photo).

An unnamed butte, one of many (Ronn Koeppel photo).

As for their geology, Brewer noted only that the buttes were "mostly of volcanic origin, directly or indirectly". If he thought it odd to find volcanos here, he didn't say (1).

Brewer is often credited with today's name: "these buttes are in a line, nine in number, and I propose to call them the Nine Sisters." But in reading his letters I found no such statement. He never called them the Nine Sisters and counted at least twenty. Claude AI found this false quote in multiple places, "copied uncritically from source to source".

Before leaving San Luis Obispo County, Brewer and a companion climbed and measured the Santa Lucia Mountains. It was a lovely day—cool and clear—and views from the crest were worthy of contemplation.

"the breakers on the shore were perfectly distinct twenty miles distant! [italics his] To the southwest and west lay all the lovely plain of San Luis Obispo, the buttes rising through it—over twenty were visible—brown pyramids on the emerald plain. We sat and contemplated the scene for over an hour before leaving."

"brown pyramids on the emerald plain" (Basar photo).

About forty years later, Harold W. Fairbanks of the US Geological Survey surveyed, mapped and described the geology of the San Luis Quadrangle (west-central San Luis Obispo County). He too was struck by the curious line of rocky peaks and ridges, which he called the "San Luis buttes".

"South of the town of San Luis Obispo there begins a line of peaks and ridges which extends northwestward for about 16 miles. It terminates in Morro Rock, lying in the ocean off Morro Bay. This series of buttes constitutes the most striking topographic feature of the quadrangle. There are about 12 ... Many of them are almost completely isolated and rise from the open valleys with bold and frequently precipitous rock faces." (Fairbanks 1904, source of quotes here).

San Luis Obispo c. 1903, with two of the San Luis buttes behind.

"Hollister Peak rises from a base but little above tidewater to a height of over 1400 feet" (Ronn Koeppel photo).

Fairbanks knew the San Luis buttes were volcanic, the rocks made that clear. He identified dacite and andesite in roughly equal abundance; dacite is now considered the dominant type.

Dacite contains visible crystals set in a fine-grained gray groundmass—classic porphyritic texture. This led Fairbanks to call the rock dacite-granophyre ("phyre" from porphyry), a term no longer in standard use. But he was correct about the porphyritic texture, and that led him to another conclusion, also correct. The San Luis buttes are volcanic plugs formed at shallow depths, not extruded magma. They were exposed later when erosion removed the softer surrounding rocks.

Dacite: plagioclase feldspar (large whitish crystals), biotite and quartz in a gray groundmass (Johnston 2021).

In the Geologic History section of his report, Fairbanks tried to place the volcanos in the greater scheme of things, but their age "could not be definitely ascertained from any observations made." It appeared that their intrusion had not deformed adjacent Cretaceous rock, and therefore the volcanos must be older. He assigned them to the early Cretaceous Period, between 140 and 100 million years ago (2). We now know they are much younger, emplaced 27 million years ago (Beck & Johnston 2011).

Like Brewer before him, Fairbanks did not try to explain why these volcanos had erupted here. It was an understandable omission. Geology was still a young science; sixty years would pass before geologists came up with widely-accepted explanations for volcanism.

Excerpt from Fairbanks's geologic map; San Luis buttes are the orange blobs from upper left (Morro Rock) to lower right (Islay Hill). Click on image to view.

The great progress geologists have made in deciphering the hows and whys of landscapes is due largely to the theory of plate tectonics. In brief, the Earth's surface consists of giant plates—on the order of a dozen large ones and many smaller. Though massive, they are not stationary. They shift, jostle, collide, rise and sink, expand and contract, and deform each other in various ways. Their movement is much too slow for us to sense, just 2 to 10 cm per year, like the growth of a fingernail. In contrast, the results are spectacular—for example mountain ranges, ocean basins, earthquakes and volcanos.

But in spite of our understanding of plate tectonics, the volcanic buttes between San Luis Obispo and the sea remain puzzling. The problem is their location. Volcanos can't erupt just anywhere; there must be a source of magma. But magma doesn't occur just anywhere. It forms with melting of the mantle, the immense mass of solid but soft rock that lies well below the Earth's surface.

Earth's internal structure (IsadoraofIbiza). The voluminous mantle is the source of volcanic magma, but only under the right conditions.

Though the mantle underlies all of Earth's crust and forms 84% of its volume, it only melts sufficiently for volcanism in special situations. The common ones are: (1) mid-ocean ridges, where two plates are moving away from each other; (2) hotspots perhaps created by rising plumes of anomalously hot mantle (they're controversial); and (3) subduction zones where one tectonic plate dives under another deeply enough to melt (Nelson 2015). The Sisters fit none of these scenarios.

California's Central Coast 40 million years ago, expected areas of volcanism circled in white. But the Sisters erupted into a thick stack of sedimentary and metamorphic rocks (from Johnston 2021; annotations mine).

The Morro-Islay volcanos all erupted into a thick stack of sedimentary rocks (3), well away from the usual tectonic settings. And the amount of magma was far too little to have been produced by a hot spot (think about all the volcanic rock in the Hawaiian islands!). So why did these volcanos erupt here? Because 27 million years ago there was a window of opportunity—specifically a slab window.

If we were to visit the coast of North America 27 million years ago and look west, we would see ocean to the horizon. But something very interesting was going on below the surface. Not far away, the seafloor was spreading along a mid-ocean ridge, with mantle rock welling up and melting, and volcanos erupting (yes, underwater!).

Mid-ocean ridge in action; orange upwelling is melted mantle (USGS).

That mid-ocean ridge was the boundary between two tectonic plates—the Pacific to the west and the Farallon to the east. The entire system was moving eastward, forcing the Farallon Plate to dive under the North American plate. This was a straightforward example of subduction until the mid-ocean ridge arrived. When it reached the subduction zone, the Farallon Plate continued sinking eastward while the Pacific Plate moved northwest. No longer connected, they opened a slab window where mantle could rise, melt, and produce the magma that formed the Sisters.

And there would be more drama—not just volcanos but also earthquakes. With the Farallon Plate gone, the Pacific Plate continued moving northwest, but now along the coast of North America. Subduction was replaced with a transform fault moving in slips and jerks, periodically wreaking havoc (earthquakes). This slab window turned out to be a major tectonic event—giving birth to the San Andreas fault as well as the Sisters!

Creation of the San Andreas transform fault (parallel but opposite arrows) with the arrival of a mid-ocean ridge (dark pink band) (USGS, highly modified).

To end this story, let's return to its beginning—to Morro Rock and the words of William H. Fairbanks. In Economic Geology, the final section of his report, he wrote:

"The buttes extending from San Luis Obispo northwestward to Morro Rock furnish excellent and durable stone for building purposes. A quarry has been opened on Morro Rock for the purpose of supplying material for the Port Harford breakwater, and blocks of any size can be obtained. It is to be hoped, however, that the grandeur and symmetrical proportions of this mass will not be marred, as equally good material can be obtained from the other buttes."

Morro Rock was quarried off and on from 1889 to 1963. It now belongs to the State of California, and has been designated both a state and historical landmark (more here). And fortunately, its "grandeur and symmetrical proportions " are still with us.

Notes

(1) Brewer's very brief discussion of the origins of the buttes isn't surprising. He was a surveyor, not a geologist. In fact his title was Principal Assistant in charge of Botanical Department. But he was an astute observer, shown by his tally of the buttes for example.

(2) Fairbanks was not convinced that the San Luis buttes were Cretaceous in age. In his Geologic History section he noted "There were at least two epochs of igneous activity during the Cretaceous, and three if the formation of the San Luis buttes be included."

(3) The sedimentary rocks intruded by the Morro-Islay volcanos are part of the Franciscan Complex— a diverse assemblage of sedimentary and metamorphosed rocks accreted to the North American plate during subduction—an accretionary wedge.

Sources

Beck, MD, Johnston, SM. 2011. U-Pb geochronology and geochemistry of the Morro-Islay volcanic complex, southern California. Abstract.

Brewer, WH. 1930. Up and down California in 1860–1864 (introduction by Francis P. Farquhar). Oxford University Press. Available at Hathitrust.

Fairbanks, HW. 1904. Description of the San Luis Obispo Quadrangle, California: Geologic Atlas. San Luis Folio 101, USGS. 7 PDFs

Johnston, SM. 2021. The Morro-Islay Volcanic Chain and what's a slab window anyway? Video lecture. Highly recommended.

Morro Bay National Estuary Program. 2024. A Geologic History of Morro Rock (includes the geology of the Sisters, with simple diagrams).

Nelson, SA. 2015. Structure of the earth and origins of magma. Lecture outlines; very clear!

Sierra Club, Santa Lucia Chapter. The Nine Sisters of San Luis Obispo County. Web Archive.

Wikipedia's Morro Rock article includes the geology of the entire Morro Rock-Islay Hill Complex.

Geohopping across Nevada

Burners at incipient plate boundary in western Nevada. Are they waving California goodbye? (original unknown)

Many times I've crossed Nevada in the company of Frank DeCourten and Norma Biggar (hereafter called D & B). Actually I've never met either one, but I know their Roadside Geology of Nevada well. That's where I learned of the state's traumatic history—torn apart, reassembled, buried in ash and welded rock, and now being torn apart again. These stories can be hard to grasp, but I've read and reread the lengthy introduction enough to be awestruck by landscapes that many travelers find dull.

Sturdily bound, with high quality paper—my copy has survived lots of use.

Maps, diagrams and photos are abundant!

In the eight years since D & B published their book, I've often parked off the highway at their suggestion to study and photograph a geologic feature. I think of this as geohopping to geostops, rather than my usual geotripping to geosights (and later blogging about it). Now it's time to give the geostops their due.

One of my favorite stretches of highway between Laramie, Wyoming (home) and the California Central Coast (home of relatives) is US 6 across Nevada. Traffic is light, towns are few, and the geology truly is dramatic!

Geo highlights along US Highway 6, May 2025.

For example, about thirty million years ago, widespread cataclysmic destruction associated with the Great Ignimbrite Flareup (GIF) created Hell right here on Earth. Supervolcanoes erupted repeatedly across today's Nevada depositing ash thousands of feet deep, much of it welded into rock by the searing heat ("ignimbrite" means "fire cloud rock"). Trying to recreate that terrifying Flareup in my mind is one of the joys of driving across Nevada.

But it's impossible to properly imagine the GIF, in part because "no volcanic eruptions ever witnessed by humans come close to rivaling these prehistoric paroxysms." And the geologic record suggests it may be one of the largest ever. Consider this: in Nevada at least 230 supervolcanoes ejected an estimated 17,000 cubic miles of lava! Here's another way to think about it: at least 30 of these eruptions each equaled 600 Mt. St. Helens eruptions!

Blue Jay Maintenance Station on left, remnants of cataclysmic destruction behind.

About 90 miles southwest of Ely, I stopped at Palisade Mesa in the southern Pancake Range. Parking is available at a small rest area next to the Blue Jay Maintenance Station. Volcanic rocks of the GIF are nicely exposed on the steep slope to the east.

Rock pancakes stacked oldest to youngest, from bottom to top.

Palisade Mesa is one of multiple gently-tilted stacks of volcanic rock that give the Pancake Range its name. The escarpment at Blue Jay shows at least four episodes of eruption, all from the immense Central Nevada caldera complex. The pale bottom (oldest) layer is a lightly-welded tuff from an ash flow c. 31 million years ago. Next is a thin black band of glassy vitrophyre—"a flow of glowing ash that became densely welded."

Vitrophyre—beautiful memento of incandescent destruction. James St. John.

The massive brown layer above the vitrophyre is a younger tuff, about 30 million years old. Being a fan of columnar jointing, it was my favorite. The summit is a 2.75 million-year-old tuff that's sufficiently welded to provide an erosion-resistant cap.

I 💖 columnar jointing—created by contraction with cooling.

The view south beckoned.

Palisade Mesa obviously deserved a longer visit, perhaps a hike along the base and up the valley to the south. But not this time. Instead I continued west.

Those who cross the middle of Nevada (e.g. east to west) soon become aware of its extensive deformation even if they have no idea what happened. For example: When I left the Pancake Range I crossed Hot Creek Valley, then the Hot Creek Range, then Stone Cabin Valley, then the Monitor Range, and then Ralston Valley before stopping in Tonopah near the crest of the San Antonio Mountains. This is typical Nevada topography—valleys and mountain ranges one after another, all trending roughly north–south. The great pioneering geologist Clarence Dutton called them “an army of caterpillars marching north from Mexico".

Left of center, caterpillars are marching across the Basin and Range Province (NPS).

The cause of this curious pattern is east-west continental stretching, which started something like 18 million years ago and continues today. Some parts of Nevada and adjacent Utah and California have nearly doubled in width! In the process normal faulting has dropped basins, leaving adjacent land standing high, as mountain ranges.

In Tonopah, I stopped for gas and groceries as I often do. Here Hwy 6 merges with heavily-traveled Hwy 95, but at Coaldale Junction they diverge, and once again I had the highway mostly to myself. This is where I stumbled upon Radio Goldfield several years ago, broadcasting very local news and interesting country-ish, old-timey, new-to-me music. It's still going strong.

At the advice of D & B, I kept an eye out for a diatomite quarry on the left, near the junction with NV Hwy 264. The white patches were obvious. This diatomite is thought to be the same age as late eruptions of the GIF, but the setting was entirely different—a shallow freshwater lake where diatoms (microalgae) basked in the sun. Now they're diatomaceous earth, a soft crumbly rock that's 80–90% silica. Among its many uses are metal polish, toothpaste, cat litter, dynamite, thermal insulation, and bonsai soil amendments.

I would have enjoyed examining the diatomaceous earth, but wasn't clear on ownership.

Diatomaceous earth up close; scanning electron micrograph by Dawid Siodłak.

After continuing west across Montgomery Pass, I dropped into Queen Valley for the final geostop of the day, parking in a large pullout not far from California. Across the valley was the north end of the White Mountains; the snowy Sierra Nevada was visible in the far distance. It was a lovely peaceful place, or so it seemed that day. But nearby were clear signs of geologic trauma.

White Mountains rise steeply above floor of Queen Valley.

Normal faulting evidenced by triangular facets (arrows).

Across the valley at the base of the White Mountains is a normal fault just 3 million years old. This is the Queen Valley fault—a tiny piece of the immense Walker Lane. I had entered a profound but vague tectonic boundary, where the Basin and Range Province meets the great Sierra Nevada.

At Walker Lane (yellow), very different tectonic regions meet. SAFZ is San Andreas Fault Zone, a critical part of this story (Carlson et al. 2013).

Walker Lane is young—just 10 million years old at the south end, and only a few million at the north. The combination of Basin and Range extension and transverse movement of the Sierra Nevada has created a complex zone of faults that's poorly understood. Even so, Walker Lane generates a great deal of excitement among geologists. Perhaps a new plate boundary is forming! Maybe California will drift away!

Like the better known San Andreas Fault to the west, Walker Lane is contributing to the slow, incessant, contrary motions of the Pacific and North American tectonic plates, which are pulling a large part of California northward. Currently the San Andreas is responsible for about 80% of this movement but Walker Lane appears to be catching up.

Fauds & Henry (2008) predict that in another 7 to 8 million years or so, the northern part of the San Andreas will join Walker Lane, extending the Gulf of California north by hundreds of miles and turning California into a peninsula along a new plate boundary. 

If this tectonic shifting continues, as the authors think it will, California will become the island that was regularly reported by explorers hundreds of years ago! This was the "famous cartographic error that appeared on many European maps from the 16th to the 18th centuries" (David Rumsey Map Collection).

"Novissima et accuratissima totius Ameriae" by Nicolaes Visscher, 1690. Large island off the west coast of North America is California. DRMC

Peering even further into the future, we may well find that California Island has become an exotic terrane (quit snickering!). As such, it could travel far and wide before being stopped at some convergent plate boundary, thousands of miles from its origin at Walker Lane.

But Emmie ... our ephemeral lives mislead us. The Earth is far from stable.

Sources

agimark 2018. Splitting North America – The Walker Lane; Part 1 – The Tectonics; Volcano Hotspot blog. Accessed Dec 2025.

Carlson, CW, et al. 2013. Kinematics of the west-central Walker Lane ...  Geosphere 9: 1530–1551.

David Rumsey Map Collection, an unbelievably wonderful resource for fans of old maps. WARNING: it's very easy to spend a lot of time here. https://www.davidrumsey.com/

DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press.

Faulds, JE, and Henry, CD. 2008. Tectonic influences on the spatial and temporal evolution of the Walker Lane: An incipient transform fault along the evolving Pacific – North American plate boundary. Nevada Bureau of Mines and Geology, Arizona Geological Society Digest 22. The future of California is discussed on page 463. PDF

Wolterbeek, M. 2020 (Feb 18). How the burgeoning Walker Lane may split the American West; in Nevada Today, UNV Reno.

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)