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.

Mono Lake—the Simple Life

When friends asked where I was going on my trip last September, I learned to say "Mono Lake". It was the only place in central eastern California they knew. I actually intended to visit volcanos, but "Mono Lake" turned out to be an acceptable answer. I stopped there most days to enjoy its peacefulness, strange rock sculptures, and oddly simple ecosystem.

Judging by the responses I got—usually something like "Isn't that where LA gets its water?"—Mono Lake is best known for the destruction wrought by the City of Los Angeles. After 1941, when the northern extension of the Los Angeles Aqueduct was completed, the lake dropped 45 feet, losing half its volume. Thanks to strong advocacy it has risen since 1994, but is still below the management level set by the California State Water Resources Control Board (more here).

To be clear, Los Angeles takes water not from Mono Lake but from the creeks flowing into it. The lake itself is much too salty, as Mark Twain noted in 1872:

"its sluggish waters are so strong with alkali that if you only dip the most hopelessly soiled garment into them once or twice, and wring it out, it will be found as clean as if it had been through the ablest of washerwomen's hands. ... If we threw the water on our heads and gave them a rub or so, the white lather would pile up three inches high."

Rather Soapy (from Roughing It)

Twain and his companions had come to Mono Lake by way of the Great Basin in Utah, Nevada and eastern California—a land of internal drainage:

"Water is always flowing into [the lakes]; none is ever seen to flow out of them, and yet they remain always level full, neither receding nor overflowing. What they do with their surplus is only known to the Creator."

I think Twain knew more than he let on. Evaporation could keep lake level constant in spite of water flowing in. And evaporation would make it "alkali". Maybe he just wanted to add another flourish to his story.

The party camped along one of the creeks flowing into Mono Lake. They rented a boat from a local rancher, and "soon got thoroughly acquainted with the Lake and all its peculiarities." Twain was not impressed. He considered it "one of the strangest freaks of Nature ... a solemn, silent, sailless sea". It was aptly called The Dead Sea of California:

"There are no fish in Mono Lake—no frogs, no snakes, no polliwigs—nothing, in fact, that goes to make life desirable. Millions of wild ducks and sea-gulls swim about the surface, but no living thing exists under the surface, except a white feathery sort of worm, one half an inch long, which looks like a bit of white thread frayed out at the sides. ... Then there is a fly ... you can see there a belt of flies an inch deep and six feet wide, and this belt extends clear around the lake."

Mono Lake's flies still "swarm up so thick that they look dense, like a cloud." (House Photography)

Today Twain is criticized for equating Mono Lake with the Dead Sea. But in spite of his disparaging remarks, he often was correct in his descriptions. Indeed there are just a few kinds of critters in the water, and they occur in abundance. In fact, they occur by the trillions! I found this fascinating—a hostile environment, a simple ecosystem, and yet so productive.

To everything there is a season—Mono Lake in May.

When I was in the area last May, Mono Lake fooled me. From atop Panum Volcano I saw a lush green field east of the lake—alfalfa?? I drove to South Tufa, where the field revealed itself to be vividly green water. I immediately thought "nitrates" ... or even worse, "sewage".

But Google assured me I needn't worry. In May, Mono Lake typically is thick with green algae madly photosynthesizing. This is truly their moment in the sun. They can flourish because the grazers are still asleep.

These algae are minute. For example Dunaliella is just 0.025 mm long. But it occurs in such abundance that it colors the water green ... until it gets eaten.

The color was unreal, even up close.

Dunaliella thrives in hypersaline environments (djpmapleferryman).

Mono Lake in September.

When I returned in September, the water was clear and flies were abundant along the lake shore—just as Twain had described. At the Mono Basin Scenic Area Visitor Center, a worthwhile exhibit explained what had happened while I was away.

As Mono Lake warmed in spring, trillions of dormant life forms on the bottom awoke. From miniscule brown cysts emerged tiny larvae, the first stage leading to adult Mono Lake brine shrimp (Artemia monica, found nowhere else in the world). The shrimps feasted on algae and reproduced at an impressive rate, reaching astronomical numbers.

Mono Lake brine shrimp, about 1 cm long. This is Twain's "feathery sort of worm" (djpmapleferryman).

At the same time, tiny eggs hatched to release larvae of alkali flies (Ephydra hians). After several stages underwater, adults emerged at the surface. They too fed on algae, reproduced, and soon achieved numbers in the trillions. No wonder the water was clear in September!

Alkali flies are fine to hang out with. They don't bite, sting, or otherwise bother humans (photo source).

Like other Mono Lake aquatic life, alkali flies are small (4–7 mm) and super abundant.

At first glance, it seems life is not so easy for alkali flies. Unlike their fully aquatic larvae, and unlike brine shrimp, adult alkali flies breath air. Therefore when they crawl down to the bottom of the lake to feed and lay eggs, they have to bring along their own oxygen! But this isn't a problem. Their dense covering of wax-coated hairs causes a bubble to form when the fly enters the water (its eyes remain exposed) (Young 2017).

Here Mark Twain again demonstrated his perspicacity:

"You can hold [the flies] under water as long as you please—they do not mind it—they are only proud of it. When you let them go, they pop up to the surface as dry as a patent office report, and walk off as unconcernedly as if they had been educated especially with a view to affording instructive entertainment to man in that particular way."

Alkali fly in its bubble (van Bruegel & Dickinson 2017).

Of course to be complete, we must acknowledge the decomposers. Without them the ecosystem would be overwhelmed and cease to function. Throughout the warmer months bacteria on the lake bottom break down and consume the dead ... algae, shrimp, and flies.

Now with bacteria added, we have a  completes list of aquatic inhabitants of Mono Lake—a very short list. Being human, of course we wonder "why?" Mono Lake's inhospitable waters probably drive ecosystem simplicity. Only a few species are adapted to survive. However not only do they survive, they thrive! A single cubic foot of near-shore water in summer contains 50–400 brine shrimp, 5,000–10,000 alkali flies and their larvae, and single-celled algae beyond counting. Is this great abundance due to lack of competition and predation? Sounds like a reasonable hypothesis to me.

After my final stop at Mono Lake I left feeling lucky, as I often do on these trips—lucky to have such wonderful public lands to enjoy, and lucky to be human and able to ponder nature's mysteries.

Sources

Unless otherwise cited, information is from the Mono Lake Committee website and exhibits at the Mono Basin Scenic Area Visitor Center.

Twain, Mark (Clemens, Samuel). 1872. Roughing It. Courtesy University of Virginia English Department.

van Bruegel, F, and Dickinson, MH. 2017. Superhydrophobic diving flies (Ephydra hians) and the hypersaline waters of Mono Lake. PNAS

Young, Emma. 2017. How alkali flies stay dry. Nature, NEWS, 20 November

Volcanic Tableland & Owen's River Gorge

Last May on the way home from the West Coast, we stopped below the steep east face of the Sierra Nevada. From the campsite I looked out across the broad Volcanic Tableland, and saw a winding incision rimmed in rhyolite glowing in the evening light. It was the Owens River Gorge, the next day's destination where we would "experience" an ancient cataclysm. Geology is wonderful that way!

The Volcanic Tableland lies just north of Bishop, California. It's about 350 square miles in extent, and as much as 750 feet thick—a humongous block of rock called the Bishop tuff. It was named in 1938 by a geology grad student from the University of California Berkeley—Charles M. Gilbert. "Since the formation covers an extensive area, has characters unique in the region, and represents a definite part of the Pleistocene epoch, the name Bishop tuff is suggested as a fitting designation, after the chief town in the vicinity."

Gilbert's study area; town of Bishop added, location approximate.

Tuff forms from volcanic ejecta, mainly ash, blown out in explosive eruptions and then lithified (turned to rock) after deposition. Ash may stay airborne and travel great distances before forming fallout deposits. The alternative is much more deadly. The eruption column can collapse, sending hot incandescent ash flows racing across the landscape, searing everything in their path.

Gilbert spent three field seasons studying the Bishop tuff. He examined and sampled it at multiple depths in the Owens River Gorge, and across its extent from Bishop north to a few miles south of Mono Lake (in a tunnel being dug for the Los Angeles Aqueduct). Everywhere he went, he found the tuff to be rhyolitic (a common type, high in silica).

In fact the Bishop tuff was surprisingly uniform in composition, given its extent. However texture varied with depth—an important clue as to origin: "the tuff, soft and porous at the top, becomes gradually less porous and harder toward the base ..." Gilbert also noted that the soft uppermost tuff did not ring with the blow of a hammer, "in marked contrast" with hardened tuff below.

Back in the lab, microscopic examination of tuff samples revealed that ash and other fragments were welded together, increasingly so with depth. Gradation was continuous, and distortion had taken place without fracturing. Gilbert concluded the cause was "vertical com­pression by the weight of the deposit after its emplacement." And judging by the extensive welding, the deposit must have been extremely hot. There was but one possible perpetrator—a nuée ardente.

Nuée ardente or pyroclastic flow, Soufriere Hills, Montserrat; Nov 2010 (USGS).

"The nuées ardentes (burning or glowing clouds) remain, then, as the agent which probably emplaced the Bishop tuff. The nuées are flows of intensely hot, discrete fragments of viscous magma, in which each fragment rapidly and continuously emits its gases. The fragments are thus enveloped and cushioned by extremely dense, hot gas, and the whole has the appearance of a dense, rapidly expanding 'cloud'. Such a cloud rolls rapidly over even a gentle slope ... [leaving] hot pyroclastic material which may mantle the surface over an extensive area and which retains its heat and continues to emit hot gases for long periods of time. Only by some such agent can the em­placement and features of the Bishop tuff be explained." (Gilbert 1938, emphasis mine)

At the time of Gilbert's publication, the idea that welded tuffs were products of nuée ardentes was not widely accepted. In his Introduction, he explained that "the author hopes that the description of one of them given in this paper will help in the recognition of others which may be found and also contribute toward a better understanding of their origin." That hope has been realized! The Bishop tuff is world famous, a geological mecca of sorts.

For the rest of the story Gilbert resorted to informed speculation. Where was the volcano? Was there just one? The Bishop tuff's uniformity suggested a single source, but how could one eruption produce such a massive deposit? He thought it likely that multiple vents in near proximity were involved. "... the writer prefers to believe that the locus of vents lay in the vicinity of Long Valley since that valley has been a center of major volcanic eruptions since Pliocene time."

Today's geologists agree.

The Long Valley Caldera produced the Bishop tuff (orange). Inset shows ashfall deposits as far east as Nebraska and Kansas—1000+ miles. USGS (highlights added).

The Owens River flows from Lake Crowley (reservoir) in the former Long Valley Caldera and crosses the Volcanic Tableland via the Owens River Gorge. Google Earth 2019.

In the nearly 90 years since Gilbert's study, much has been learned about the Volcanic Tableland and Bishop tuff. They formed just 760,000 years ago (geologically young) during a series of eruptions in the area of today's Long Valley, as Gilbert suspected. It must have been spectacular!! In less than a week the vents disgorged 150 cubic miles of tuff—equal to the volume of Mt. Shasta and far more than any eruption in recorded history.

During this massive outpouring the ceiling of the magma chamber collapsed, creating an elliptical depression about 10 x 20 miles in extent—the Long Valley Caldera. Continuing eruption filled much of it, but not all.

The Volcanic Tableland is crossed by the Owens River Gorge—about 18 miles long, 200–800 feet deep, and generally less than 150 feet wide at the bottom. It provides terrific opportunities to examine the Bishop tuff, but also presents a geological puzzle: how did such a deep narrow gorge form?

For some 600,000 years post-eruption, water and sediment running off the Sierra Nevada ponded in the Long Valley Caldera. Then about 100,000 years ago, perhaps during a cool wet glacial interval, the lake breached the caldera rim on the southeast side and today's Owens River was born. What it did from there is unclear.

Lake Crowley in Long Valley; pale deposits upstream are from former Long Valley Lake. (Hildreth & Fierstein 2016, labels added).

Streamflow may have been vigorous enough to downcut rapidly through the relatively soft Bishop tuff (Sharp & Glazner 1993/2002). But meanders in the gorge suggest the Owens River was first a shallow winding stream. Perhaps with subsequent rising and tilting of the land (this area is tectonically active), the river downcut more rapidly but was constrained by established meanders (Hildreth & Fierstein 2016, 2017).

Whatever the scenario, the Owens River has incised a deep narrow gorge that exposes the innards of the Volcanic Tableland. Let's go have a look.

Owens River Gorge looking north; note meanders (Hildreth & Fierstein 2016).

The Gorge is accessible via roads and trails; directions are available online. It's very popular with rock climbers and anglers. But I was blessed with a parking spot close to the locked gate, on the road to the Los Angeles Dept. of Water & Power's Middle Gorge Power Plant.

Ways & Means (mine on right).

Pinkish-gray blobs are pumice fragments.

The rock at the parking area was extremely porous due to remains of tiny bubbles from the explosive eruption, and contained many randomly scattered fragments of pumice. It was surprisingly light, almost airy. With tuff in hand, I read the guidebook's horrific account of this place 760,000 years ago, when a searing racing cloud of molten magma fragments and volcanic gases exploded out of the Long Valley Caldera and came to rest here. Initially it was hot (1500º F!), plastic, and porous throughout. But that changed as the deposit cooled.

As we descended through the pyroclastic flow, the tuff became more dense, and pumice fragments more deformed. I matched rocks with guidebook descriptions while my field assistant waited in bits of shade. And I thought of CM Gilbert in whose footsteps we walked, wishing there were some way to tell him of the many geologists who pass this way every year, awestruck by the Bishop tuff.

Not far from the parking area, the tuff was denser but pumice fragments still obvious. There were irregular holes where fragments had fallen out. There also was a line of perfectly round holes—relics of paleomagnetic studies critical in developing plate tectonics theory in the 1960s.

Further down the road the tuff darkened, was denser (chunks were heavier), holes disappeared, and pumice fragments were blackish and much flattened.

Pink tuff with elongate blackish pumice fragments.

At a really obvious color change, we entered full-on welded tuff. It was uniformly dark gray and no pumice fragments were visible; they had been severely deformed and even remelted under that great burden of extremely hot volcanic debris. I lifted a chunk—it was much denser and heavier than the piece I hefted at the parking area.

My camera's meter struggles with dark rock in bright sun.

Then we turned around and walked back up the road.

Tuff recapitulation going up: dark gray to pink to almost white; welded and dense to porous and light.

Back at the top I looked across the Gorge and contemplated the spectacular columnar jointing, formed when the tuff shrank and cracked with cooling. The upper columns were slender and well-defined. Those below, which formed in slower-cooling densely-welded tuff, were larger and barely columnar, almost amorphous.

Here we have yet another puzzle. Usually columnar jointing is oriented perpendicular to the surface. But as Gilbert noted, "[in the Bishop tuff] columns may have any attitude whatever, from vertical to hori­zontal". He included a photo of a spectacular example.

Note curving columns center right; from Gilbert 1938.

I was thrilled when I realized that the photo in Gilbert's paper was taken very close to where I parked. In fact, it's a popular shot.

The Smithsonian's Global Volcanism website includes the same curved columns in the Bishop tuff.

My photo—a rock flower born of fire and water.

These odd arrangements of columns curving down to a common point have been called joint rosettes or rock tulips. They're attributed to fumaroles (vents through the tuff) that released volcanic gases enhanced with steam, perhaps where the pyroclastic flow overran and vaporized the ancestral Owens River. Indeed there are many fossil fumaroles on the Volcanic Tableland in the vicinity of the Gorge.

Bumps on the Volcanic Tableland are fossil fumaroles, typically 10–20 feet tall.

Atop a fossil fumarole. Tuff cemented with minerals from volcanic gases is more resistant to erosion.

Relaxing after a hot but fascinating day in the field.

Headings in this post use the Herculanum font, named after Herculaneum, a Roman city destroyed by pyroclastic flows from Mount Vesuvius.

Sources

Gilbert, CM. 1938. Welded tuff in eastern California: GSA Bull. 49:1829–1862.

Hay, RL. 1989? Memorial to Charles M. Gilbert, 1910-1988. GSA PDF

Hildreth, W, and Fierstein, J. 2016. Long Valley Caldera Lake and reincision of Owens River Gorge. USGS Sci. Inv. Rep. 2016–5120. https://doi.org/10.3133/sir20165120 

Hildreth, W, and Fierstein, J. 2017. Geologic field-trip guide to Long Valley Caldera, California. USGS Sci. Inv. Rep. 2017–5022–L. https://doi.org/10.3133/sir20175022L

Sharp, RP, and Glazner, AF. 1993 (2002). Geology Underfoot in Southern California. Mountain Press Publ.

USGS, California Volcano Observatory. Long Valley Caldera.