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| Do you see wreaths of vapor? Or the lurid light of molten lava? (D. Mayer photo) |
Last September I toured the Mono Basin in the company of the great pioneering geologist Israel Russell, author of
Quaternary history of Mono Valley, California. He wasn't there in person of course, having left this world more than a century ago. But I had read his report. Russell was an able writer and contagiously enthusiastic about his subject, so his spirit was very much with me.
The Mono Basin lies in far eastern California between the east slope of the Sierra Nevada and the California–Nevada state line. Israel Russell first visited in 1881, in the employ of the US Geological Survey. His stay was brief, being incidental to reconnaissance of Lake Lahontan, the great Ice Age lake. Even so, he became sufficiently acquainted with "the more prominent features of Quaternary history" to know he had to return.
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| Pleistocene–early Holocene lakes of the Great Basin (Russell 1889). Red box marks glacial Lake Mono. It was later renamed Lake Russell (1). |
In the fall of 1882, after finishing his Lake Lahontan project, Russell traveled to the Mono Basin for further study. But he was too late; "the storms of winter compelled a postponement of the undertaking." The following summer he returned with topographer Willard Johnson, who completed a survey of the Basin, and JB Bernadou who made field sketches and assisted in various ways.
Quaternary history of Mono Valley was published seven years later, in the 8th annual report of the USGS (2).
In his report Russell invites the reader to travel with him "in fancy" from the mining town of Bodie down into the Basin, along the shores of "intensely alkaline" Mono Lake, and then up to the crest of the Sierra Nevada and the summit of Mt. Dana. I joined him just south of Mono Lake, at the north end of the Mono Craters.
"between the observer [in the Basin] and the steep face of the Sierra there is a range of volcanic cones that attract the eye ... These are the Mono Craters. So perfect are their shapes and so fresh is their appearance that the eye lingers about their summits in half expectation of seeing wreaths of vapor or the lurid light of molten lava ascending from their throats." (All quoted text is from Russell 1889 unless otherwise cited.)
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| Mono Craters, looking south. Panum volcano at north end, Sierra Nevada in distance. USGS 1971. |
Russell knew the Craters were volcanic, but some of their features were strange. "The Mono Craters are composed entirely of ejected matter. Lapilli (a general name for small rock fragments thrown out by volcanoes) form the most conspicuous portion of the cones. There are also several coulées of volcanic rock which flowed out in a molten condition and consolidated on cooling." It was the coulées that puzzled him (3).
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| Obsidian Coulée rises steeply behind Russell's mule—"the most practicable method of carrying forward work". |
The Mono coulées were clearly lava flows, but they were very short, quite thick, and made of rhyolite. "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."
In fact these coulées didn't flow very far at all, rarely beyond the foot of their cones. And they were 200–300 feet thick, with steep sides and fronts. Russell rightly concluded that the lava had been quite viscous.
"One of the most striking features illustrated by the lava streams of the Mono Craters is that the molten rock came forth in a viscid or semi-fluid condition and cooled rapidly. ... The extruded lava was apparently sufficiently heated to be pasty or semi-fluid, but the temperature was not raised high enough to produce what is usually termed fluidity and thus permit rapid flow."
The Mono Craters also include plugs and domes made of rhyolite. Russell considered them "incipient coulées which were congealed before a definite flow in any direction had been established." He was right about this too.
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| Looking east at steep fronts of several Mono coulées (Russell 1889). |
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| Same coulées from Nature Trail stop on US 395. High point on left is Crater Mountain, a rhyolite dome. |
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| North Coulée from south shore of Mono Lake; left end was the front of the flow. |
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| North Coulee at a worthy geostop on CA 120. "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." |
Today we're taught that magma viscosity is a major factor in its behavior and the resulting landforms. We also learn that viscosity varies with silica content (e.g., Fisher et al. 1997). For example magma containing < 55% silica flows easily, making the familiar "basic" lava flows Russell mentioned. At the other end of the spectrum, magma with > 70% silica flows with great difficulty if at all, and is too viscous even for gases to escape. Instead pressure builds until the magma explodes. Hot incandescent ash races across the landscape searing everything in its path before finally stopping and cooling to form massive beds of welded rhyolite.
As Russell noted, the Mono coulées are rhyolite, high in silica. So why did silici magma ooze out instead of exploding, as we're told it does? Russell didn't have to struggle with this question because so little was known about volcanoes then. It was something for future geologists to puzzle over, and Russell was optimistic they would.
"The range is unlike any other known to the writer, and, so far as can be judged from the reports of explorers, is the only one of its kind in the United States. 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".
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| Mono domes and coulées "formed of acidic lavas" (Marcaida et al. 2019), |
If Russell's spirit indeed visits the Mono Basin, it must be a happy one. From the time
Quaternary history of Mono Valley was published, "Geologists have been gripped by Mono dome fever ... the chain well deserves the attention succeeding generations of geologists have lavished on it." (Sharp & Glazner 1997).
Those feverish geologists have shown unequivocally that the Mono volcanoes are young. The first may have erupted as early as 90,000 years ago, but most are less than 10,000 years old. In that short time at least 28 rhyolite domes and coulées have emerged (Marcaida et al. 2019). Those in the middle of the chain are oldest. The youngest (and my favorite) is Panum at the north end, a mere babe just 700 years old.
But the question remains: Why did silica-rich magma ooze out of the Mono Craters instead of exploding? A common suggestion is that Mono volcanoes erupted in two stages. In the first, small fragments (lapilli) are ejected and deposited to form a crater-like cone. Some magma remains in the chamber, specifically magma depleted of explosive gases and still hot enough to flow. In the second stage, "a mass of thick, pasty [rhyolitic] glass oozes up within the crater to form a dome ... The dome may grow so large that it fills its crater and occasionally breaches the ring of explosion debris to flow away as a stream of molten glass, a coulée." (Sharp and Glazner 1997). Wouldn't that be great to see!
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| Tiny dome inside Panum crater, perhaps an "incipient coulee". Is it still flowing? |
As volcano guidebook authors must, Russell addressed the unavoidable question: Are the Mono volcanoes extinct or only sleeping?
He explained that some are quite young, having erupted after geologically-recent events. "Their last eruption took place after the glaciers had retreated up the cañons of the Sierra. They also are, in part, more recent than the ancient beaches to be seen about the border of the valley, which record former high water stages of [glacial] Lake Mono."
The Mono Craters may have erupted recently geologically-speaking, but for most of us, ephemeral creatures that we are, they're relics of the past. Russell ended his report by tackling this misconception.
"The [Pleistocene–early Holocene], as compared with the present, appears to have been a time of greatly expanded water surface, increased glacial action, and more energetic volcanic activity. In making such a statement, however, it is evident that we are comparing the events of a day with a whole volume of history. Could we look into the future with as much accuracy as we are able to review the past, it would be evident that changes are now in progress that in time will equal the apparent revolutions [of the past]." (emphasis mine)
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| Israel Russell (source). "his physique gave to the eye little suggestion of that capacity for sustained effort and endurance without which his more strenuous exploration would have been impossible." (Gilbert 1906) |
Notes(1) There also is a Glacial Lake Russell in Washington, also named for Israel Russell.
(2) Many readers have been captivated by Russell's writing. Early residents of the Mono Basin liked it so much that they paid for a separate printing of his report, to use to attract tourists (Gaines 1984). It was reprinted in 1984 by Artemisia Press, but again sold out. I found a used copy online.
(3) "Coulée" is a Canadian French word derived from the French "couler"— to flow. Beyond that, its meaning varies widely. Coulées can be lava flows, like those of the Mono Basin. In eastern South Dakota coulées are draws, usually narrow with steep sides, where water flows or has flowed. In Washington (state) coulées are huge channels scoured into basalt bedrock by Ice Age floods.
Sources
Fisher, RV, Heiken, G, Hulen, JB. 1997. Volcanoes: crucibles of change. Princeton U Press.
Gaines, D. 1984. Mono Lake's Poet-Geologist. Preface in 1984 reprint of Russell's 1889 report (see below).
Gilbert, GK. 1906. Israel Cook Russell. J. of Geology 14:663-667.
Marcaida, M., et al. 2019. Constraining the early eruptive history of the Mono Craters rhyolites ... Geochemistry, Geophysics, Geosystems, 20, 1539–1556. https://doi.org/10.1029/2018GC008052
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 (now out of print).
Sharp, RP, and Glazner, AF. 1997 (4th printing 2003). Geology Underfoot in Death Valley and Owens Valley. Mountain Press.
A revised second edition of this guide was published in 2022 (Glazner, Sylvester, & Sharp). Based on the areas I've visited, it seems more science light and the tours more cursory. But of course maps and illustrations are far better. Perhaps buy both.
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).
