Monday, August 13, 2018

Boxelder—the Odd Maple


The boxelder I’m following this year, the one that grows in a nook formed by warehouse walls, has changed only subtly since last month’s report. It has more leaves—not that I’ve counted them or measured their cover, but I can see new ones emerging, even though the first signs of summer’s end have appeared. Must be an optimistic tree.
The sand dock along the warehouse wall is done storing energy underground, and is now a brown tangle of dead leaves (Canada thistle sticking up through).
Boxelder’s oppositeness—stems as well as leaves.
Fresh new leaves … in September!
The miniature leaves emerging at the tips of the branchlets had interesting forms, surprising actually. But in the morning shade it was impossible to capture them in focus, so I took a branchlet home.
Interesting! These two young leaves look deeply incised and pinnately-lobed (keep this in mind for the trick question coming up soon).

Q: "How many leaves in the next photo?"
A: "Just one." This is a little trick we ranger naturalists would use on nature walks. Most people would answer five. Then we explained how actually this is a single leaf, i.e., from one bud, and that it split into leaflets as it grew. At least one former ranger I know took it a step further, overlapping the leaflets to show that the boxelder is indeed a maple, a member of the genus Acer.

Even so, doesn’t it seem a bit odd that boxelder ended up in the genus Acer—that a tree with compound leaves was thrown into a group of simple-leaved brethren (1), especially given the proclivity of taxonomists for splitting? If you agree that this is odd, you’re not alone. In fact, you’re in the company of some of the most esteemed pioneering North American botanists.

The boxelder is native to North America, but by the mid 1700s, it had become common in Europe as well:
“ … it is in great request for adorning pleasure grounds, on account of the rapidity of its growth, and the beauty of its foliage, whose bright green forms an agreeable contrast with the surrounding trees.” (Michaux & Hillhorn 1819)
So it's not surprising that Carl Linneaus included the boxelder in the first edition of his Species Plantarum (1753), in which he formally introduced our binomial system of nomenclature. He assigned the scientific name in use today: Acer negundo (the complete name has “L.” appended to it because Linneaus is the authority). Why did he place boxelder in the genus Acer? Probably because it has the same distinctive fruit as maples: two-winged two-seeded samaras that spin in the wind (aka wingnut, helicopter, whirlybird, whirligig and spinning jenny).
Acer negundo L. (Michaux & Hillhouse 1819); samaras (keys) lower left.
When Andre Michaux published the first North American sylva in 1810 (2), boxelder was Acer negundo, following Linneaus’s lead. And so it remained for some thirty years, during which time Thomas Nuttall published periodic updates, adding new species that he and others discovered.

But by the time of Nuttall’s 1849 edition, boxelder had moved up in status (see p 90). It was in its own genus, Negundo, the boxelders, which included two species and a rumored third. The common boxelder became Negundo aceroides. N. californicum was the California boxelder, discovered by David Douglas, a Scottish botanist and major contributor to the North American flora. “It is remarkable for the almost tomentose pubescence of its leaves” (densely fine hairy), most of which had just three leaflets. Even the samaras were pubescent. Nuttall also mentioned a possible third species, christened N. mexicanum by Swiss botanist Augustin Pyramus de Candolle (3), but declined to recognize it until more material was available for study.
California boxelder, Negundo californicum (Nuttall et al. 1865).
I had hoped to find out why the boxelders were segregated into their own genus. But I wasn't able to solve that mystery. The Plant List considers Negundo aceroides an unresolved name, attributable to either John Torrey or Conrad Moench. Some sources hold Nuttall accountable as well. In any case, Torrey and Asa Gray, two of our great botanical luminaries, included it and N. californicum in their North American flora, published 1838-1840 (pp 49-50).

But Negundo turned out to be a passing fancy. At some point, boxelders were lumped back in with the maples. However, the California and Mexican boxelders were not abandoned—they’re now varieties. In fact, seven varieties of Acer negundo are recognized; the distributions of all but var. mexicanum are shown here (source):

My boxelder is Acer negundo var. violaceum, because its branchlets are glabrous (no hair) with a glaucous bloom—hence the bluish-green stem in the photo below.

Notes

(1) Boxelder is not the only odd maple. The great majority have simple lobed leaves, but there are several with either compound or simple unlobed leaves (details here).

(2) A sylva is all the trees growing in a specific region; also “silva”.

(3) Negundo mexicanum may have been based on material collected by botanist Jean-Louis Berlandier, a student of DeCandolle, who joined a Mexican scientific expedition in the late 1820s at the recommendation of his mentor (source).

Sources

Biodiversity Heritage Library Flickr Album for The North American sylva v.2 (1865).

Constantino, G. 2018 (March 29). Exploring the First American Silva. Biodiversity Heritage Library Blog.

Michaux, FA, and Hillhouse, AL. 1819. The North American sylva, or A description of the forest trees of the United States, Canada and Nova Scotia; considered particularly with respect to their use in the Arts , and their introduction into Commerce [English translation]. Paris, printed by C. D’Hautel. Available online at Biodiversity Heritage Library.

Nuttall, T, Michaux, FA, and Smith, JJ. 1865. The North American sylva; or, A description of the forest trees of the United States, Canada and Nova Scotia. Considered particularly with respect to their use in the arts and their introduction into commerce, v. 2. Philadelphia, Rice, Rutter & co. Available online at Biodiversity Heritage Library.

For more news about trees from various parts of the world, visit the monthly tree-follower gathering kindly hosted by The Squirrelbasket. New members always welcome!


Monday, August 6, 2018

From Art Nouveau to Wildflowers

Margaret Armstrong often used plant motifs, in this case the Coast Range Mariposa Lily (1); source.

By 1913, American graphic artist Margaret Armstrong had all but abandoned her successful career in cover design. It's true that Art Nouveau, whose flowing undulating lines she found so inspiring, was falling out of fashion, and that cheaper printed paper dust jackets were replacing decorative covers. But these trends were largely coincidental. Armstrong gave up her career for a very different reason. She was writing a book about her passion—the wildflowers of the American West.
Armstrong family c. 1910s; Margaret lower right (source).
Margaret Neilson Armstrong was born in New York in 1867 to a wealthy and artistic family. Her father, a diplomat, studied oil painting while in Italy, and also worked in stained glass, as did her sister Helen. Margaret started her career as a graphic designer in the 1880s, and by 1890 was specializing in book covers and bindings. She produced covers in her popular distinctive style for at least 270 books, including works by well-known authors such as Washington Irving, Henry Thoreau, Henry Van Dyke and John Greenleaf Whittier, as well as the popular botany books of Francis Theodora Parsons (aka Mrs. William Starr Dana).
Armstrong marked her covers with the monogram “MA” (upper right corner). Winding banner reads: "New leaf, new life, the days of frost are o’er" (source).
By 1908, Armstrong’s output began to decrease noticeably as she devoted more and more time to her passion for plants. This wasn't terribly surprising to those who knew her, for she had been an enthusiastic naturalist since childhood (2). Things got exciting in 1911, when Armstrong (then in her mid-forties) and three gal friends toured the American West, seeking adventure as well as plants. They rode horses down into the Grand Canyon, and crossed the Victoria Glacier above Lake Louise to climb to the Mitre Col (see her 1912 article). While they weren’t the first women to accomplish these things (3), they were operating far outside the norm, and enjoyed themselves immensely for it.
“We found the rope very much in our way, but we saw the necessity of it when we came to a crevasse: a wonderful, shining, blue-green crack, its walls fringed with icicles, cleft in the bosom of the glacier. It was a long, dizzy step across …”

Armstrong continued to travel the western United States—studying, sketching and collecting wildflowers. None of the existing field guides adequately covered the area west of the Rocky Mountains (4), and she was eager to produce one herself. But as she noted, “the field is vast, including within its limits all sorts of climate and soil, producing thousands of flowers, infinite in variety and wonderful in beauty, their environment often as different as that of Heine's Pine and Palm” (quotes here and below are from Armstrong's Field Book). Out of this multitude, she chose some 550 of the more common species for her book—from Washington, Oregon, Idaho, California, Nevada, Utah and Arizona.
Field Book of Western Wild Flowers was published in 1915 (available online here). At first glance it might appear academic. It's organized by family, and includes a technical family key, genus as well as species descriptions, and a glossary of technical terms. Both common and scientific (Latin) names are used, the latter following accepted nomenclature of the time (5). Species descriptions address all relevant characters.

Yet Armstrong intended the guide to be “mainly for the general public.” Toward that end, it was “fully illustrated” with 500 line drawings and 48 color paintings, all done by Armstrong from live specimens. And she didn’t restrict herself to dry technical descriptions. She added vignettes about the beauty, oddness, enchantment, and even danger that one might encounter in meeting these wildflowers.

Let's look at some of her chosen subjects (images from BHL Western Wild Flowers Flickr album unless noted):

White Evening Primrose, Pachylophus marginatus, is today’s Oenothera cespitosa. “There is a patch of these wonderful flowers in the Grand Canyon on Bright Angel trail, halfway between the rim and the plateau, where in a shaded spot beside a great rock the pure blossoms seem to shed a moonlight radiance.”

Yellow Avalanche Lily, Erythronium grandiflorum, was also called Dogtooth Violet in Armstrong’s day. “A patch of these flowers bordering the edge of a glacier, as if planted in a garden-bed, is a sight never to be forgotten. Pushing their bright leaves right through the snow they gayly swing their golden censers in the face of winter and seem the very incarnation of spring.”
Erythronium grandiflorum pushing "right through the snow" (source).

Armstrong's Bi-colored or Parti-colored Lupine, Lupinus stiversii, is now called Harlequin Lupine. “One of the prettiest and most conspicuous kinds [of lupines], for its coloring is unusual … fragrant flowers, over half an inch long, with rose-colored wings and a yellow standard, changing to orange in fading. The combination of pink, orange, and yellow is very striking.”
Lupinus stiversii (source).

Pride of California, Lathyrus splendens, “has such glorious flowers, so superb in color and form, that it is by far the handsomest of its kind and not to be mistaken for any other.” (Metropolitan Museum of Art)

Snow Plant, Sarcodes sanguinea, is “a strange plant, widely celebrated for its peculiar beauty … The plants are shaded with red all over, from flesh color, to rose, carmine, and blood-red, and are translucent in texture, so that when a shaft of sunlight strikes them they glow with wonderful brilliance, almost as if lighted from within. …They are pointed out to tourists by Yosemite stage drivers, but the government forbids their being picked, for fear of extermination.”

Indian Pipe, Monotropa uniflora, is “all translucent white, beautiful but unnatural, glimmering in the dark heart of the forest like a pallid ghost, mournfully changing to gray and black as it fades” (source).
Monotropa uniflora (source).

Armstrong was quite impressed by this cholla, Opuntia fulgida—now called jumping or chain fruit cholla, Cylindropuntia fulgida. “A horrible shrub, or dwarf tree … The distant effect of this plant is a pale, fuzzy mass, attractive in color, giving no hint of its treacherous character more like a wild beast than a plant! The joints suggest a very ferocious chestnut-burr and break off at a touch, thrusting their spines deeply into the flesh of the unwary passer-by, so that the Indian story, that this plant flings its darts at wayfarers from a distance, might almost as well be true, and the barbs making the extraction difficult and painful.” (source).
Cylindropuntia fulgida (source).

I learned of Margaret Armstrong and her wildflower book only recently, by way of Flickr where the color plates have been uploaded by Biodiversity Heritage Library. They’re available for download, are copyright-free, and have been tagged (partly by yours truly) so that they will come up in searches (scientific name, common name, artist name and more). If you’re looking for a fascinating citizen science opportunity, checkout BHL on Flickr for more information.

Notes

(1) The Coast Range Mariposa Lily, Calochortus vestae, was C. luteus var. oculatus in Armstrong’s day. She considered mariposa lilies to be the most characteristic flowers of the West. “They grow freely all through the West, as far north as British America, and down into Mexico, but they never get east of Nebraska, so these gay and graceful flowers may be considered the peculiar property of the West.” C. vestae was “one of the most beautiful of all the Mariposas.”

(2) Armstrong had no formal training in botany.

(3) Armstrong and her companions are sometimes referred to as the first white women to reach the bottom of the Grand Canyon. But from her account of the trip, it’s clear that others had gone before. She included a story told by their guide of a female tourist who refused to ride back up to the rim, and had to be hauled out on a litter.

(4) In her Preface, Armstrong states that hers “is the only fully illustrated book of western flowers, except Miss Parsons's charming book, which is for California only” (Wild Flowers of California by Mary Elizabeth Parsons, 1900). She doesn’t mention Rocky Mountain Flowers by Frederic and Edith Clements, published in 1914—possibly too late to be noted in her book. In any case, Armstrong didn't include species found only in the Rockies.

(5) Armstrong recruited a small army of botanical experts for help with identification and nomenclature. In her acknowledgements, she noted that “Professor J. J. Thornber, of the University of Arizona, is responsible for the botanical accuracy of the text and his knowledge and patient skill have made the book possible.” She thanked eight experts on the flora of the western United States for “most valuable assistance in the determination of a very large number of specimens …” and eight others for advice and assistance (including luminaries such as Alice Eastwood, WL Jepson, Marcus Jones and NL Britton).

Sources

Armstrong, Margaret. 1912. “Canyon and Glacier” in The Overland Monthly ser. 2 v. 59. Available here courtesy HathiTrust.

Armstrong, Margaret. 1915. Field book of western wild flowers. London,C. [sic] P. Putnam's Sons. Available here courtesy Biodiversity Heritage Library.


Friday, July 27, 2018

Sunstones at Sunstone Knoll

Sunstone Knoll in western Utah’s Black Rock Desert (1); Bryant Olsen CC BY-NC 2.0
From the highway, Sunstone Knoll looked unimpressive—hardly worth stopping for. But I did stop. And after I explored it, imagined its fiery eruptions, and experienced its sparkles firsthand (including ten delightful minutes in the company of a small boy), I was impressed enough to take home a handful of mementos to fix the visit firmly in my memory.
Sunstone Knoll is one of many volcanic features in the Black Rock Desert near Fillmore, Utah (see recent post). Less than 100,000 years ago, it was an active cinder cone. Then it disappeared under the waters of Lake Bonneville, a huge ice age lake that covered much of northwest Utah. Erosion took its toll. Now just remnants of the cinder cone remain, and its lava flows are buried under Lake Bonneville sediments.
Remains of a compound cinder cone.
It’s neat to stand on Sunstone Knoll and ponder the dramatic ways in which the Earth changes—once a fiery cinder cone, then a lake for as far as the eye could see, and now a high desert. But history isn't the main attraction here. This is a rockhounding site. The basalt contains clear to pale yellow crystalline xenocrysts (inclusions) called sunstones.

While a true rockhound might feel the urge to put a rock hammer to basalt to find sunstones, it’s hardly necessary. Weathering and erosion have covered the ground around the knoll in gravel-sized rock fragments, and it doesn’t take long to spot sunstones flashing in the sunlight. Most of the larger ones (to 5 cm) have been carried off, but small ones are easy to find. In fact, they're unbelievably abundant. Many rockhounds have stopped here—the site is well-known and just off the highway. Even so, I quickly found a handful of sunstones.
Dime ~1.5 cm in diameter.

Sunstone Knoll is said to be a rockhounding site but properly speaking, sunstone is not a rock. It's a mineral—labradorite. Therefore this is my first mineralogical post. And like so many new things geological, it turned out to be more complicated than I expected. On the positive side, I learned quite a bit.

First a refresher:
“A mineral is a naturally occurring homogeneous solid with a definite (but not generally fixed) chemical composition and a highly ordered atomic arrangement, usually formed by an inorganic process (2)” Nelson 2013/2017
In other words, a mineral is a solid that’s not manmade, and is composed of a single kind of chemical compound, i.e., can be described with a chemical formula. Also, the atoms are orderly enough to form crystals. In contrast, a rock is an aggregate of various minerals (sometimes a single kind) or of rocks fragments or of shells (more here). As the USGS explains:
“A good way to think about it is if a chocolate chip cookie was a rock, then the flour, sugar, butter, chocolate chips are the minerals that make up that rock!”
Though probably not of general interest, I would be remiss if I didn’t provide the chemical formula for labradorite (source). It illustrates the “definite (but not generally fixed) chemical composition” of minerals described by Nelson, which means there are variable amounts of elements with atoms of similar size and charge:
(Na,Ca)(Al,Si)4O8 with Na (30-50%) and Ca (70-50%)
Mineral names usually end in “-ite”. They’re named after people most commonly (45%), followed by location of discovery (23%), chemical composition (14%) and various other things (source). As you probably guessed, labradorite was discovered in Labrador, Canada—specifically on the Isle of Paul in 1770 by a Moravian missionary (source).

When I searched Google for labradorite images, I found only a few that looked like the sunstones of Sunstone Knoll. Most labradorite specimens posted online exhibit labradorescence—beautiful flashy colors given off when light enters the specimen and is reflected from internal structure (rather than off the surface). But even though this phenomenon is named after labradorite, many labradorite specimens do not labradoresce—including Sunstone Knoll sunstones.
Labradorescing labradorite, from Madagascar; Géry Parent CC BY-ND 2.0
Sun shines through labradorite from Sunstone Knoll.
Labradorite usually is associated with mafic igneous rocks, commonly basalt and gabbro. At Sunstone Knoll, it occurs as inclusions in basalt. This basalt and that of other volcanoes nearby is tholeiitic in composition, unlike most volcanic fields in the Basin and Range Province. Johnsen et al. (2010) point out that tholeiitic magmatism is common in continental rift zones (as well as oceanic ridges), and suggest a rift might be developing here! Or the unusual composition may be due to the field's location between the extending Basin and Range Province and rotating Colorado Plateau (see previous post).
Labradorite xenocryst in basalt.

When I arrived at Sunstone Knoll, I first investigated one of the rocky crests, and found a sunstone in a chunk of basalt (above). Then I strolled around the base, where the ground was littered with small rock fragments washed down from outcrops above. Here my search was much more productive, especially after I discovered that if I walked at the proper angle to the sun, small flashes of light revealed the sunstones.
Sunstones seemed to be especially abundant in an abandoned anthill—collected preferentially? Arrows below mark a fraction of sunstones in the frame.
It was a lonely search. But then a minivan arrived and unloaded three generations of rockhounds. One of them—a boy maybe ten years old—ran toward me shouting: “Have you found any sunstones?! Where are they?! Show me!!” I explained my method and he went to work. After about ten minutes of collecting, he yelled to a man on the hill, “Grandpa, grandpa, they’re down here!!” But Grandpa knew Sunstone Knoll, and had a few tricks of his own. He returned from the crest with several very nice specimens in chunks of basalt.
View from Sunstone Knoll—all deep under water just 15,000 years ago!

Notes

(1) This is not the better-known Burning-Man Black Rock Desert of northwest Nevada.

(2) Traditionally minerals have been limited to compounds formed through inorganic processes, but mineralogists are reconsidering this rule: “… this eliminates a large number of minerals that are formed by living organisms, in particular many of the carbonate and phosphate minerals that make up the shells and bones of living organisms. Thus, a better definition appends "usually" to the formed by inorganic processes. The best definition, however, should probably make no restrictions on how the mineral forms.” (Nelson 2013)

Sources

Johnsen, RL, et al. 2010. Subalkaline volcanism in the Black Rock Desert and Markagunt Plateau volcanic fields, in Carney, SM, et al., eds., Geology of south-central Utah. Utah Geological Association Publication 39.

Millard County Travel. Day Trips in Millard County Utah—great guide to geo-sites and other things; available online (PDF, 17.4 MB) and at museums and agencies in the county.

Nelson, SA. 2013, updated 2017. Introduction and Symmetry Operations, Definition of a Mineral. Tulane University EENS 2110 Mineralogy (online lecture notes).


Friday, July 20, 2018

Volcanoes in Utah? How can that be?!

Black Rock Desert volcanic field in western Utah.
Volcanic eruptions have been big news lately, with graphic accounts from Hawaii, Bali and Guatemala, and fearsome stories of much greater destruction not so long ago. Yet most of us consider volcanism no threat to us personally. And we’re right. In the greater scheme of things, volcanoes are rare.
A volcano won’t erupt in your cornfield unless you farm in just the right place (Paricutín 1943).
Volcanoes are born when magma forces its way to the surface and becomes red-hot oozing lava, or a fiery fountain of ash and fractured rock, or a racing incandescent cloud that hugs the ground and incinerates everything in its path. But magmas don’t form just anywhere. They—and therefore volcanoes—require special circumstances.
Magma was once thought to flow from Hell, e.g. via Iceland’s Mount Hekla, the Gateway to Hell (source).
Magma is liquid rock, specifically silicate rocks (rich in SiO2). Therefore none of the layers of the Earth qualifies as a direct source of magma. Crust and mantle have the proper composition but are solid. The outer core is liquid, but the composition is wrong—iron and nickel. Therefore magmas must be melted mantle and crust (1). But what is the source of heat for melting? It’s here that debates rage.
Earth’s structure can be described by physical properties or by chemical composition; see (2). For this post, only “mantle” and “crust” are used, as is common in less technical discussions. Modified from Nelson 2015.
Global distribution of volcanoes coincides with certain types of plate boundaries (USGS via wikimedia).
A world map of active volcanoes reveals a suggestive pattern. Most line up along plate boundaries—where the shifting plates that make up the Earth’s surface collide, override, jostle and split in the dance of plate tectonics. This is an appealing pattern because several types of plate interactions could facilitate melting to form magma. Decompression melting probably occurs at divergent boundaries such as mid-oceanic ridges and rift valleys. At convergent boundaries (subduction zones), addition of water could lead to flux melting (Nelson 2015; OSU Volcano World).

But not all volcanoes conform to the pattern. Some erupt far from any plate boundary, and these intraplate volcanoes are difficult to explain.
Above and below, Ice Springs basalt flow in the Black Rock Desert, the most recent volcanic eruption in Utah. But why here?
Looks fresh enough to have erupted last year!
The Black Rock Desert volcanic field (3) is located about 120 miles south of Salt Lake City, Utah, well into the interior of the North American plate. It covers almost 2700 sq mi (7000 sq km), and includes shield volcanoes, cinder cones, lava domes, lava flows, maars and possibly a caldera. All are Quaternary in age, having erupted in the last 6 million years—most in the last 2.7 million. In other words, this field is young—and may still be active.

The field sits at the eastern edge of the Basin and Range Province, just west of the Colorado Plateau. Herein may lie an explanation for the intraplate volcanism. Though far from a plate boundary, this part of North America is hardly stable. Big changes are underway.
Quaternary volcanic fields are common in the Basin and Range Province and on the margins of the Colorado Plateau; modified from Valentine et al. 2017.
West of the Black Rock Desert, the Basin and Range Province has been expanding east-west for the last 30 million years, increasing the distance to the West Coast by 250 miles. Crustal extension may explain, at least in part, the Province’s thin crust; the mantle (asthenosphere) is only about 17-18 miles below the surface. And crustal thinning may explain (partly) the many Quaternary volcanic fields (map above).

Just east of the Black Rock Desert is the Colorado Plateau, which is very different from the Basin and Range Province. It’s a big chunk of crust that is thick (25-30 miles) and relatively undeformed. Volcanism is mostly restricted to the margins. Yet the Plateau is changing too. Precise GPS measurements show it's slowly rotating clockwise (4).

Most intriguing is the boundary between the extending Basin and Range Province and the rotating Colorado Plateau (5). It just happens to be lined with Quaternary volcanic fields, including the Black Rock Desert! Surely there’s a story here!!
Note volcanism along Colorado Plateau margins (Spence & Gross 1990).
In fact, there are multiple stories, all based to some degree on speculation. The earliest invoked a rising plume of hot mantle material over which the continent drifted, producing a line of progressively younger volcanism. While a “hotspot” model works well in Hawaii, it would be awkward to apply to the Colorado Plateau, requiring multiple hotspots and/or varied direction of movement.

But there may be no need to invoke mantle plumes; plate dynamics may be enough (6). Rotation and/or extension could be fracturing the crust sufficiently to create conduits for magma, especially where the crust is thinner (DeCorten 2003; see also Recent Volcanic Activity in Northern Arizona which includes discussion of western Utah). Or perhaps magma is rising through fractures in reactivated ancient crustal sutures (see my Jemez Lineament post). Ballmer et al. (2015) suggest that the difference in thickness between Basin and Range crust and that of the Colorado Plateau may affect mantle flow and cause decompression melting.
Looking north from Tabernacle Hill across basalt flows toward Pavant Butte, a large tuff cone.

Whatever the cause of magma generation, it seems likely that Black Rock Desert volcanism will continue. Little if anything has changed since the last eruptions, just 700 years ago. Basin and Range crust is still stretching, the Colorado Plateau is still rotating, and local mantle flow likely hasn’t changed much.
“There is really no more reason to believe that the epoch of basalt has closed in this region, than that it has barely begun; and it is certainly probable that the few centuries we can know by history and tradition, belong to one of the intervals of quiet, such as separate the more or less convulsive efforts of volcanoes; an interval to be terminated sooner or later by a renewal of activity.” Grove Karl Gilbert, 1875
Pavant Butte, from GK Gilbert’s 1890 Lake Bonneville monograph (USGS).
“Lava from valley of lower Sevier Utah” (Black Rock Desert; Gilbert 1875).

Utah’s Black Rock Desert lies just west of Interstate Highway 15 near Fillmore. Geo-tripping is convenient and fun. Most sites are on public land, and accessible by gravel and passable dirt roads. A wealth of geo-info makes the area especially interesting, ranging from the excellent Millard Country travel brochure to the scientific literature. In May I spent a week there, which wasn't nearly enough. More posts from the trip will be up soon.
"Volcanic District near Fillmore, Utah" (Gilbert 1890).

Notes

(1) Properly speaking, magma forms from partial melts—of mantle rock most often and occasionally crust.

(2) Two systems are used to describe Earth structure: physical properties and chemical composition. The resulting units are not equivalent. For example, based on physical properties, the outermost layer—the lithosphere—is solid brittle rock. But the lithosphere includes two different rock types based on chemistry—crust and uppermost mantle. Another example: the mantle is thought to be uniform chemically, but has three zones based on physical properties: the solid brittle part in the lithosphere, a solid but ductile asthenosphere directly below, and the solid mesosphere (source).

(3) This is not the Burning-Man Black Rock Desert of northwest Nevada.

(4) Estimated rotation rate for the main body of the Colorado Plateau is 0.103 ± 0.017° per Ma. Rates at the margins appear to be affected by Basin and Range extension (Kreemer et al. 2010).

(5) The western margin of the Colorado Plateau is sometimes called the Basin & Range Colorado Plateau Transition Zone (map courtesy Utah Geologic Survey).
(6) For more about the raging plume-ist vs. plate-ist debate, see Jack Share’s The Geologic Evolution of Iceland—specifically “A NEW GEOLOGICAL PARADIGM” about 2/3 of the way through the post.


Sources

Ballmer, MD, et al. 2015. Intraplate volcanism at the edges of the Colorado Plateau sustained by a combination of triggered edge-driven convection and shear-driven upwelling. Geochem. Geophys. Geosyst., 16: 366–379. doi:10.1002/ 2014GC005641.

DeCourten, FL. 2003.  The Broken Land: adventures in Great Basin geology.  Salt Lake City, Utah: University of Utah Press.

Gilbert, GK. 1875. Report upon the geology of portions of Nevada, Utah, California, and Arizona, examined in the years 1871 and 1872, in Wheeler, GM, Report upon United States geographical surveys west of the one hundredth meridian, v. 3, Washington DC:GPO. https://www.biodiversitylibrary.org/bibliography/49058

Gilbert, GK. 1890. Lake Bonneville. USGS Monograph 1. Washington DC:GPO. https://pubs.er.usgs.gov/publication/m1

Johnsen, RL, et al. 2010. Subalkaline volcanism in the Black Rock Desert and Markagunt Plateau volcanic fields, in Carney, SM, et al., eds., Geology of south-central Utah. Utah Geological Association Publication 39.

Kreemer, C, et al. 2010. Present‐day motion and deformation of the Colorado Plateau. Geophys. Res. Letters 37: L10311. doi:10.1029/2010GL043374

Nelson, SA. 2015. Structure of the Earth and the origin of magmas. Tulane University EENS 2120 Petrology (online lecture notes).

Spence, W, and Gross, RS. 1990. A tomographic glimpse of the upper mantle source of magmas of the Jemez Lineament, New Mexico. Journal of Geophysical Research 95 B7:10,829-10,849.

Valentine, GA, et al. 2017. Lunar Crater volcanic field (Reveille and Pancake Ranges, Basin and Range Province, Nevada, USA) Geosphere13: 391-438. https://doi.org/10.1130/GES01428.

Volcano Hotspot. 2018-01-09. Recent volcanic activity in SW Utah.