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).

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.

Tree Report, Road Report

Not far from my house, halfway down a dirt road to the Laramie River, a boxelder grows in a nook formed by warehouse walls. This is the tree I'm following this year. I visit it early each month and report on what I find at the monthly virtual gathering of tree-followers, kindly hosted by The Squirrelbasket.

Since my visit in June, there's been a lot of plant growth in general. On disturbed soil and old railroad ties along the dirt road, pioneering plants were glowing in the morning sun. It’s impressive where they can grow and flourish! We should appreciate their ability to provide ecosystem services where nothing else can, but too often they’re considered “just weeds.”

Scotch or Cotton Thistle

Nuttall's Evening Primrose—some would say it's not weedy because the flowers are so pretty.

Foxtail Barley

It's a good year for Yellow Sweetclover! (there's a dog in there somewhere)

Several native prairie grasses have become established here too, probably from the small prairie near the river.

Needle and Thread

Indian Ricegrass

Next we crossed the dirt parking lot (empty, as it was a weekend) to visit the boxelder in its protected nook. It’s thriving, and looks so different from the spindly little tree of winter! I suspect rain runs off the roof, and that this spot is more hospitable than it appears.

Then ...

... and now.

These days the boxelder is all about leaves. Their tiny green factories (chloroplasts) are furiously gathering sunlight and cranking out energy for growth and storage.

Compound leaves, weird for a maple (genus Acer); more on this next month.

The boxelder’s neighbors are thriving too. Along the base of the warehouse wall, “weeds” have been growing fast, determined to reproduce before the season ends.

Yellow Sweetclover with Canada Thistle on either side.

Tumble Mustard (pale yellow flowers) surrounded by Canada Thistle. Cheatgrass front center.

Dock’s beautiful red wings are brown, but it’s still photosynthesizing, storing energy in its rhizomes.

After visiting the boxelder, I checked on the new road under construction. I’ve been following it too, after getting hooked by the amazing Gomaco 6300, which extruded curb-and-gutter like a pastry bag extrudes cake decorations. There's been a lot of progress, most recently signal lights, stripes and road signs. Best of all, in the evenings after the work crew has left, and on weekends, we can walk across the new bridge!

Then (Gomaco 6300 on left) ...

... and now.

New stop sign waits in a patch of Common Kochia, one of our tumbleweeds. Kochia is extremely common along the new road, and I anticipate lots of tumbleweeds in my yard next spring.

Nearing the crest of the new bridge.

Decorative street lights are a nice touch.

View south from the crest.

The horizontal green line is the terribly inadequate old bridge, to be torn down soon. Good riddance!

The road is scheduled to open by the end of July. Then the pleasure of a quiet stroll high above the railroad tracks will come to an end. But I’m not complaining. There's a sidewalk and a bike path too, and it will be a blessing to finally have a safe route across the tracks to the east side of town.

Join us … all are welcome!