Two Trees and a Rock on a Trail

First, the Weather Report.

It's time for a final report about the trees I followed this year. But they haven't changed since last month, and it's cold and wicked windy. I'd rather think about that nice October day when I met two trees and a rock on the trail to Lamoille Lake, in the Ruby Mountains in northeast Nevada.

Rock huggers.

I imagined these gnarled old trees as seedlings emerging above ground in a protected spot next to the giant boulder. For years they grew taller from apical buds, and wider by adding new vascular tissue (plant nerds—see diagram further on). They were symmetrical saplings until they bumped into the rock. But they kept growing anyway, taller and wider ... except where they couldn't.

Limber pines, Pinus flexilis (named for tough flexible branchlets).

Trees grow taller at the top, not from the base (a common misconception).

This is my contribution to the monthly gathering of Tree Followers kindly hosted by The Squirrel Basket. Consider joining us for some low-key good times (more here). Now I need to go find another tree!

Late Season Early Morning Beauty

Prickly poppy, Argemone sp.

When I visited northeast Nevada in October, the days were mild, the nights cold, and the light low and warm. There were few flowers, but still much to see, for death and drying reveal forms overlooked earlier in the season. These backlit examples were growing along a dirt road in the Snake Range.

Prickly poppies are common roadside plants, with large showy white flowers. After the petals fall, and the plant turns brown and dry, the interesting structure of the capsules becomes apparent. A prickly poppy capsule opens by splitting from the apex down about ⅓ of its length, producing segments called valves. The valves spread to reveal a framework of dried vascular tissue, joined at the top by the persistent style and stigma (female flower parts).

Prickly indeed!

Wild buckwheat, Eriogonum sp. The diffuse inflorescence is roughly 30 cm across.

Inflorescences of wild buckwheats (Eriogonum spp.) range from tightly globose to spike-like to diffuse, like the one above. All species have tiny flowers, which are nicely displayed in plants with diffuse inflorescences. I don't know this plant to species, but the hanging flowers suggest the common E. deflexum or something close to it.

Within the buckwheat inflorescence, flowers are clustered in cup-shaped structures called involucres. In the next photos, dried white-and-pink flowers emerge from the pendant involucres. Each flower contains an achene—a single seed wrapped in dried tissue of the ovary.

Involucres are about 2.5 mm long.

Finally, a classic view of my field assistant, who is as fascinated by plants as I am :)

A Glimpse of the Underworld, high in the Ruby Mts

Trailer Park Rock, part of the Harrison Pass pluton.

Plutons are large bodies of rock, specifically igneous rock and therefore once molten. Like their namesake, the Greek god Pluto, they reside in the Underworld. Yet I spotted one on the crest of the Ruby Mountains, in northeast Nevada. Obviously there's an interesting story here, and fortunately I had several guides along to explain.

Ruby Mountains in northeast Nevada; note Harrison Pass (modified from Snoke et al.1997).

Trailer Park Rock started as molten magma far below the surface, in a place very different from the Earth we know. This is where the familiar lava and ash of volcanos originate as well, so it be must super hot. Could it be Hell itself?! Let's descend to the Center of the Earth. Maybe we'll find out.

Downward

Dante at the Gates of Hell on Easter Sunday, 1300 (Doré 1890).

When Dante Alighieri stood at the Gates of Hell, en route to the Center of the Earth, he hesitated. Shrieking, pain, death, fire, and other horrors lay ahead. But with the assurance of his guide, he entered. We, however, needn't worry. We're not being sent by a vengeful God. We are descending to gain understanding, a righteous mission. So relax and keep an eye out for rock similar to Trailer Park Rock—granitic (1) but molten.

Here's a diagram of the territory ahead—a refresher for people like me who find classification of the Earth's interior complicated. Note that there are two classifications—one based on chemical composition, and one based on physical properties. This can be confusing, especially for the outer layers.

Two classifications of the Earth's interior—chemical composition (right) and physical properties (left) (modified from Nelson 2015).

We first descend through the rigid lithosphere, comprised of large shifting plates that jostle and collide to create our landscapes (plate tectonics). Physically the lithosphere is a single layer, but chemically it is two: crust and the uppermost part of the mantle.

Since we're on land far from any ocean, we pass through continental crust—low density light-colored "felsic" rocks high in silica (2). This seems like an excellent magma source, as the Harrison Pluton is largely felsic. In contrast, the other layer of the lithosphere, the uppermost mantle, is peridotite, a high density dark "ultramafic" rock with much less silica and more iron and magnesium. But neither layer is liquid, so we will keep searching.

Below the lithosphere lies the asthenosphere, also part of the mantle. It's solid but soft enough to flow (ductile). It's thought that this flow drives plate tectonics—that lithospheric plates are rafted about on currents of sorts in the asthenosphere. However, it too is made of ultramafic peridotite, so we continue on.

Next we pass through the mesosphere, where the mantle is noticeably more rigid and composition is uncertain. Then, finally, we arrive at a truly liquid place, the outer core. But the composition is all wrong—mostly iron with a bit of nickel. As we approach the inner core, we see through sinister swirling shadows that it's solid rock. Probably best to turn back here.

Confronting denizens of the dolorous world (modified from Doré 1890).

Upward

But before we ascend, we must ask, "Why we didn't we find what we were seeking?" There has to be magma somewhere! Were we blinded by our expectations? Perhaps solid rock is fine if it can be melted. Maybe peridotite can somehow be turned into granitic rocks.

In fact, we passed suitable rocks near the beginning of our trip, in the lithosphere (crust + uppermost mantle). As my guides explain, melted lithosphere is likely the source of most magma (3). But what heat source can melt solid rock? Actually, rock doesn't have to be heated until it melts. Instead, its melting temperature can be lowered, commonly by the introduction of water or carbon dioxide (4).

While it's generally agreed that plutons originate as melted lithosphere, the question of which layer—crust or uppermost mantle—is debated. For granitic rocks such as Trailer Park Rock, continental crust is closer in composition. But geologists have learned that peridotite also can be a source of granitic rock. Molten peridotite can become more siliceous as it melts or as it rises or both, the iron and magnesium dropping out (e.g., Meldahl 2013; Nelson 2012).

Once back in the lithosphere, we wander around until we find some molten rock—magma—and follow it upward. It squeezes through fractures, melts adjacent rock, and engulfs large chunks of crust (stoping). It will ascend as long as it can, fueling volcanos if it reaches the surface. But our magma cools and becomes too viscous to move. It solidifies underground, forming a pluton. Because this could take a million years or more, we return to the surface.

Emerging from the Underworld (Doré 1890).

If this were the end of a pluton's journey, we creatures of the surface would never see one. But some of the most dramatic landscapes of the American West are developed on plutons! So there must be more to the story.

California's Sierra Nevada is a gigantic batholith—an agglomeration of many plutons.

Sherman batholith in the Laramie Mountains (Wyoming) is thought to be derived from mafic mantle rocks.

Back to Harrison Pass

Obviously some plutons don't stop below the surface. After a hiatus underground, they rise again, this time carried by the crust around them. After enough uplift and erosion, they are revealed. The Harrison Pass pluton has been rising with uplift of the Ruby Mountains, which started after the pluton formed (Barnes et al. 2001) and continues today.

The Harrison Pass pluton was emplaced about 36 million years ago, in a transition zone between highly deformed rocks of the northern Ruby Mountains, and largely unaltered sedimentary rocks to the south. Multiple pulses of magma were involved, and mapping the various rock units and deciphering the pluton's history have been challenging. Younger rocks appear to be crustal in origin, whereas magma of the oldest rocks probably came from both the mantle and crust (Barnes et al. 2001).

The igneous rock exposed at Harrison Pass is granodiorite, a granitic rock felsic to intermediate in composition, perhaps reflecting mixed origins—continental crust and mantle. It's one of the older rock units in the pluton, emplaced early on.

Trailer Park Rock is the largest outcrop along the Harrison Pass Road by far. I hit the brakes when I rounded a curve and suddenly saw it in full view! It was named for the recreational vehicles and trash that accumulate in a large pullout below. In fact, that trailer park was what kept me from getting any closer. Next time I will avoid hunting season.

A seriously cropped photo, shot from across the road.

NOTES

(1) "Granitic" is a category of rocks; granite is one member of the group.

(2) Plutonic igneous rocks often are classified based on silica content. In decreasing order, types include felsic, intermediate, mafic, ultramafic, and gradations in between (source). The mantle is ultramafic; the crust includes both felsic rocks (continental crust) and mafic rocks (oceanic crust).

(3) One possible exception is volcanic hot spots, where thin plumes of magma are thought to rise from deeper in the mantle (Nelson 2015).

(4) This is flux melting, in which volatiles such as water and carbon dioxide are introduced into hot rock at depth, lowering the melting temperature. It's thought to be common in but not restricted to areas of plate subduction (e.g., Nelson 2015, Johnson et al. 2021).

SOURCES

Thanks to Mike the Rock Guy for his continued help and patience.

Barnes, CG, et al. 1997. Grand Tour-Part 4: Geology and geochemistry of the Harrison Pass pluton, central Ruby Mountains, Nevada (in Snoke et al. 1997, 283-292). Available here.

Barnes, CG, et al. 2001. Petrology and geochemistry of the Harrison Pass pluton. J. Petrology 42:901-929.

DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology.

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

McGrew, AJ, and Snoke, AW. 2015. Geologic map of the Welcome Quadrangle and an adjacent part of the Wells Quadrangle, Elko County, Nevada. Text and references accompanying Nevada Bureau of Mines and Geology Map 184. Available here.

Meldahl, KH. 2013. Rough-Hewn Land, a geologic journey from California to the Rocky Mountains. U. Calif. Press.

Nelson, SA. Lecture Notes (various classes). Dept. of Earth & Environmental Sciences, Tulane University. Accessed 2017.

Snoke, AW, et al. 1997. The Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1-Introduction & Road Log. Geology Faculty Publications. 39. Available here.

Treefollowing: Wordless Wednesday (nearly)

Flash the maple is thick with dried samaras.

Spike the hawthorn still sports a bit of red and green.

~🍂~

Though the beech is golden

I cannot stand beside it

mute, but must say

"It is golden," while the leaves

stir and fall with a sound

that is not a name.

It is in the silence

that my hope is, and my aim.

...

The Silence by Wendell Berry

 

This is my brief contribution to November's gathering of Treefollowers. Consider joining us—as you can see, it doesn't have to be a lot of work. Photos would have been enough, but like Berry, I "must say".

Finding Petrophytum & Sereno Watson's Ghost

Petrophytum caespitosum mat on limestone. Dark spots are shadows cast by flower clusters.

In a narrow rocky canyon on the east side of the Ruby Mountains, I was stopped in my tracks by Petrophytum caespitosum—rockmat. It's not that I was surprised. There's plenty of limestone in eastern Nevada. Rather I was pleased. It's always a treat to come across this striking rock dweller.

It was first collected by the great pioneering botanist Thomas Nuttall, probably in 1836, probably somewhere in the southeast quarter of Wyoming. His specimen—two small fragments, each with a flowering stalk—now resides at Harvard University (HUH; closeup below). Nuttall's yellowed handwritten label in the corner is terse: "Spiraea (Petrophyta. Caespitosa. Platte [River] sources." We shouldn't blame him. The expedition was on the move; time and space were limited. Large specimens and detailed location information were out of the question. Furthermore, it was an exploratory expedition. The country was still being figured out.

Spiraea caespitosa collected by Thomas Nuttall. (HUH; lower right corner of herbarium sheet; scale line added).

Nuttall's description of the new species was published in 1840. He named it Spiraea caespitosa, for its caespitose (matted) growth form. Most species in the genus Spiraea (meadowsweet) are erect shrubs, but this one was "A singular dwarf alpine plant (1), with scarcely the habit of Spiraea."  It was distinctive enough that Nuttall put it in its own section, which he called Petrophytum (rock plant). He also noted that "The taste of the plant is scarcely perceptible."

Sixty years later, Per Axel Rydberg of the New York Botanical Garden would argue that the rockmat was distinctive enough to warrant its own genus. He called it Petrophytum caespitosum, as it remains today (2) (Rydberg 1900).

The next known collection was made by William Whitman Bailey, botanist with Clarence King's Geological Exploration of the 40th Parallel. He found it in September of 1867, in the West Humboldt Mountains in western Nevada. His collection also resides at Harvard. But Bailey went further. That winter, he made a sketch of the rockmat eponymously sprawling across rock. It was added to the herbarium sheet, showing what can't be seen in a dried pressed specimen.

WW Bailey's Spiraea caespitosa (one of six specimens, from various collectors, on a single sheet; HUH).

Bailey's sketch of Spiraea caespitosa in the West Humboldt Mountains (HUH).

A year later, rockmat was again collected by a botanist with the King survey, this one an unlikely hire. Sereno Watson was in his early 40s, not a young man, with limited botanical training at most, and no previous expedition experience. But after a series of unsatisfying careers (teacher, medical student, insurance company secretary), he was desperate for change. In the spring of 1867, he sailed from New York to the promised land—California.

From San Francisco, Watson took the train as far east as it went, and then walked across the Sierra Nevada to King's survey camp. After presenting a letter of introduction from a mutual friend, he begged for a job. He would help in any way he could. King hired him as an assistant topographer and general camp helper, for "nominal" pay—basically a "volunteer" (Brewer 1900).

But the gods soon smiled on Sereno Watson. Bailey, the official botanist, began to have health problems, so Watson became his assistant. When Bailey quit in early 1868, Watson was appointed expedition botanist, with a salary. It was the beginning of a successful and very satisfying life collecting and studying plants (3). 

In early May of 1868, the expedition "took the field again and worked eastward from the Washoe through the Trinity, West Humboldt, Havallah, and the several other mountain ranges to Ruby River [possibly the Franklin River in the Ruby Valley], and from there the East Humboldt Mountains were explored" (Brewer 1900).

It was in the East Humboldt Mountains that Watson collected Petrophytum caespitosum. At that time, the East Humboldt range included the Ruby Mountains, which is where I saw it. Was the ghost of Sereno Watson nearby?

In his catalogue of collections, Watson provided a specific location: "Cliffs above Camp Ruby 7,000 ft." On my visit, I camped at the foot of the Ruby Mountains just 7.5 mi north of old Camp Ruby (also called Fort Ruby). I found the rockmat in a canyon immediately west. Sereno and I definitely were in the same area! And though I don't believe in ghosts, I do see them on occasion.

Petrophytum caespitosum at base of limestone cliff, east side Ruby Mountains; October 2021.

Caespitose rockmat is now known from many sites scattered across the interior western US and into Mexico. It seems to be restricted to limestone and limy sandstone. Though widespread and sometimes locally abundant, its distribution is curiously patchy. It's often absent from what looks like perfectly good habitat. Challenges in dispersal or establishment might be the explanation.

The growth form of Petrophytum caespitosum is striking. Its prostrate stems intertwine to form dense rock-hugging mats, sometimes a meter or more across. Short stems rise above the mat, with cylindrical flower clusters 1–4 cm long. The flowers are tiny, the white petals just 1.5–2.5 mm long.

Stems rise from clusters of leaves 3–12 mm long.

Inflorescences can be branched, but usually are simple cylindrical racemes.

As well as sprawling across rock, the rockmat can grow in vertical cracks, or even hanging from cracks!

Petrophytum caespitosum in a crack high up on the cliff (tip of arrow).

"The top of this two foot long plant is attached to the rock wall; the rest of the plant swings gently with any breeze." Photo ©Al Schneider, http://www.swcoloradowildflowers.com  

I end this story with a lovely shot by Andrey Zharkikh, who shares his many plant photos on Flickr. With the right setting, and a knowing eye, rockmat can be exceptionally photogenic!

Petrophytum caespitosum, with butterfly; Wasatch Range, Utah. Photo by Andrey Zharkikh.

NOTES

(1) In Nuttall's day, "alpine" did not necessarily mean the highest elevations, above treeline. It was sometimes used for lower montane sites with no trees—a common situation in the arid American West.

(2) The genus is sometimes called Petrophyton, said to be an "orthographic variant (misspelling)".

(3) Among other things, Sereno Watson became Curator of Gray Herbarium at Harvard in 1888, a position he held until his death in 1892 (Coulter 1892).

Sereno Watson, a "thorough and painstaking" botanist, working at Gray Herbarium (Coulter 1892).

SOURCES

Once again, I'm immensely grateful to the Biodiversity Heritage Library (BHL) for providing such easy access to original literature!

Brewer, WH 1903. Biographical memoir of Sereno Watson, 1820-1892. National Academy of Sciences. PDF 

Coulter, JM. 1892. Sereno Watson. Bot. Gaz. 17:137–141, Plates VI, VII. Available online courtesy BHL.

Nuttall, T. 1840. Spiraea cespitosa in Torrey, J, and Gray, A. Flora of North America v. 1, 417-418, Available online courtesy BHL.

Rydberg, PA. 1900. Catalogue of the flora of Montana and the Yellowstone National Park, vol. 1:206–207.  Available online courtesy BHL.

Watson, S. Catalogue of botanical collections made in Nevada and Utah, in 1867-9. Harvard University Botany Libraries. Available online courtesy BHL.

Williams, RL. 2003. A Region of Astonishing Beauty: The botanical exploration of the Rocky Mountains. Roberts Rinehart.

Crystalline Beauty Amid the Garbage and the Flowers

On my recent geotrip in eastern Nevada, I often stopped at road cuts—those man-made features so beloved by geologists. A cut removes weathered rock, accumulated sediments, and those pesky plants. It often reveals rocks and structures below the surface. And when Fortuna smiles, a cut is in just the right place to expose something astonishing!

Of particular interest to me were road cuts in the Ruby, Schell Creek, and Snake ranges of northeastern Nevada. In these mountains there are rocks that have been around for a long time, and have suffered multiple episodes of deformation. Here's a condensed version of their history, to give a sense of what they've been through (DeCourten and Biggar 2017, DeCourten 2003).

Formation on a passive continental margin. Roughly 500 million years ago, much of eastern Nevada was a warm shallow ocean where sand, mud, and limey muck accumulated. With pressure, cementation, and enough time, the sediments became rock—thick layers of sandstone, shale, and limestone. This went on for hundreds of millions of years. Then Nevada's idyllic coastal setting came to an end.

Deformation caused by uplift and intrusion. By about 250 million years ago, the sea had disappeared. It was replaced with land pushed up and contorted due to the jostling of lithospheric plates far to the west. This also caused production and rise of magma, much of which cooled and hardened before reaching the surface. When magma was intruded into the old sedimentary rock layers, they were deformed and often metamorphosed (one reason there are so many productive mines in Nevada).

Deformation due to continental extension. Currently much of western North America is stretching, as it has been for maybe 30 or 40 million years. Nevada is about twice as wide as it was thirty million years ago! The old marine sedimentary rocks have been disturbed once again—uplifted and often tilted. As a result, mountains have risen and intervening land has sunk, forming the Basin and Range Province.

Northern Basin and Range Province (NPS). The many more-or-less parallel mountain ranges looked like "an army of caterpillars marching toward Mexico" to pioneering geologist Clarence Dutton.

In some of the areas I visited, extension and deformation have been extreme. Rock layers weren't just uplifted and tilted. Older rocks arching up from below broke the layers into huge chunks, which slid or were pushed for miles, greatly mangling the rocks (1). Geologists often say they read the rocks to understand the past. But rocks as mangled as these can be difficult to decipher.

With my limited background, I found this type of road cut cryptic, even with a guidebook. No matter, I poked around just the same. That's how I came upon this treasure amid the garbage and the flowers (2).

One of many beer cans for scale.

The closer I looked, the more beauty I saw. "Fractal" came to mind. Only after many photos did it concern me that I had no idea what this structure was, though somehow it reminded me of a geode. Turns out that's not far off.

The rock exposed here is mostly old limestone that formed when eastern Nevada was a shallow sea. Since then it has been greatly altered. Intruded magma probably played a role, for there is mineralization nearby (copper, silver, lead, zinc, and gold). The limestone also has been subjected to extreme extension, fracturing into huge chunks that moved for miles. In the process, the rock was broken into angular fragments, which were then cemented back into rock in various ways to form limestone breccia ("breccia" is Italian for rubble or rubbish).

Calcite veins in fractured limestone.

Calcite veins are common in this limestone, and sometimes broad cracks or cavities allowed the growth of larger crystals, which explains my treasure. This is the geode connection. Typically "geode" is used for a spherical or rounded chunk of rock containing a cavity lined with crystals. But other cavities qualify too, even caves, such as the Cave of the Crystals in Chihuahua, Mexico.

Gypsum crystals in the Naica cave, a giant geode. Alexander Van Driessche photo (source).

A geode may contain several kinds of crystals, and I wonder if my treasure is strictly calcite. It looks diverse to me, but according to Google, calcite can be clear, white, yellow, pink, purple, green, and more. Colors often are due to presence of metallic ions, referred to as chemical impurities. [Minerals and crystals are new topics for me. If you can add or fix something, please Comment.]

There were a few other things of interest that I recognized.

Fragments of limestone with quartz veins in a matrix of ... ??

Red coloration was common, due to iron oxide according to the guideboook.

Another puzzling but beautiful spot, with calcite, iron oxide, and an unknown rock.

And finally, there were flowers. Rabbitbrush, Ericameria nauseosa (many still call it Chrysothamnus nauseosus), was blooming along many roadsides.

NOTES

(1) This is a metamorphic core complex. I visited two on my trip, and may try to put together a post about them, but they intimidate me. It seems they're poorly understood, even by experts.

(2) From Leonard Cohen's Suzanne, a song that will always be with me.

SOURCES

DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology.

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

Tingley, JV, and others. 2010. A Geologic and Natural History Tour Through Nevada and Arizona Along U.S. Highway 93. NV Bureau of Mines & Geology.