Devil's Playground—another intrusion, this one with wildflowers

Late afternoon at the Devil's Playground in northwest Utah—my kind of #vanlife.

I ended my tour of Utah and Nevada last May with a stop at the Devil's Playground north of UT Highway 30 near ... well ... near not much of anything. It's at the south end of the Grouse Creek Mountains and about 40 air miles west of the north end of the Great Salt Lake. The Utah Geological Survey provides directions here.

Like several other stops on the trip, this one featured an igneous intrusion—the Emigrant Pass pluton, emplaced 41 to 34 million years ago in three phases. Rock in the Devil's Playground area is part of the youngest phase (Egger et al. 2003). This pluton is especially interesting to geologists studying metamorphic core complexes (MCCs), for it is in the southern part of the Albion–Raft River–Grouse Creek MCC (ARG in map below).

Black blobs are MCCs; arrow points to Emigrant Pass pluton. After Strickland et al. 2011.

Note multiple plutons in Albion, Raft River, Grouse Creek Mts. After Egger et al. 2003.

The diagram below shows a textbook metamorphic core complex (actually there may be too much variation to have a classic example). During continental extension, rock strata stretched, broke, and slid along low-angle faults, revealing older deeper rocks which domed upward. But continental extension has occurred across much of the Basin and Range Province without making MCCs ... hmmm, puzzling.

Based on Peterson & Buddington 2014, DeCourten & Biggar 2017.

As I learned during my trip to the Ruby Mountains, MCCs are controversial. Among topics debated is why they're clustered in this part of North America (thickened crust?). Another is the role of plutons in MCC formation. Some geologists argue that plutonism is the main driver, supplying heat that softens rocks and facilitates extension. After all, plutons roughly contemporaneous with extension "are ubiquitous in many of the core complexes" in this region. But other geologists disagree, arguing that plutonism plays a minor role at most (see Introduction in Egger et al. 2003 for more discussion).

I like metamorphic core complexes very much, largely for their mystery. But they're difficult. It's challenging just to spot one, even with a guidebook. These are giant structures, visible only as parts exposed here and there. Plutons are much easier to understand, fairly common, and yet still worth contemplating. I became a fan when I realized that if I can see a pluton, something dramatic must have happened.

It seems plutons are often sculpted into intriguing forms, like the Harrison Pass pluton in the Ruby Mountains.

Tors carved from one of our local plutons, at the crest of the southern Laramie Mountains.

Notch Peak intrusive at the base of the west face of the House Range; photo by Mike Nelson.

Emigrant Pass pluton—a tilted world. Did it tilt during uplift? 

Plutons, like the god Pluto, reside in the Underworld. They form when magma solidifies well below the surface, where it cools slowly and forms visible crystals. So unless we make a Dante-esque excursion miles underground, we can only see plutons if they've been exposed in some way. When the Grouse Creek Mountains rose about 13 million years ago, during Basin and Range extension and faulting (Ege 2006), erosion set in. That's probably when the "devils" of the Playground were born.

Rocks in this part of the Emigrant Pass pluton have been called granitic and granitoid. These are handy terms because the range of granitic rock types is broad and hard to subdivide neatly. Egger et al. (2003) are more specific: "a virtually homogeneous coarse-grained biotite granite". The granite is criss-crossed with aplite and pegmatite dikes, which formed when still-molten magma—hydrous and therefore last to crystallize—was injected into fractures in the solidifying pluton.

Aplite is more resistant to erosion so dikes stick out a bit from the granite.

My field assistant pointed out a dike in a different kind of granitic rock.

Weathering of the pluton may have started underground, with groundwater enlarging fractures. In any case, today's wonderfully enigmatic forms are largely products of physical and chemical weathering above ground (Ege 2006 explains this nicely). These processes will continue until eventually the alcoves, spires, arches, fins, and other devils disappear.

Fine example of spheroidal or onion-skin weathering at the Devil's Playground. Photo courtesy scienceteacherexplorer (click link for more great shots).

Geocacher enjoying spheroidal weathering (source).

Are these young devils, recently emerged? Or elderly ones, to dust returning?

In the company of plants and rocks :)

Among the devils were spring wildflowers, a nice touch. Perhaps the most common was Stenotus acaulis, the Stemless Mock-goldenweed. It grows on rocky soils in drier areas across the western US. This is a DYC—"damn yellow composite"—but only because we find yellow composites difficult to identify. Seems to me that's our damn problem.

Stenotus acaulis; those of us who have been around for awhile may know it as Haplopappus acaulis.

Another yellow composite (Compositae is the old name for the sunflower family) caught me by surprise—Balsamorhiza hookeri, Hooker's Balsamroot, a plant of the Great Basin. I know Arrowleaf Balsamroot well, but didn't recognize this plant as a balsamroot. Based on online specimens and discussions, the plants here might be hybrids; more research needed.

Hooker's Balsamroot seems so different from Arrowleaf Balsamroot. Most strikingly, it is short (these plants are 10 to 15 cm tall), and can thrive on very dry rocky sites.

With so much sagebrush in the area, it wasn't surprising to find its common parasite—paintbrush; this one is Castilleja angustifolia, the Northwest Paintbrush. The low gray-green shrub next to it in the photo is sagebrush. Like DYCs, paintbrushes are difficult to identify to species. But I've never heard anyone damn them. Thanks to markegger for the identification, via iNaturalist.

Paintbrushes are hemiparasitic. They can photosynthesize, but by tapping into sagebrush roots they grow more vigorously. This ability may vary among species, perhaps explaining conflicting reports online.

Such a lovely parasite!

Sources

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

Ege, Carl. 2006. Geosights: Devil's Playground, Boxelder County, Utah. Utah Geological Survey Survey Notes 38 no. 1, January 2006.

Egger, AE, et al.  2003. Timing and nature of Tertiary plutonism and extension in the Grouse Creek Mountains, Utah. International Geology Review, 45:6, 497-532. https://doi.org/10.2747/0020-6814.45.6.497

Peterson, J, and Buddington, A. 2014. A geological study of the McKenzie Conservation Area, Spokane County, Washington. Conference Paper.

Strickland, A, et al. 2011. Timing of Tertiary metamorphism and deformation in the Albion–Raft River–Grouse Creek metamorphic core complex, Utah and Idaho. J. Geol. 119:185–206. https://doi.org/10.1086/658294

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.