To the Land of Volcanic Enchantment (LoVE)

New Mexico’s official nickname is “Land of Enchantment”—because of its rich scenery, long record of human history (for the US), and the many diverse cultures. Other nicknames popular enough to appear on license plates reference Cactus, Spanish, Sunshine, Delight Makers, Opportunity, Heart’s Desire, and most recently Chile Capital of the World. But oddly, none mention volcanoes.

This is so wrong! Much of the state is covered in volcanic rock, from basalt flows to massive beds of welded tuff. The most recent activity was just 45,000 years ago, and it isn't over yet. For example, only 20 km below the town of Socorro, an area of intense microearthquakes, a huge magma body lies waiting. The state really is a volcanic wonderland … which is why I'm going there later this week.

New Mexico is a geotripper’s paradise in general. Landforms are diverse. Rocks range from ancient crust (1.5+ billion years old) to the recent volcanics. Due to a dry climate, vegetation is sparse and plants don't obscure the landscape. Thus we often can make sense of what we see. However, my specific destination is an exception. The Jemez Lineament is one of the most prominent volcanic features in the state, and yet it may be the most mysterious. In 1984, volcanologists Smith and Luedke called it “the most spectacular phenomenon of its kind in the US … and not easily understood.” Thirty years later, their words still apply.

Volcanic fields of the Jemez Lineament, with Socorro Magma Body added (source unknown).

The Jemez Lineament is an obvious 800-km line of volcanic fields that spans northern New Mexico—from the Raton-Clayton volcanic field in the northeast corner, to the Zuni-Bandera field near Grants, and on into Arizona. It crosses multiple major tectonic provinces: the Great Plains, southern Rocky Mountains, Basin and Range Province, and Colorado Plateau. Features include cinder cones, lava flows, calderas, maars, shield volcanoes, composite cones, volcanic necks and more. The rocks are relatively young, erupted in the last nine million years. Most are basaltic (the one major exception is the Valles Caldera).

Capulin Volcano lies near the northeast end of the Jemez Lineament (L. Crumpler; source).

Aerial view of Valles Caldera—so large that it’s hard to grasp from the ground (L. Crumpler; source).

The Zuni-Bandera field near Grants includes the youngest eruptions in the state, such as McCarty’s flow (view from 20,000 ft; L. Crumpler; source).

Early on, some geologists assumed the Jemez Lineament was a hotspot chain—a line of volcanic activity resulting from the continent moving over a hotspot in the mantle. It parallels a line of volcanism to the north in Idaho and Yellowstone, considered by many to be a hotspot trace. However, the Jemez Lineament doesn't fit the hotspot model because there’s no time progression. Younger and older features are lined up in no particular order. [Even though the hotspot hypothesis has been rejected, Wikipedia refers to the Jemez Lineament as the “Raton hotspot trail.”]

Currently, the most popular hypothesis invokes events from the deep past, when North America was a smaller continent. From 1.8 to 1.6 billion years ago (Paleoproterozoic), several large crustal fragments and/or volcanic arcs drifted north and collided with the southern margin of proto-North America, enlarging it by 1500 km in just 200 million years.

The American Southwest in the Paleoproterozoic Era (source, modified).

The suture between the young continent (aka Wyoming craton, green in map above) and the Mojave and Yavapai terranes is known as the Cheyenne Belt. I live just south of it, and it's nicely exposed in the Medicine Bow Mountains west of town. Very different rocks occur on either side of a narrow zone of contorted rocks mangled during collision; this is the ancient suture. In contrast, the boundary between the Yavapai (pink) and Mazatzal (blue) terranes is “diffuse and elusive” (Magnani et al. 2005). It's considered a broad transition zone (purple in map) where rocks from both terranes are exposed. Even the boundaries of the transition zone are hard to pinpoint; the sharp lines on maps are misleading.

Adding the Jemez Lineament to the map of Paleoproterozoic terranes reveals a seductive pattern (below). The volcanic fields of the Lineament (black blobs) line up with the Yavapai-Mazatzal transition zone! It's tempting to think that crustal collision 1.6 billion years ago is still shaping the landscape. Perhaps the old suture has provided pathways through the crust for magma to reach the surface.

A pre-existing structure shows up in many explanations of the Jemez Lineament:

“magma leaked through the broken crust” (interpretive sign)

“… a zone of apparent crustal weakness geologists call the Jemez lineament” (Price 2010)

“Although there’s no simple answer to this question, clearly the answer must involve some major structural feature deep within the earth’s crust” (Muehlberger et al. 2003).

“Some geologists consider the Jemez lineament to be a reactivated Precambrian suture or boundary; … some structural or tectonic features of the Jemez lineament must penetrate through the crust into the mantle, because the most significant volumes of [Quaternary] volcanic rocks in New Mexico are erupted along this zone (Goff 2009).

“It may coincide with a boundary between Proterozoic lithospheric age provinces (a suture zone), which in effect exerts control on the present-day disassembly of the lithosphere” (Baldridge 2004).

Baldridge’s suggestion that the crust is coming apart (“disassembly”), perhaps along an ancient suture, is not just some wacko idea. Much of New Mexico has been undergoing extension for the last 30 million years. The most obvious evidence is the Rio Grand Rift, which has widened to almost 100 km in places (map below; source). Maybe thirty million years of stretching has loosened the Yavapai-Mazatzal suture enough to allow magma to ascend to form the Jemez Lineament.

In 2005, Magnani and colleagues reported that near Las Vegas, New Mexico, the ancient Yavapai-Mazatzal suture does indeed lie beneath the Jemez Lineament (reflection seismology profile below). They also noted that a mantle anomaly (hotter) had been documented in the same area. In other words, there’s now suggestive evidence for both a magma source and a conduit beneath the Jemez Lineament. Perhaps we’re starting to piece together the story of this “most spectacular phenomenon.”

The magic of reflection seismology requires humongous amounts of computer processing time (source (free)).

If you go …

Though the state nick-namers have ignored New Mexico volcanism, other agencies give it the attention it deserves. The Bureau of Geology and Mineral Resources, and the University of New Mexico Press have published excellent affordable books for those who want to see and learn about the state’s volcanic features and geology in general (flagged ** in Sources below). In addition, several very cool interactive websites can help you plan your trip: Volcanoes of New Mexico and Virtual Geologic Tour of New Mexico.

Sources (in addition to links in post)

** less technical but full of information, with photos, figures, maps and more

Baldridge, WS. 2004. Pliocene-Quaternary volcanism in New Mexico and a model for genesis of magmas in continental extension, in Mack, GH, and Giles, KA, eds. The geology of New Mexico, a geologic history. New Mexico Geological Society Special Publication 11.

Dunbar, NW. 2005. Quaternary volcanism in New Mexico, in Lucas, SG, MOrgan GS, and Zeigler, KE, eds. New Mexico’s ice ages. NM Institute of Mining and Technology Bulletin 28:95-106.

** Goff, F. 2009. Valles Caldera, a geologic history. University of New Mexico Press.

Magnani, MB, et al. 2005. Seismic Investigation of the Yavapai-Mazatzal Transition Zone and the Jemez Lineament in Northeastern New Mexico, in Karlstrom, KE, and Keller, GR, eds. The Rocky Mountain Region—an evolving lithosphere: tectonics, geochemistry, and geophysics. (2005), AGU, Washington, D. C.Geophys. Monogr. Ser. 154: 227-238. Free.

** Muehlberger, WR, Muehlberger, SJ, and Price, LG. 2005. High Plains of northeastern New Mexico, a guide to geology and culture. NM Bureau of Geology and Mineral Resources.

** Price, LG., ed. 2010. The geology of northern New Mexico’s parks, monuments, and public lands. NM Bureau of Geology and Mineral Resources.

Smith, RI, and Luedke, RG. 1984. Potentially active volcanic lineaments and loci in western conterminous United States, in Explosive volcanism: inception, evolution, and hazards. Washington DC, National Academy Press, Washington: 47-66.

Pseudoflowers—Trick or Treat?

A flower-less rockcress.

Every spring some of our rockcresses forego flowering, and instead grow terminal clusters of fragrant yellow leaves dotted with sugary goo. But why? Generally plants produce color, fragrance and nectar to lure pollinators, which carry male gametes (pollen) off to where female gametes (ovules) await fertilization. But the yellow-leaved rockcresses have no flowers, no pollen, no ovules. And yet these plants are all about sex—fungal sex that is.

Rockcresses (Boechera spp.; formerly Arabis) are members of the mustard family. Most are perennials, with a few biennials. They usually produce white to pale pink or purple flowers, but the yellow-leaved versions are common enough to frequently confuse wild-flower enthusiasts.

“Almost every spring, someone brings me a picture or a plant of a strange little flower they’ve never seen before, and can’t key out or even begin to guess the family for.” Irene Shonle

“Strange little flower” (source).

Normal rockcress, and infected rockcress with pseudoflowers (Cano et al. 2013).

The yellowed rockcresses are infected with Puccinia monoica—mustard flower rust. Rust fungi are obligate plant pathogens, and include some of the most destructive agricultural pests (e.g. wheatstem rust, coffee rust). Some have extremely complex life cycles, involving five spore types and multiple host species in a single life cycle! (details here)

The life of the mustard flower rust is simpler, requiring three spore types and one or two hosts (full story here). If a wind-blown basidospore (which has a single haploid nucleus) is lucky enough to land on a suitable host plant, it germinates. Hyphae grow into the stem, tapping into the plant’s nutrient supply. But living happily ever after on a rockcress is not part of the rust's plan. Sex is its goal. Mustard flower rust is heterothallic, meaning opposite mating types are produced by separate “individuals” (rust infections) on separate rockcress plants. Opposite mating types need to get together somehow.

Puccinia monoica solves this problem by creating pseudoflowers. Like real flowers, they attract pollinators (mostly insects) by way of fragrance and the promise of sweet reward. How impressive that a simple little fungus has evolved to to grow such features! … except that’s not what happens, at least not directly. The real story is even more amazing. The plant grows these novel features … under the direction of the rust!

In addition to siphoning off nutrients, the rust reprograms the host plant, somehow changing which genes are expressed when. As a result, the infected rockcress never makes the transition from vegetative growth to flowering. Instead it elongates, grows extra leaves, and produces yellow pigment, fragrant compounds, sugary liquid, and wax. The resulting structure looks, smells and tastes enough like a flower that foraging insects show up, partake of a bit of sugar, and hopefully carry off the spore-like spermatia to receptive hypha on other rockcresses.

Bumps are spermagonia, which contain spores waiting to be dispersed and super-sweet liquid.

Pseudoflowers may mimic other wildflowers, like this nearby sagebrush buttercup (speculation for now).

With today’s molecular analysis techniques and model organisms (Arabidopsis thaliana, the thale cress, is a close relative of rockcresses), it’s possible to delve deeply into pseudoflower biology. In 2013, Liliana Cano and her colleagues looked at developmental changes in rockcresses infected with mustard flower rust. They found that for at least 31 genes, activity was significantly altered (enhanced or reduced), affecting leaf, stem and flower development; metabolism and transport of sugars and lipids; synthesis of volatiles (fragrant compounds); and wax production.

These changes can be interpreted as beneficial to the mustard flower rust. For example, consider wax production. Cano and colleagues suggest that the waxy leaves induced by rust infection serve to reduce water stress. Water-stressed plants often have shorter stems and fewer leaves—not what the rust needs. Perhaps the waxy leaves of infected plants allow taller leafier growth.

Gravelly soil drains rapidly, making for dry habitat. Looks like waxy leaves weren't enough to compensate.

Whatever the mechanisms, by enabling fungal sex, infection clearly benefits the rust. And the rockcress clearly suffers—no flowers, no sex. But what about pollinators? Are they beneficiaries or unsuspecting dupes? Some botanists consider pseudoflowers to be tricksters, luring insects into service with little reward. However in a 1998 paper, Robert Raguso and Bitty Roy pointed out that the super sweet liquid of rockcress pseudoflowers is popular with many kinds of insects, including bees, ants, butterflies and flies. And given how many sugar-oozing spermagonia there are on each yellow leaf, infected rockcresses may actually produce more yummy calories than uninfected plants. If so, then for pollinators, pseudoflowers are not a trick but a treat.

Foraging ant (in a hurry).

Puccinia monoica on Boechera sp. is the latest addition to my iNaturalist project, Plants of the Southern Laramie Mountains (two observations—one for the rust, one for the plant). To identify the rockcress to species, I have to wait until uninfected individuals are in fruit.

I found infected rockcresses scattered through this sagebrush grassland.

It’s still early spring at Blair (8000 feet elevation)—not much flower action.

Sources

Thanks to Elio Schaechter of Small Things Considered who recently blogged about Boechera pseudoflowers, which I’ve long ignored.

Caro, LM, et al. 2013. Major transcriptome reprogramming underlies floral mimicry induced by the rust fungus Puccinia monoica in Boechera stricta. PLoS ONE 8(9): e75293. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0075293 (free).

Raguso, RA, and Roy, BA. 1998. ‘Floral’ scent production by Puccinia rust fungi that mimic flowers. Molecular Ecology (1998) 7, 1127-1136.
http://onlinelibrary.wiley.com/doi/10.1046/j.1365-294x.1998.00426.x/abstract