Going to Maars in New Mexico
March 05, 2010
No, that is not a typo. A maar is a type of volcano. New Mexico is infested with them, statistically speaking. By now you may have noticed that I seem to talk about volcanoes and volcanic features rather often. It’s impossible not to do so – New Mexico should have been called the Volcano State rather than the Land of Enchantment. You can hardly look out your window anywhere in New Mexico without seeing something volcanic. Here’s a link you have to check out: Volcanoes of New Mexico. As the article says, New Mexico has “one of the largest numbers, the largest diversity of type, the largest range of preservation, and some of the best type examples” of volcanoes in the North American continent. We even have a bun in the oven, so to speak, smack dab in the middle of the state: the Socorro Magma Body. This is a mid-crustal sill of magma that is slowly, but actively, inflating beneath the city of Socorro, New Mexico and surrounding areas, one of only three such features in the United States – and the only one that hasn’t expressed itself at the surface. Yet.
A maar is shallow, flat-floored volcanic crater formed by violent steam explosions, caused when ascending magma meets water at or near the Earth’s surface. The eruption at the surface is confined to these explosions, which toss out great quantities of loose, water-sodden sediment mixed with shattered fragments of chilled magma and the occasional bit of exotic rock torn from the deeper crust. No great cone of lava is built, and in fact the low crater, usually only a mile to two across, typically fills with water to form a shallow, circular lake. Such relatively modest features don’t last long here at the surface, where weathering and erosion work relentlessly, so if you find a maar it’s probably pretty young. Geologically restless New Mexico has a world-class collection of maars, in all stages of preservation, and just west of Santa Fe, the Rio Grande River and its short tributaries have cut canyons right through an entire pock-marked field of these things, preserved by burial under the lavas of the Caja del Rio Volcanoes.
One of these short tributaries is the beautiful canyon of the Rito de Frijoles, which forms the centerpiece of Bandelier National Monument west of Santa Fe.
This lovely canyon, which is mostly cut in the orange and pink deposits of the Bandelier Tuff, is accessible from end to end, nearly, by trails which start at the Visitor’s Center. The most popular trail heads up canyon to the Ceremonial Cave, with its 140 feet of ladders pinned to the cliffs and the restored kiva in its alcove far above. But if you head down canyon, you will be treated to a number of wonderful sights along the so-called Falls Trail: meadows full of towering Ponderosa pine, the chortling music of the Rito de Frijoles, two waterfalls, a remarkable transition from woodlands to arid canyon vegetation, and some beautiful color in the autumn. You can follow this trail all the way down to the Rio Grande if you like, although the last bit is in a sloggy delta covered in dead junipers (once flooded by the lake behind Cochiti Dam) that I prefer to avoid.
For years I hiked down this trail and wondered at the tall cliffs of contorted lava above the waterfalls, which protect a softer wall of obviously stratified material, orange, buff, white, and grey, that could not contrast more strikingly with the somber rocks above. And this stratified stuff didn’t fall into any easy categories of sedimentary rocks I’d seen before: no water-cut channels, no dune or bar-like features, a weird regularity of bedding and the oddest mix of volcanic particles with regular sand, and – strangest of all – rough boulders of basalt sitting right in the middle of the beds, with the layers below bent down and contorted, as if someone had just thrown them there.
Finally I learned that I was actually walking inside of a volcano. Frijoles Canyon has cut a perfect cross section into the flanks of a maar, and the stratified beds are the remains of the wet sediment and shattered lava flung out by explosion after explosion of steam caused by an injection of magma into the floodplain of an ancestral Rio Grande. The gently sloping layers are punctuated by volcanic bombs ejected by explosions beneath the riverbed and hurled down onto the flanks of the growing tuff ring, as it is sometime called. That explained my mysterious boulders.
But it gets even better. Look at this photograph:
Just below the Upper Falls, you can actually see the curving interior of the crater of the maar, which has been filled with layers of lava, interbedded with scoria. The shallow crater filled with small lakes of lava! And if you turn around you will see this:
It’s very likely that the Upper Falls is cascading down the lava-choked throat of the vent that fed the maar in the first place. Amazing!
All of these features are preserved by thick flows of contorted andesite that form the cliffs above:
Andesite is a lava rather closely related to basalt, but with a higher silica content. Don’t quote me on this, but I think of andesite as ‘contaminated’ basalt – basalt that has incorporated lighter material from the crustal rocks through which it has leaked upwards. You can see in the picture that these lavas have a ‘sticky’ component, by the way they are thickened and contorted, rather than showing the flattened ‘runny’ layers characteristic of basalt lavas. But I always found these particular rocks puzzling, since they are nearly as dark as basalt and full of tiny crystals of olivine – that component of the Earth’s mantle whose presence nearly always shouts ‘basalt’! Oh well, no one said igneous petrology was straightforward.
In any case, what an opportunity it is, to be able to see a volcano from the inside out, as you can do here on a lovely trail not far from Santa Fe. It’s just one more reason to visit Bandelier National Monument when you come out to see us, here in maar-velous New Mexico.