12 July 2014
One of the interesting things about inviting a geologist to any sort of historic site is the inevitable moment when they get distracted by the stones that have been used to build whatever fabulous architectural treasure it is that you’re admiring. Case in point: When I was invited to go to the New Horizons Symposium in the Chimalistac neighborhood of Mexico city, I spent at least a few minutes each day taking photos of the walls (much to the amusement of my fellow conference-goers).
If you’re a volcanologist (or just get excited about volcanoes), you’ll understand why. The Casa de Califorinia’s La Casona, a mansion formerly owned by publisher Ignacio Cumplido and now used as a conference center by the University of California in the Chimalistac neighborhood of Mexico City, is built from two types of volcanic rock: a beautiful tuff and a vesicular basalt.
Mexico City sits in the middle of the Trans-Mexican Volcanic Belt (TMVB), an east-west aligned zone that extends more than 1,000 km from the Pacific Coast to the Gulf of Mexico. (According to the cab driver who took me to the airport after the meeting, it’s also called the “cinturón de fuego”, or belt of fire – very appropriate!) The TMVB contains stratovolcanoes as well as an abundance of monogenetic volcanic structures like scoria cones, and southern Mexico City is, in fact, built on top of some basaltic volcanic centers.
This makes identifying the likely source of La Casona’s basalt foundation pretty easy: my bets are on the Xitle volcano, a 2000-year-old scoria cone in the Chichinautzin volcanic field that produced multiple basaltic lava flows. The Universidad Nacional Autónoma de México (UNAM) campus is built on the distal end of a 13-km-long flow in this series, and in many places around the campus (and near our hotel) there are actually archaeological sites in the lava. (According to Delgado et al. 1998, there are remnants of pyramids from the Cuicuilco Culture.) The flows are composed of very fresh-looking olivine basalts, and show both a’a and pahoehoe morphologies in various places.
The tuff (a lithified pyroclastic density current depost) was a little harder. When you’re looking at older buildings it’s often correct to assume that the stone would have been quarried locally – this is a good bet in the case of the basalt – but the only pyroclastic deposit associated with the Xitle volcano is an air-fall tephra composed of basaltic lapilli and lithic fragments. The tuff used in La Casona contains far too much pumice and completely the wrong kind of lithic fragments (these are too light and clearly contain large plagioclase phenocrysts). So I had to do a little more hunting.
When you can’t easily source a building stone and there’s not a lot of information available on the building it’s used in, it sometimes helps to look at what’s been written about more famous structures nearby. A study of weathering and rock properties at the Templo Mayor, Palace Heras Soto and the Metropolitan Cathedral (Wedekind et al. 2011) lists some candidates for tuffs, and their description and photos of the andesitic Gris de los Remedios tuff (Gris de los Remedios) seems like a decent candidate.
“The volcanic rock of Remedios represents a lapilli tuff, supported by an ash matrix. The color of this tuff is gray to light gray. Two types of lapilli fragments are distinguished, one is white in color, and the other is dark. The ash matrix is more or less gray. Within the matrix, dark spots occur that can be traced back to mafic minerals. The mafic minerals often show prismatic crystal shapes and represent mostly idiomorphic developed hornblende crystals. The white lapilli fragments are dominated by pumice.”
This is the sort of thing I would call an ash-flow tuff, which is a term that’s commonly used to describe this kind of deposit in the Western United States. “Lapilli tuff” isn’t a very common term, but I think the authors were trying to draw attention to the fact that the tuff has an ashy matrix supporting pumice and lithic fragments. The authors say that the stone comes from quarries in the mountains to the northwest of Mexico City, but don’t give any more information about the origin of the stone; if this location is within the Los Remedios National Park, then it could possibly be a deposit of the Nevado de Toluca volcano, which is andesitic to dacitic and had four major explosive eruptions in the late Pleistocene. Unfortunately, I can’t say for sure because all the information I can find outside of that paper and one other by the same authors is from ‘dimension stone’ websites, which are very unhelpful.
At any rate, Mexico City is built both on and from the products of volcanic eruptions, and continues to see active volcanism today (Popocatepetl is currently having ash-and-gas explosions and seems to be in the process of building a new lava dome after destroying one in February).
One of the things I love about being a geologist is that I can find a connection to my science just about anywhere – even in the walls of the building I’m in. What’s even more fun is explaining to people why I’m doing things like taking photos of the walls, and getting a “that’s pretty cool” reaction from them when I can say something (even if it’s speculative) about where the rocks in the walls formed and how it happened. And I think it’s nicely fitting to give a talk about volcano hazards in a building made out of tuff and lava flows!
Delgado et al., 1998, Geology of Xitle Volcano in Southern Mexico City – A 2000-year-old monogenetic volcano in an urban area. Revisita Mexicana de Ciencias Geologicas, v.15, no.2, p. 115-131
Siebe et al. 1995, Quaternary Explosive Volcanism and Pyroclastic Depsoits in East Central Mexico: Implications for Future Hazards. GSA Annual Meeting 1995, Field Trip Guide Book #1
Wedekind et al., 2011, Natural building stones of Mexico–Tenochtitlán: their use, weathering and rock properties at the Templo Mayor, Palace Heras Soto and the Metropolitan Cathedral. Environmental Earth Sciences, v.63, no.7, p. 1787-1798