Geologic History of Oaxaca December 12, 2022
The Catholic feast days come fast and furious in December, and they feature more Virgin sightings than I can keep track of—Virgin de Juquila on the 8th, Virgin de Guadalupe on the 12th, Virgin de Soledad on the 18th. At least we are spared the ten days of explosions that erupt from San Cristobal’s Templo de Guadalupe at this time of year.
Still, the frenetic cityscape encourages us to get out of town for some peace and quiet. It is fortunate that my good friends Steve and Sue have found me yet again in foreign terrain, since it gives us a break on taxi fares to escape the city. In just a few days, we manage to cover Huayapam and Mitla to the east, as well as Monte Alban to the west, giving my guest geologist an opportunity to sample the rock of the hinterlands. I have been eager to finally hear the rock story of this place.
“The geologic history of Oaxaca is impossibly complicated,” says Sue, the geologist. The end. For the sake of absolute empirical accuracy, she refuses to be implicated in the narrative that follows, but I have a deadline and so must push forward, without bias or discrimination.
Much of the complexity is due to the fact that the Isthmus of Oaxaca is affected by not one but two oceanic plates, each subducting beneath the southern sliver of a continental plate. This creates a land of upheaval and sharp contrasts, spanning at least 200 million years.
The two sides of this large valley present a good study in contrast. To the west, the yellow hills are primarily Cretaceous sandstone, with an age in the neighborhood of a hundred-million years. These include the terraced mounds of Monte Alban and other ancient sites. These sandstone strata experienced substantial uplift from tectonic compression, and, sometime around 40-million years ago (Eocene), explosive volcanos formed, which covered the area with ash.
To the east of the city lies older rock of a different character. Cactus dot the jagged escarpments that ring the valley in Mitla. Much of the dark stone is Jurassic gneiss and schist, but some pale ridges are rendered level by thick layers of volcanic tuff. This provided the builders of Mitla with ample material to create their massive columns and lintels. Filed smooth and perfectly contoured, the stone nevertheless reveals traces of gas bubbles, a reminder of its explosive origins, long before the Virgin of Guadalupe had so many pyromaniacal adorers.
So what is to come for the geologic future of Oaxaca? Sue is silent about such speculation, because, as she knows, a scientist could always be wrong. But Our Man in Oaxaca is never wrong if he is true to the story, which goes like this: The earthquakes will continue until the encroaching oceanic plates have had their fill, volcanic cataclysm will return to this state with a vengeance, and Mexican Virgins will keep showing up in the darnedest places, but only in December.