A blood-red deer figure stands out on the cave wall, a prehistoric hematite finger-painting drawn by indigenous peoples of Baja, Mexico. I’m standing in a gaping cavern at the end of a lava-rock canyon; it lies hidden past pink volcanic boulders the size of cars and across a stream that bubbles from the natural spring ahead. The site, the cave paintings of La Trinidad, is a UNESCO protected world heritage site. It’s hidden at the end of a long, deep gorge, surrounded by a spiny desert, tucked away in a private goat ranch, and is only accessible at a price and with a guide.
The minerals in the paint itself, the wall on which they were painted, the canyon, and the development of the native culture on Baja peninsula, were all shaped by Baja’s geologic history, which I’m studying for my undergraduate thesis project. The research project brought me to Baja, and the cave paintings are a separate weekend excursion. But it doesn’t take a major mental leap to see how dramatically our lives, the development of species and cultures, and the existence of cultural artifacts like these paintings, are governed by geology.
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The cave paintings of La Trinidad are an hour or so outside of Muleje, a city 40 miles south of Santa Rosalia, where I’m staying in an active copper mine, Minero Boleo. We drove through the desert all morning to get here. The trek involved wakeup call at 6:30 for the announcement that our guide was stuck 6 hours south in La Paz, an hour or so of waiting in the mine for a new plan, a couple hours of miscommunication and tacos in town, a couple hours of driving south down Highway 1 to Muleje, and an hour or so of struggling up and down a sandy rollercoaster road, getting stuck in ditches and peeing on cacti in the windblown desert plains.
We started off the morning right: The freshest fish tacos in the sweet seaside village of Santa Rosalia. The day dawned overcast, a cold white-gray sky. The fish taco stand in the center of town was up and buzzing by 8am, serving streaming hot coffee and tossing fish, shrimp and scallions. We sat at picnic tables outside the truck and piled guacamole, salsa verde, Mexican coleslaw, radishes and cucumbers into our corn tortillas. The houses surrounding us were still decked out in Christmas decor; it will stay up until Three King’s Day, the official end of the twelve days of Christmas. On Three Kings Day, Mexicans hide tiny plastic baby Jesus’ in cakes and everyone has their choice of slice. Whoever has the bad luck of choking on a Jesus has to throw a party for everyone else in the room sometime in February.
The road from Mulege to Rancho Trinidad isn’t long, but it takes a few hours to crawl over the rocks and through the dust, and a sandy forest of enormous cacti waving thick, olive-green arms. Wild horses on the side of the road shake their manes in the dust and then whip around the cacti in an elegant canter. Ranch houses drift into the distance, pastel adobe with wide wooden porches, cowboy-hatted ranchers relaxing on porches. Lumpy red cliffs of lava slowly reveal themselves in the distance and we enter a landscape of tall red volcanic cliffs, remnants of the Comondu volcanic eruptions.
The Comondu volcanoes were born in a subduction zone, which transitioned to a rift zone that created Baja Peninsula, irreversibly changed the landscape, and affected species distribution and cultures we see today. The Comondu were a volcanic chain similar to the Andes that initiated around 24 million years ago, when the Farallon oceanic plate subducted under the North American continental plate. As the Farallon plate subducted, it pulled the East Pacific Rise spreading ridge towards North America until it made contact near Southern California. The irregular shape of the subducting Farallon plate caused the central section of the plate to submerge completely under North America by ~12 million years ago.
When the Farallon plate submerged under Mexico, it brought the East Pacific Rise spreading center with it, which completely disrupted the status quo. The East Pacific Rise continued to spread underneath Mexico, shoved Baja off to the west, and formed a rift and proto-sea akin to the Red Sea rift. Rifting shut off Comondu volcanism, formed a new landscape of rift basins and hills, caused biological species divergence and future indigenous cultural isolation between Baja and mainland Mexico, and introduced rift volcanism. True seafloor spreading began between 6 and 2 million years ago with the complete transfer of the Pacific-North American tectonic plate boundary to the Gulf of California. My undergraduate research project is to better understand the transition from subduction to rifting in the landscape, and the timing of rift initiation by dating the two types of volcanic rocks here – subduction-related and rift-related.
After becoming a peninsula, Baja was shaped by continued faulting, volcanism, and major climactic changes. Today, in 2015, I’m scrambling up and over massive boulders made of subduction-related volcanic rocks that have been funneled through this gorge cut into the Comondu volcanics -- at some point when there was enough precipitation in Baja to allow flooding that could move a three-meter boulder. Teetering at the top of one, I peer down at the glassy surface of the spring-fed stream that runs along the gorge today, a stream with the power to move a pebble, but not something like this. To get to wetter times we have to rewind again, to 20 thousand years ago. The Last Glacial Maximum began, temperatures dropped, sea level dropped, and there was more precipitation in Baja. A river probably incised the canyon carved through the Comondu volcanics that hosts the Trinidad cave paintings.
Lower sea levels during that time are the basis for the theory that Native Americans migrated from Siberia over an exposed Bering Strait. Around 12 thousand years ago, humans began appearing in the Americas, and around two thousand years ago, the Cochimi people inhabited central Baja. Being a peninsula, Baja eventually fostered the isolation of cultures, leading to unique traditions separate from mainland Mexico and California. The Cochimi were probably attracted to the canyon because of the natural spring water, and occupied this cave, drawing the cave paintings using hematite-based paint on long-cooled Comondu lava flows. Relative to geological time, the timeline of the Cochimi people is short. When Cortez appeared on Baja in the mid 1500s, the Spanish introduced diseases that quickly nearly exterminated the Cochimi. By the year 2000, only 80 people spoke the Cochimi language. The cave paintings will outlast the Cochimi culture. The canyon itself and the lava flows, though, will surely outlast humankind.
So will the Gulf of California. And right now, I’m pretty grateful to be here. The gulf is breathtakingly gorgeous, and in the hot afternoon, shone a brilliant turquoise. It’s also home to some of the best seafood I’ve ever had. After our cave expedition, my group had dinner in Muleje and I ordered a fresh lobster, avocado salad, and mango daiquiri with local fresh mango. We ate in an eccentric restaurant with big windows and decorations that ranged from corny plastic Christmas ornaments, stuffed and mounted animal heads and sombreros, to rifles, crosses, miniature shrines, and exotic plants. Our group was the only one in the restaurant, so the waiters turned the TV to American football in our honor. Viva Mexico!
Resources checked to write this:
Atwater, Tanya, and Joann Stock, 1998. "Pacific-North America plate tectonics of the Neogene southwestern United States: an update." International Geology Review, 40.5, pp. 375-402.
Lonsdale, Peter, 1989. "Geology and tectonic history of the Gulf of California." The eastern Pacific Ocean and Hawaii: Boulder, Colorado, Geological Society of America, Geology of North America, v. N, pp. 499-521.
Stock, J. M., and K. V. Hodges, 1989. "Pre‐Pliocene extension around the Gulf of California and the transfer of Baja California to the Pacific Plate." Tectonics, 8.1, pp. 99-115.
Umhoefer, P.J., 2011. “Why did the southern Gulf of California rupture so rapidly?—Oblique divergence across hot, weak lithosphere along a tectonically active margin.” GSA Today, v. 21, no. 11, pp. 4-10.
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