In 2006 I made my first trip to Bolivia as a new student in the PhD program at Oregon State University working with Dr. Shan de Silva. Universidad Mayor de San Andrés geologitsts Néstor Jimenez and Victor Ramirez, their students (Paola Ballon and Wilbur Acarapi), and drivers (Eugenio Roque and Victor Quispe) joined us for the trip to the field. We didn’t get too deep into the Lípez region of the Potosí department in the remote southwest corner of Bolivia before we ran into some trouble. First, a large container of gasoline fell from the roof of one of our two vehicles (while we were driving) and was too heavy to lift back up until our driver, Hector, got a mouth full of petrol and topped up the tanks.
Petrol fail. I mean fall.
The goal of our project was to refine our knowledge of the vast ignimbrites that blanket southwest Bolivia. Ignimbrites are the deposits of ground-hugging avalanches of hot glass, crystals, and gases that erupt explosively from volcanic vents. Southwest Bolivia is entirely covered with ignimbrites and lavas.
A number of these ignimbrites were deposited during catastrophic eruptions that were amongst the largest our planet has ever seen. Fortunately, the arid climate has preserved most of these ignimbrites perfectly. Unfortunately, the arid climate has limited erosion so it is very hard to see which ignimbrite is laying on top of the other. In only a few places are the contacts clearly visible.Such as below where the 8.41 m.y Vilama ignimbrite is overlain by the 5.45 m.y. Chuhuilla ignimbrite.
In order to differentiate the numerous ignimbrites, we collected pumice and matrix samples from about 80 locations across the Lípez region. We stayed in small hostels that mostly cater to the region’s tourists.
After getting back to Oregon, the biotite and sanidine crystals were separated and analyzed for their 40Ar/39Ar ages at Rare Gas Geochronology lab at the University of Wisconsin by Dr. Brad Singer and Dr. Brian Jicha. The 40Ar/39Ar ages were instrumental in correlating the ignimbrites as they all kind of look the same and have very similar compositions. But now we know (pretty close, anyway) the ages and volumes of some of the largest eruptions on Earth.
Below is the summary image showing caldera locations and ignimbrite distributions per 1.5 million year timeframe in the Lípez region of Bolivia and neighboring Chile and Argentina. Image from Salisbury et al., 2011.
Width across each panel is about 200 km.