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The Geology of the Valle de Mars

The Mars Valley North of San Pedro de Atacama was a morning hike called "Cuchabrache" to the source of the Rio San Pedro in late December 2017. After a 30-minute van drive north along the Rio San Pedro up an old dirt road, Alvaro, my guide, and I disembarked and hiked up the mountain along an astonishing ridge line, with breathtaking views of the Andean range and the Rio Sand Pedro below us. Ridge lines over ridge lines with alluvial fans at the bottom reminded me of evidence of massive amounts of water that flowed millena ago, carving the landscape of the desert. The Atacama is one of the driest places on Earth, classified as BWh/BWn (hot desert climate and mild desert climate) according the Köppen climate classification system, with some parts receiving less than 15 millimeters of precipitation per year. That equates to about 15 liters per square meter - a 2-minute shower, once a year. The global precipitation average over land, for comparison, is 715 millimeters per year. Evidence suggests that the Atacama has not received any significant rainfall between 1570 and 1970, for 400 years. Thanks to the Andes, the Atacama is in the rain-shadow of the Andes to the West and the Amazon rain forrest to the East being one of the wettest places on Earth.

Looking back towards San Pedro

Along the ridge line of our hike the majestic Andean range and its mountains accompanied us. You could see distant clouds caught on the peaks from the Bolivian side, shielding the desert from precipitation.

Rio San Pedro Valley and Andes Range

The Rio San Pedro has carved a valley through the dry desert landscape. The stratified layers indicate an age of the surrounding area of about [__ Mio?] years with volcanic ash sedimentary on top of the layers. The alluvial fans indicate a strong flow of water in more ancient times. In the distance you can see the two stratovolcanoes Sairecabur (from Spanish saire: “rain” and Cabur “mountain”), roughly 43 km away at 5,976m and Licancabur ( from Spanish lican: "people", or "pueblo" and cábur: "mountain"), roughly 36 km away and 5,916 m high in the Andes range. Both stratovolcanoes are part of the Central Volcanic Zone of the Andean Volcanic Belt. Licancabur has been active during the Holocene, after the ice ages about 12,000 to 10,000 years age. The Holocene is our current geologic period.

What is difficult to discern from the image above, is the proximity to the volcanoes and the impact of lava flow on the region we hiked through. To the rescue comes Google Earth (you can also turn on the "satellite" option in Google Maps), with an image section of the volcanoes and topical indications of lava flow as well as the ravines, the Rio San Pedro and the alluvial fans at the base of the mountain ranges, a truly astonishing image captured with the Copernicus/Landsat satellites.

Source: Google Earth. Annotations are my own.

Now you might wonder: "If this is the driest place on Earth, where did all the water come from that these basins and alluvial fans leave as evidence?" Good question. I asked myself the same thing. Hundreds of years doesn't mean much in terms of geological changes, thousands of years might get closer, tens of thousands of years even better. On a place like Earth, where tectonic plate shifts and movements constantly recycle the upper most layers, we have to look back thousand maybe 10,000-20,000 years. In my online research, I wasn't able to find many papers that discuss the geological formative times of this region (maybe they are all in Spanish) but I remembered from an online lecture about the Solar System by Professor Mike Brown of Caltech and the countless images about Mars and its history (volcanoes and ice ages) we looked at during class that there might be a similarity. This region isn't called the Mars Valley by coincidence. Mars was once a wetter planet and had plenty of flowing water on its surface. So I went down the rabbit hole of trying to find papers that discuss the extent of the ice sheets during the previous glacial maximum, around 18,000 BP (Before Present), more precisely the range of 25,000 to 15,000 BP, and indeed I did find a paper (link in the footnotes) and the map below (26 and gray indicating "ice sheet and other permanent ice"):

Source: Ray, N. and Adams, J.

Do we want to rely on a single paper to proof the claim about the source of the water in the Rio San Pedro Mars Valley? Probably not. Does it all make sense? If you assume that the volcanic activity of the region, in particular Licancabur, and remember, it did not have its maximum until after the last glacial maximum and the ice mass was pretty substantial, then volcanic activity (heat) mixing with ice (lots of it) and converting that to water, could very well explain the big basins, alluvial fans, salt lakes, etc.

What did/does the planet Mars look like that has people compare this region to it? Below is an image from ESA's Mars Express mission and a picture of Reull Vallis, west of Hellas Planitia from 2012. The scale is about 50x larger than the Google satellite image above, but nevertheless a good comparison:

Credit: ESA

Back to the small stuff: the proximity of the volcanic range results and volcanic activity from 10,000 years ago splattered our path with tuff and volcanic bombs. Andesite is the primary ejecta from Licancabur and dark grayish in color. If you look at the smaller volcanic bomb in the picture below you can make out the grayish color of the tuff. The outer coating is light brown and reddish, resulting from weathering. The air pocket of the volcanic bomb forms when magma is rising through the volcano chimney and ejected at high velocity into the open where the material randomly encapsulates air and solidifies quickly.

The amazing eruptive power of a volcano is further evidenced by this large volcano bomb. Around 1.5 cubic meters, we guessed, it weighs about 1000 kg. An enormous air-pockmarked projectile.

The following pictures shows the stratigraphic of the surroundings are with discernible whiter layers. My guide explained that the upper layer is from an eruption from about 11,000 years ago the one below from about 33,000 years ago. All in all, an amazing place, well worth of it being used as a baseline comparison to the soil sample results from Mars.

Footnotes and further links:

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