A Trip to the Valley of Rainbows
By Ryan Thornhill, Comprehensive Geology Major
Today, July 21st, a group of nine of us took a tour to Rainbow Valley and an ancient Atacaman petroglyph site near San Pedro de Atacama, Chile. We started our tour in the morning at Rainbow Valley, and passed an area called ‘Refuge of Chulchul’ by the Atacamans that means “hidden”, and is believed to have been a place used for spiritual ceremonies by the ancient peoples of the Atacama Desert. Our first stop on the Rainbow Valley tour, our tour guide pointed out that there was a fault, and we could see intrusive, coarse grained granitic rocks that developed at a very late stage of the magma crystallization. These rocks are called pegmatites and these specific ones had large crystals of muscovite. We hiked about 50 meters further and came to an area where there used to be a waterfall, but is now dry. The local call this the ‘Dolina’ and erosive patterns in the rock clearly demonstrate that water ate away and eroded the clay structure over time, and left about a 20 meter drop off. In Rainbow Valley, there is also an abundance of gypsum which is an evaporite mineral like salt. The gypsum here likely formed when this area was under ocean waters high in calcium and sulfate, then evaporated over time due to the dry climate, leaving behind sedimentary gypsum.
The mountain range where the Rainbow Valley is located called the Maco Mountains and began forming about 150 million years ago when the Nazca plate converged and subducted under the South American plate. There are believed to be three sedimentary formation episodes that occurred in the Maco Mountain range that are the result of the erosion of the moutains as they were uplifted. The first episode the native people calle ‘Tonel’ due to all of the crystallized gypsum. The second formation episode, ‘Purilactis’ was formed with sandstone. Lastly, the third episode is formed by conglomerate and is called ‘Barros Arana’. An interesting facet of Rainbow Valley is the fact that there aren’t any volcanic rocks to be found here, instead the igneous rocks are plutonic rocks, meaning the rocks crystallized from magma cooling slowly below the surface as opposed to on the surface. These plutonic rocks we found in the valley have no exact age but other rocks in the area have been dated to be of Jurassic age. These rocks are considered granites and are composed of actinolite, chlorite, hornblende, plagioclase, and orthoclase. These were interesting rocks and had a mainly green color due to the presence of actinolite and chlorite. Another fascinating characteristic about this area was the fact that an ignimbrite formed on the top of the mountains due to a pyroclastic flow, and was about 780 degrees when deposited. This is the only volcanic rock in the area and the source of the eruption is very far from the Maco Mountains. We had a chance to see these rocks up close at our last stop in the Rainbow Valley. This was really interesting because there were two different colors of ash, which is sand size particles of crystallized lava, purple and white. The purple ash is from oxidized silver and the white ash is from oxidized titanium in the rocks.
Next, we took a drive to an ancient Atacaman petroglyph site. The area only recently became restricted and private so tourists couldn’t vandalize any of the petroglyphs. The authentic Atacaman petroglyphs are easily distinguishable from any recent “fake” ones because the originals are oxidized and are almost outlined in white. The first petroglyphs we stopped at featured llamas, flamingos, and foxes. Petroglyphs of llamas show how important they were to the Atacaman people because llamas were the only animals that could travel across the desert and transport materials for trade and are a major food source. The petroglyphs of the foxes represent hunting and slyness to the Atacaman people. The petroglyphs of the flamingos represent fertility because these birds can only lay one egg a year.
Overall, it was a really great tour and our guide did a fantastic job of leading us and explaining the history of the Atacaman people. It was a great experience and introduction into the history of the people and geology of the Atacama Desert. I would highly recommend this tour and easily do it again!