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History Channel's How the Earth Was Made is a wonderful earth science documentary series. The producers of the documentary set up each episode as a type of detective story, with evidence shown to back up a hypothesis.
The episode titled "Driest Place on Earth" describes the unique interplay of phenomena that created the Atacama Desert of northern Chile, a location that receives virtually no rainfall. Modern investigation reveals the desert to be the product of subduction of the earth’s crust, uplift of a previous ocean floor, special location along the Tropic of Capricorn, the adjacent cold Humboldt Current, and the rainshadow of the Andes Mountains. The Atacama is so barren and lifeless that it serves a proxy for the study of potential life on Mars. This episode is available on YouTube and DVD.
The video worksheet is a one-page, double-sided handout consisting of 56 multiple choice questions that track the progress of the video. This format enables the students to pay attention to the video while quickly recording their answers. In this way, the students are not bogged down in writing long responses, and they can better enjoy watching the video. A key is included, and the files are provided in both MS Word and PDF formats.
History Channel’s How the Earth Was Made: Driest Place on Earth Overview
The Atacama Desert of northern Chile is considered the driest place on earth. A unique set of circumstances conspired to create the characteristics of this region. The Atacama is roughly the size of Iowa, and is located between the Pacific Ocean to the west, and the Andes Mountains to the east (the Atacama is positioned within the “rainshadow” of the Andes, and any moisture from the adjacent Amazon region is spent on the sides of the mountains before reaching northern Chile).
The desert features relative humidity values of 10%, and receives less than 1 mm of rainfall per year! Groundwater in the desert is located over 200 ft. below the surface. In some places, the surface is barren and moon-like with little to no apparent vegetation.
Atacama, and earth’s other deserts, lie along the special lines of latitude, 23° north and south of the equator, known as the Tropics (Cancer and Capricorn). The Tropics feature sinking air that has been depleted of moisture. Along with its tropical location, other circumstances conspire to further remove water from the Atacama environment.
The Atacama has existed for millions of years under approximately the same conditions. This was demonstrated with two lines of evidence: gypsum boulders and helium isotope abundance in mineral crystals.
Gypsum, a rock that would readily dissolve in rain, was found on the surface. Due to the pristine state of the gypsum, it was reasoned that the mineral had not experienced any significant rainfall. Fossil analysis of adjacent rocks reveal a staggering age of 150 million years! Here was a surface that existed during the time of the dinosaurs, long before the advent of humans on earth.
Other research indicates that some parts of the Atacama have experienced the same climate extremes for millions of years. In other deserts, sporadic rainfall creates flash floods, which disturb and overturn surface boulders. Some boulders in the Atacama appear to have not been disturbed for millions of years. This result was obtained by measuring the concentration of helium-3 in pyroxene crystals in rocks. The helium-3 is generated as cosmic rays from space strike the mineral crystals. Based upon the abundance of helium-3 in the crystals, boulders in the Atacama appear to have been in the same positions for at least 23 million years! Like the gypsum boulders mentioned earlier, this implies that no significant rainfall has occurred in this immense time period, and that the desiccated climate of Atacama has persisted for a really long time.
The gypsum mentioned above was the product of sea water evaporating as the land was raised to the surface. The desert floor we see today once lay on the bottom of an ocean! What force could raise a sea floor? In this location, subduction of earth’s plates provides a reasonable explanation.
Evidence for subduction, the process where one of earth’s tectonic plates is forced under another, was identified at El Tatio, the largest area of geyser activity in the southern hemisphere. Heat from molten magma deep in the earth’s crust powers the geysers and volcanoes of this region. This magma is the result of the subduction of earth’s tectonic plates, and this same force had originally heaved the ocean floor up into the air to form the Atacama Plateau (Altiplano). Chile’s volcanoes were once located near the coast, and the video documents extinct, coastal volcanoes that date back 23 million years. Eventually the melting zone was pushed further to the east to its present location 140 mi inland, at a depth of 60 mi, and the volcanic area is now located much further inland. (The combination of subduction and a dry climate fostered the development of the massive open pit copper mine of Chuquicamata, one of the most valuable places on earth. After geologic forces concentrated copper ore in the near surface rocks, the copper was retained due to the lack of water erosion.)
Despite being a desert, the Atacama climate usually stays under 80° F, and is much cooler than other deserts such as Death Valley in California. The South American coast lies under the influence of the cold Humboldt Current, and surface waters are usually 20° F cooler than the adjacent land. The cool Humboldt water fosters communities of diverse sea life near the Chilean coast, including Humboldt penguins! The interaction of the cool surface air of the Humboldt Current with the warm, descending tropical air creates a temperature inversion, a situation where the air becomes trapped and depleted of moisture. In other words, the presence of the Humboldt Current further exacerbates the dry Atacama climate.
Despite the extreme Atacama climate, it has been shown that early humans once lived in the desert under milder conditions. Some areas of the Atacama appear to have experienced greater rainfall, and these conditions correspond to the retreat of northern hemisphere continental glaciers approximately 11,000 years ago. The enhanced rainfall created small oases and wetlands where diverse animal life, including humans, and plant life flourished. Stone tools dating back 11,000 years have been found in these once wetter regions of the Atacama. Adjacent chinchilla rat nests include pristine samples of now-vanished grasses. Radiocarbon dating of the grasses provided the age of 11,000 years.
The last section of the video documents the efforts of NASA scientists to study the Atacama Desert in hopes of applying lessons learned here to the future exploration of the planet Mars. In the relatively lifeless desert, green microbes were found to flourish inside large concentrations of mineral salt, halite, lying on the surface. The microbes survive on atmospheric water extracted by the salt from brief periods of higher humidity. Spacecraft observations indicate that Mars also has atmospheric moisture and halite on its surface. Based upon the study of Atacama microbes, future explorers of Mars might be prompted to search inside of rocks for similar forms of life.