When asked what the driest desert on Earth is, many people might think of the Sahara, the Gobi, or perhaps a location such as Death Valley. They might be a little surprised then to learn that the driest desert on Earth is one that is near the Pacific Ocean, in Chile, South America.
The Atacama Desert has been recognized and confirmed as the driest desert in the world for some time now. Ongoing research is taking place to determine why it is so dry and how long it has been that way, as well as to answer many other puzzles. Some of the results so far have greatly surprised many Geologists.
What has been known for some time is that Atacama receives on average less than .004 inches or .01 cm of rain a year. This is less than a drop of rain per year. Some areas in this desert haven't received any recordable precipitation for centuries. To put this into perspective, Death Valley receives a little less than 2 inches or 5.08 cm of precipitation per year, and the Sahara receives half again as much rainfall.
The task for geologists has been first to figure out how long the desert has been dry. This endeavor was helped when a thin layer of very soft rock was found by accident. The rock layer is primarily gypsum, which is very soluble in water. Judging from the structure and by dating rocks above and below it, scientists now conclude that at least in the one area, the ground has been dry for 25 million years.
Perhaps an even more startling conclusion has been that at one point in the past, the area was underwater in a shallow sea. Gypsum forms when water is evaporated, leaving the minerals that were dissolved in the water, behind. This means that sometime over 25 million years ago, the desert was not a desert, but was covered by water.
Not so coincidentally, this about the same time frame when two tectonic plates collided, raising the Andes Mountains just to the east of the desert. In fact, much of the desert currently has an altitude of several thousand feet, raised by the same processes.
This information has helped scientists figure out why the desert is so dry, too. The Andes hold part of the key. Since the mountains have much higher elevation, moisture-laden winds blowing from the east to the west must climb up over the Andes. This happens in many parts of the world.
As the air moves up over the mountains, the air becomes denser and moisture forms droplets that become too heavy to remain in the clouds. Rain and snow fall, removing water from the air. When the air drops down the other side of the Andes, it gets progressively hotter and drier. This same phenomenon spawning the Santa Anna winds of California. By the time the winds reach the Atacama Desert, they are hot and parched.
Why doesn't the desert pick up moisture from the Pacific? Actually, it does, it just doesn't pick up much moisture. The water currents are cold off the coast of Chile in this area. Still, fog does form. Remember though that the desert is several thousand feet in elevation. Without a hot updraft as is normal with warm currents, the fog seldom has the energy to reach high enough to give moisture to the desert.
The desert is caught between two dry extremes.
There is no doubt that the Atacama Desert is the driest on Earth, and now we know why as well as for how long. There are still some puzzles to unravel, though. Research done over the coming decades into the Atacama Desert is going to be filled with curiosities and surprises. This is going to be an interesting ride.