Understanding Alluvial Fans
The next time you get to enjoy an airplane ride, especially over a mountainous desert region, take a look out the window. If it’s a clear day, you will likely enjoy the sight of many alluvial fans.
Alluvial what? A wonderful triangular morphologically fanlike depositional geoformation resulting from the accumulation of clastic alluvial sedimentary materials often of fluvial derivation, of course.
“Huh?” you may be asking. “Please! Speak English!”
Oh, very well!
Simply said, alluvial fans are piles of rocks, pebbles, sand, and silt that emerge from a narrow canyon, especially in an arid or semi-arid environment like a desert. As sediments erode from mountains and into a confined channel and then suddenly emerge onto a flat plain, they spread out in a cone- or fanlike triangular shape. The narrow point from which the sediments emerge is referred to as the “apex.”
The accumulation of an alluvial fan may happen a little at a time or in big flash flood events referred to as debris flows. The resulting fans may be only a few yards across, or as much as 500,000 yards or more! One near Xinjiang in the Chinese Taklamakan Desert is 25 miles long and wide. The very biggest alluvial fans are said to be found along the Himalayan Mountains. This fact is not surprising given how that mountain front is rapidly rising and just as rapidly erodes and dumps out loads and loads of sediment.
Alluvial fans are typically composed of rather coarse-grained deposits, namely, gravel. When several join together at the foot of a mountain range, they are referred to as an “apron” or a “bajada.” Such fans and aprons have been seen not only here on Earth but in photos sent back by satellites taking images of the surface of our sister planet Mars, particularly around the rims of meteorite craters. This evidence tells us that rains once fell on Mars just as they do on our own good Mother Earth. Next time you’re in that airplane (or flying in a spaceship over Mars), be on the lookout for an alluvial fan.