Alluvial Fan: Definition & Formation

Alluvial Fan: Definition & Formation
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  • 0:40 Formation
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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

Alluvial fans are geologic features that are easy to spot, once you know what they are. So, what are alluvial fans and how are they formed? Learn the answers to those questions here.

Definition

Some people live on an alluvial fan. I know that sounds exotic, like part of a Spanish dancing costume, but it's actually a familiar sight in the Southwest U.S. where geologic time is practically on display when you look at the terrain from an airplane window. This particular alluvial fan seems stable, but it is an indicator of much more excitement in the past.

Alluvial fan in Death Valley, California
Death Valley alluvial fan

An alluvial fan is a cone-shaped feature in the landscape where silt, gravel, sand, and sometimes boulders, have been deposited. That mix of stuff that has been deposited is called alluvium.

Formation

The way alluvial fans form is not complicated, but it does need a particular set of circumstances. The recipe includes flowing water, sediment, a narrow passage between hills, mountains or canyon walls, and an open plain where the water and sediment exit the narrow passage. When water is flowing through the passage, it picks up the loose sediment and carries it along until it hits the open plain. The open plain allows the water to slow down and spread out, leaving alluvium behind. As the water slows down, the harder-to-move pieces of alluvium, usually the boulders and gravel, are the first to stop. Further down the alluvial fan, the lighter sediment, like silt and sand, is deposited.

Alluvial fans have some similar characteristics. The pointy part of the triangle, where the water and sediment have come out of the narrow passage, is called the apex or sometimes the fan head. The bottom part of the triangle, where the mixes spread out, is the apron, or tail, of the alluvial fan. The chunky deposits are closer to the apex, and the fine deposits are concentrated in the apron.

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