Granular Segregation in Nature

I recently took a trip to Spiti in Himachal Pradesh, a region on the leeward side of the mountain range that separates it from Manali, which is lush green. As you may see from the photo below, the mountains of Spiti are greatly barren and devoid of vegetation. I could not spot a single tree. This is primarily because of the lack of rainfall. Naturally, the landscape is very granular. Having spent some time learning about granular materials during my Masters, this place appeared very relatable to me.

In the photo below, you'll see something interesting. Landslides! These are not the typical landslides you see on mountains where a chunk of landmass just scrapes off the side of a mountain. These are shallow landslides with polydisperse grains. But you know, what is even more interesting? Segregation! Observe that the bigger rocks and boulders are down at the bottom and the fine particles are up the slope, which clearly shows segregation in grains!

You'll also spot a small landslide somewhere mid-slope where a similar pattern is repeated. But why does this happen?

Well, it's not a new discovery, unfortunately. Scientists and researchers have spent years figuring this out! One may check the following review article if they desire to go in-depth.

Otherwise, here is an explanation for a laywoman/man.

When a granular mixture moves down a slope, the bumpy base adds excitation to the flow in the direction normal to the base against gravity. Additionally, different layers of the flow move at different velocities due to frictional resistance from the base and this is called shearing. Because the grains collide with one another due to the energy inputted, the smaller grains keep falling into the voids of that form beneath them. As a result, the bigger grains come up to the top with the landslide progressing downwards.

Bigger grains have greater inertia and are hence slowed down less due to frictional effects. They move ahead and reach the bottom of the slope much faster. On the other hand, the small particles experience a lot of retardation and lag behind the bigger ones, forming the graded pattern as in the photo.

There are other extra-terrestrial instances of granular segregation as well. For example, asteroid Itokawa.

There are slightly heavy scientific terms like the Brazil Nut Effect, Kinetic Sieving, and Squeeze Expulsion which explain these phenomena occurring under a combination of gravitation, convection, and other physical parameters but that is for another day!