Stratification & Segregation
Avalanches

The most common kind of avalanche happens when accumulated snow abruptly rushes down a mountain slope. An avalanche may also occur when part of the mountain itself suddenly gives way sending a mixture of rock, gravel, and sand rushing down. Such an event may be triggered by heavy rains, an earthquake or when accumulated stresses reach a point where the formation suddenly becomes unstable. The pile of debris that forms is generally not uniform: the largest rocks tend to come to rest further down the slope with the smaller rocks and gravel higher up. How can we explain this?

Sorting by size

Picture a rock rolling and bouncing down a hillside. The slope has some irregularities. The rock may get stuck in a low spot and roll no further. Like the rocks and gravel in an avalanche, the sizes of the irregularities on the slope also vary in size. A particular depression may be large enough to trap a small pebble, but a large rock will roll right over it and head further downhill. What looks like a smooth surface to a boulder appears very rough to a tiny grain of sand. The smaller a piece of debris, the more likely it is to get trapped higher up the slope. The large rocks are more likely to make it all the way to the bottom.

Waterborne debris may also sort out naturally by size. In CyberGeologist, Ian Bryant and Peter Tilke describe how "point bars" are formed in the bends of rivers. Fast-moving water erodes the outer bank in a bend. Gravel, sand and silt are carried towards the inner bank. As the water slows, the larger pieces sink first leaving a characteristic layering in which finer grains are on top and coarser grains below.

So rocks and gravel can naturally separate out according to the size of the pieces. It would not be surprising to find sandstone with finer grains above and coarser grains below. But why stripes? What natural process could result in alternating layers?

Sorting by shape

Would a round ball or a rectangular box roll downhill more easily? The more spherical an object, the more easily it rolls. Round grains of sand or gravel tend to get further downhill than irregular or "faceted" particles, which have a greater tendency to get stuck in the irregularities of the slope.

So, larger grains tend to get further downhill than smaller ones. Rounder grains tend to get further downhill than faceted ones. But what if we have a mixture of larger faceted grains and smaller rounded grains? Will the large faceted grains tend to go downhill because they're large or stay uphill because they're faceted? Will the small round grains be more likely to come to rest further uphill because they are small or roll further downhill because they are round?

We have a contradiction. How does it get resolved? Do the grains get confused and stay jumbled together? It turns out that certain mixtures of larger faceted grains and smaller rounded grains will form a kind of layering, or "stratification". Hernán Makse can show you how it happens with something called a Hele-Shaw cell which he made with equipment and materials that you, too, can build and use yourself. Click here to go to that activity.