Stratification & Segregation
Spontaneous Stratification & Segregation
Here we see Hernán Makse with the experimental setup he used to explore how certain mixtures of two types of grains spontaneously stratify and segregate. The device is known as a quasi-two-dimensional Hele-Shaw cell. It consists of two sheets of Plexiglas separated by spacers to be about 5 mm apart.
How spontaneous stratification occurs
The mixture Hernán is slowly pouring into the cell consists of large, red, cubic sugar grains with a typical diameter of 0.8 mm and smaller spherical glass beads, which have a diameter of about 0.19 mm. The stratification is apparent in the alternating red and while stripes. While the slope forms from Hernán's simulated avalanche it alternates between two phases:
- The mixed grains form a bump on the top of the slope.
- When this pile gets too steep it suddenly collapses. As this mixture moves downhill the small round grains sink lower in the fluid mix and come to rest in a layer under the large cubical grains.
Meanwhile, as you continue to pour more mixed grains into the cell a new pile starts to accumulate again on top.
The sequence of images below are taken from a computer simulation of this process. It assumes a steady stream of grains being poured onto the highest point of the slope, just as Hernán is doing in the picture above. Purple represents grains of mixed sizes that are flowing downhill. Red represents the large cubical grains that have come to rest. Green represents the small round glass beads that have come to rest.
|Sequence of stratification for large faceted and smaller spherical grains|
In most sandstone, stratified or segregated, the areas of different grains do not show up so clearly as they do in these demonstrations. The grains were chosen to have different colors as well as different shapes and sizes. In nature, the colors are likely to be very much the same.
How spontaneous segregation occurs
What happens if we pour a mixture of large round grains and small faceted grains into the cell? When a mixture of small, black, faceted sand grains (typical size 0.3 mm) and large, white, spherical glass beads (typical size 0.8 mm) are poured into the quasi-two-dimensional Hele-Shaw cell, the large grains tend to move downhill because they are both large and round, while the small grains tend to segregate uphill because they are both small and faceted.
Why is this important?
It's interesting to figure out how things got to be the way they are. But scientists are also interested in Stratification & Segregation for practical reasons. Much of the world's oil is in the pores of sandstone. How much oil a particular sandstone can hold depends upon its porosity, which in turn, depends upon the size and shape of the grains. Understanding how Stratification & Segregation occur helps us get a more complete picture of underground geological structures.
The Rock from Petra
So now we know how this rock from Petra got its stripes. Or do we? Actually we are not sure. The stripes could result from the stratification phenomenon we just examined, or from some other process. Further investigation is needed to be sure. But there are rocks, formed from ancient sand dunes, in which we are sure that stratification has occurred.