Columnar microstructures in the weathering of sand and gravel mound

by Quirino Sugon Jr.

Columnar microstructures in the weathering of sand and gravel mound

Yesterday, I passed by Blue Eagle Gym in Ateneo de Manila University.  And I saw a sand and gravel mound beside a large mahogany tree.  Upon closer examination, in the mound are columnar microstructures, some about 3 cm tall and 1 cm in diameter.  I wonder how they were formed.

It has been raining these past few days.  Two days ago, the classes were cancelled in the afternoon because PAGASA, the Philippine weather bureau, forcast that the rains would also be as heavy as the morning.  But there was no heavy rain.

A strong rain would destroy those columns.  I think the mahogany tree has filtered the strong wind and rain, so what the mound received are just drops of water from the tree’s leaves.  I think these water drops have not yet traveled at terminal velocity.   Let us see: at 10 m, the freefall motion of the droplet without air resistance takes about .  The speed of the droplet upon reaching the ground would be .  This is too fast: the largest possible raindrop is 5 mm in diameter with terminal velocity of 9 m/s  (see tables of raindrop sizes and terminal velocities).  So I am wrong: the raindrops would already have reached terminal velocity when they reached the ground and hit the mound.

Maybe the effect of the mahogany tree is to make the drops fall more or less vertically downwards.  The mound is shielded from the East by the gym and from the west by the tree.

Close-up view of the columnar microstructures in sand and gravel mound. Notice that each column has a gravel stone on top.

Why do columnar structures form?  Gravel stones are larger than sand particles.  Gravel stones can withstand a raindrop as long as the sand beneath them won’t give away. From the close-up view, we see that parts of the mound that has no gravel stone on top would be washed away, leaving the columnar structures intact.    This process is called weathering, the process of breaking up of rocks and soil due to the action of air, water, and plants.  But maybe there is an ideal sand-gravel mixture and the type of sand used also makes a difference.

You may also like to duplicate these columnar microstructures by mixing sand and gravel and pouring water from a water sprinkler.  If you see the columnar structures, send me a note and link your pictures in the comment box.