Originally posted by reaper13
Does anyone know why the sink hole a perfrct circle?
Well, not all sinkholes are perfect circles — even these Guatemalan specimens aren't "perfect" circles. I've seen a lot of sinkholes that are
highly irregular, more like crevasses than holes
; however, you may have noticed that everything in the Universe tends to form in circles and
discs and spheres. A circular or spherical shape is the simplest (and not coincidentally strongest) shape in Nature.
Try an experiment... Take a big transparent plastic water jug and partially fill it with sand. Then turn it upside down with one hand capping the
bottle neck. Now remove your hand and watch inside the jug as sand escapes. You will notice that a circular depression forms in the sand, which
deepens into a conical depression until all the sand is emptied.
It doesn't escape in a triangular shape, nor a square, nor anything more complex than a circle. It's the same physical reason that sand also
accumulates on the ground
in a circle (cone) as you pour out the jug. The circle is formed by material being more or less evenly distributed
in all directions — it's the simplest shape.
Same thing with these big sinkholes in Guatemala City... The ground beneath the city is mostly compressed volcanic ash. If, let's say, a swollen
bored through the ash 400 or 500 feet below the surface, it would remove a large portion of the ash down there, leaving a
cavity with nothing to support the ash above it
. Like the sand in the jug, the ash just starts escaping
into the subterranean cavity,
pulling in ash from all directions — which creates a circular depression up topside at first, then a steeper and steeper conical depression, until
all the ash drains away.
This action can happen incredibly
rapidly when you're talking about millions of tons
of ash thundering down under the force of gravity.
— Doc Velocity