I was just scrolling and saw a video of a sink-hole forming and just swallowing a rather large area of water/grass. Like all of us I’ve seen photos of the gaping holes in cities etc.
But how do they work? Why do they form? Where did all the mass that was there before them go? Where does the stuff they swallow go? Mass can’t just disappear.
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Rainfall seeps through the soil, absorbs carbon dioxide, and reacts with decaying vegetation, creating a slightly acidic water. That water moves through spaces and cracks underground, slowly dissolving limestone and creating a network of cavities and voids.
So the limestone dissolves similar to erosion above ground, but underground.
No, mass doesn’t disappear. Water flow underground washes the soil and stuff away, making a small hole underground grow larger until the ground above the hole can’t hole itself up any more and it collapses.
You can check out youtube for demonstrations. Small scale models of the ground with water flow and see how it causes a cavity in the ground to grow.
Sinkholes are usually the result of an underground river or what source washing out the dirt and sediment below the area. Eventually, it creates a big enough void that cause the area above to collapse.
If the water source is close enough to the surface, you can see the water, however sometime its deep enough where the dirt/ground above slow enters the void and continues to be washed out. Think of it like sand in an hourglass, but the bottom half has a river washing any sand that falls through
What others have not yet mentioned, many sink holes results from illegal mining activities and detonations used, for them, to get to a particular layer below the surface
This might destabilize or expose some lower vulnerable layers to water tables, rains or running water bodies, ( cf, limestone caves at Carlsbad, resulting from dissolution of CaCO3 in water).Sometimes improperly filled mines , with sand, oil etc. also give away in terms of rains and floods, so the top layers collapses into the vacant space underground.
If there’s a void underneath (sometimes natural and the loading on top can become too heavy, or from say tunnelling activities) or something has happened to mean the soil and water pressure up is way less than the gravity pulling it down, sinkhole.
But also, not sure if you saw the same video I saw with the grass and water where it was entering two locations close together? That wasn’t actually a sinkhole like the post was labelled, it was a drainage culvert. There was a flood and the water was rushing into that big big drainage pipe. No sinkhole, but it’s going to be a nightmare to unclog after the flooding stops!
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