Gypsum and salt: the rock that dissolves fastest

Why a small share gets its own tier

Limestone dissolves over thousands of years. Gypsum dissolves roughly a hundred times faster, and rock salt faster still, quickly enough that a leaking pipe, a pond or a disposal well can open a void within a human lifetime. That is why SinkholeOdds puts a county in its top tier once a quarter of it sits on evaporite, even in a dry climate where limestone would be slow.

Where it is

271 counties have at least 1% of their land over evaporite rock on the USGS map, and 73 have a quarter or more. The states with the most of those are Texas (24), Michigan (18), Oklahoma (16), Kansas (6). The largest shares are in King County, TX, Stonewall County, TX, Childress County, TX, Cottle County, TX, Caddo County, OK, Harmon County, OK.

Shallow beds and deep basins are different

Some county pages also name a deep evaporite basin, such as a salt basin thousands of feet down. Deep beds rarely cause a surface sinkhole on their own. Where they have, it has usually been through people: solution salt mining, brine wells, or oil and gas wells that let fresh water reach the salt. If a county page names a basin, the question to ask is whether there are old wells or mines nearby, not whether the ground will fall in tomorrow.

Before you buy or build

On gypsum or shallow salt, water is what starts trouble. Ask where roof drains, septic fields and pools discharge, and whether the lot has had a pipe leak. Your state geological survey is the place to check for known evaporite sinkholes in the area.

Where to go next

See how it varies: Izard County, AR, Southampton County, VA, Elmore County, AL.

Florida sinkhole insurance: what a standard policy covers · Signs of a sinkhole under a house, and how inspection works