Gypsum tumulus in Germany
The great Jurassic limestone platform that covers much of Western Europe was born when the sea levels were higher due to the breakup of the supercontinent Pangaea. As new oceans opened at the nascent spreading ridges, entire ocean basins rose due to the buoyancy imparted by the heated roiling mantle below that was fuelling the supercontinental split. This meant in practice that much of the land was covered by sea as the water was pushed out of its basins in very slow motion, and these ubiquitous limestones and the younger Cretaceous chalk (as in the White Cliffs of Dover) are the most obvious remnants of this process.
As the seas receded during the last 65 million years paralleling the cooling and gradual sinking of the heated oceanic crust, land emerged, and the forces of erosion started to play their part. Rainwater, acidified by the CO2 in the atmosphere dissolves limestone into karst landscapes, replete with caves and their attendant speleothems (such as stalactites and stalagmites). Complex chemical interactions often occur, with minerals being repeatedly dissolved and precipitated, and climatic oscillations (such as ice age cycles) imprinting their records.
Where the limestone (made of calcium carbonate) contains a proportion of the evaporite mineral gypsum (hydrated calcium sulphate, which results from shallow supersaturated water evaporating, as in a salt pan), these interesting mounds can form. They are most common in semi arid environments (which limestone regions often are, as the rainfall all percolates into the extensive cave systems below). They result from the repeated combination of heating and cooling with drying and wetting during the sun's daily and yearly cycles. This causes the gypsum to change to its dehydrated cousin anhydrite and back.
In time, the overall volume of the gypsum expands, as its crystal structure disaggregates a bit and more water is retained in the structure. The gypsum layer then peels off the underlying sediment and rises, forming a hollow dome. This specimen was snapped in the Harz Mountains, where they were once thought to be the residences of dwarves and gnomes.
Image credit: Gerhard Schuster
