Paleocollapse is a rock structure resembling the karst landform, but is formed essentially by the dissolution of underlying

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

. It has also been called paleo-karst collapse. This has the effect of collapsing the formerly intact rock above, forming extensive fractures, debris pipes, and open

caverns A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

. Normally, the process was started and completed in the geologic past. The mechanism of its formation is relatively simple. As in the illustration, a deeper layer of

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

(or other

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...

) is dissolved, through some process. The support for the upper rock vanishes, and starts the collapse process. This is much like the subsidence associated with old

coal mines Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron fro ...

. Eventually, the caving process reaches the surface, and can be associated with debris pipes, rock fractures, and open caverns. At some later date, undisturbed

sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity In physics, gravity () is a fundame ...

, or

glacial till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

may fill the collapse zone. Paleocollapse geology can be remarkably stable (unlike karst), but poses some serious challenges to

engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...

or environmental geology. Primarily, the collapse zone is extremely

permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...

. This can provide a conduit for groundwater, or contaminant transport.Paleocollapse structure as a passageway for groundwater flow and contaminant transport: W.F. Zhou
/ref> As well, these zones may only be marginally stable, in that they can be reactivated by human activity, or events such as earthquakes. In China, underground mines have become suddenly flooded, due to paleocollapse features. In order to fully characterize the hazard, investigations may be undertaken, using dye tracers, or

exploration geophysics Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods, such as seismic, gravitational, magnetic, electrical and electromagnetic at the surface of the Earth to measure the physical properties of ...

. Of particular importance is understanding the true stability of the region. The paleocollapse process may continue at some future date, due to deep groundwater flow changes. Or perhaps, the reason that a certain evaporite zone has dissolved, is because of underlying faults in the bedrock.

References

{{reflist Stratigraphy Landforms