Paleocollapse is a rock structure resembling the
karst
Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ...
landform, but is formed essentially by the dissolution of underlying
sedimentary rock. 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 (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 acting on the particles. For example, sa ...
, 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 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
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Stratigraphy
Landforms