September 29, 2003
A: FROM MENTOR JOAN LUSK IN RI

For a sinkhole to form, something has to hollow out a space in the earth into which the surface can form. On Prof Salvati's web site (quote below), he summarizes causes as a combination of the structure of the earth (for example, limestone dissolves easily, leading to caves, which can collape), movement of tectonic plates (earthquakes that can open up holes), groundwater circulation (that can erode rocks like limestone and move sand around - flooding from Isobel would have saturated the earth with water, intensifying its effects) and human activity (like leave the earth full of holes and tunnels from mining, or removing oil or water that was supporting the earth above it.) When a cavity in the earth does collapse to form a sinkhole, it happens pretty fast, though I'm not sure as fast as "instantly".

Here's an excerpt from one abstract that discusses sinkhole collapse and says it can be gradual or sudden. Floods, of course, would trigger sudden collapse.
http://pubs.usgs.gov/circ/c1148/appendix1.html
Randall C. Orndorff and Scott Southworth,
U.S. Geological Survey, 926A National Center, Reston, VA 20192
Sinkhole collapse, which involves the sudden vertical mass movement of residuum and bedrock, is a hazard in areas of carbonate bedrock in the central Appalachians and needs to be considered in engineering and geotechnical studies and in land-use planning. Both natural processes and man's influence contribute to the risk. For example, a common cause of collapse is a drop in the water table due to extended droughts or excessive pumping of ground water. In the study area, subsidence typically occurs over long periods of time by solutional enlargement of joints, faults, and bedding planes. However, sudden collapse of residuum and bedrock does occur. Since 1992, extensive property damage has resulted from collapse sinkholes in Clarke County, Va., and Frederick and Carroll Counties, Md.

Roberto Salvati, Universisty of Akron

In particular, the relationship among geological-structural setting, tectonic activity, groundwater circulation and human impact that are generally accepted as the main triggering issues of such phenomena in the wider international references are completely unknown.

http://www.geophysicalsurveys.com/sinkhole.htm - Some ways of looking for sinkholes