How is a sinkhole primarily formed?

A sinkhole is primarily formed by the collapse of a cavity, which typically occurs in limestone or other soluble rock formations. Over time, acidic rainwater can seep into the ground, dissolving the rock and creating underground voids or cavities. When the ceiling of these cavities becomes too thin or is unable to support the weight of the material above, it collapses, leading to the sudden formation of a sinkhole at the surface. This process illustrates the characteristic interplay of erosion and geological stability in karst landscapes, where sinkholes are commonly found.

The other options do not accurately describe the primary mechanism of sinkhole formation. Volcanic activity primarily results in features like craters rather than sinkholes. Erosion from water flow typically leads to surface features like valleys but does not directly involve the collapse of underground structures. Glacial retreats can sculpt the landscape but do not create the dissolution-driven cavities necessary for sinkholes to develop.