SURFACE COLLAPSE
AND SUBSIDENCE

Vicki M. Harder

Introduction

Subsidence is the sinking or collapse of a portion of the land surface. The mechanisms of collapse, and sometimes the conditions existing before the collapse, result from natural physical processes. Groundwater facilitates ground collapse through dissolution of limestone and other carbonate rocks. These rocks are nearly insoluble in pure water but are readily dissolved by carbonic acid, a common constituent of rainwater. The weathering attack occurs mainly along fractures and other partings and openings in the carbonate bedrock. The resulting features include caves, sinkholes, and karst topography. In many cases the conditions leading up to a subsidence event are exacerbated or even created by human actions.

Caves, Sinkholes, and Karst Topography


Cave formation begins with dissolution by percolating groundwater along a system of interconnected open fractures and bedding planes, developing along the most favorable flow route. The rate of dissolution and cave growth increases with increasing veolocity of groundwater flow. It has been estimated that the development of a continuous passage may take up to 10,000 years and a fully-developed cave system and additional 10,000-1 million years.


Carlsbad Caverns, NM
 In contrast to a cave, a sinkhole is a large dissolution cavity that is open to the sky. The formation of sinkholes can be due either to the sudden wholesale collapse of the roof of a cave (stoping) or by a more gradual downward movement of unconsolidated material into an open, chimneylike passageway (raveling) . Raveling eventually leaves the roof materials unsupported; surface fractures begin to develop and the roof eventually collapses.

Both caves and sinkholse are enlarged whenever groundwater levels are high, but the pressure of the water in the caverns and passageways helps support the weight of the overlying rocks. Collapse results from the lowering of the water table due to drought and/or excessive pumping of water wells which leaves underground spaces and passageways unsupported, facilitating the collapse of the overlying rocks.

The terrain that is characterized by the presence of caverns and sinkholes is called karst topography. The most common type of karst terrain is sinkhole karst, a landscape dotted with closely spaced circular collapse basins of various sizes and shapes. Several factors control the development of karst landscapes:

  • The topography must permit the flow of groundwater through soluble rock under the pull of gravity.
  • Precipitation must be adequate to supply the groundwater system
  • Soil and plant cover must supply an adequate amount of carbon dioxide (to make carbonic acid from rainwater)
  • Temperatures must be high enough to promote dissolution

Sinkhole karst.

Exercise

The Edwards Aquifer Homepage 		DIRECTIONS: Click here to go to the questions page. Once there, print out the page. It will help you to follow the directions and answer the questions. Once you have a printed copy of the questions you will be ready to complete the exercise. Click here to go to The Edward's Aquifer Homepage Link.

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