Karst topography forms in areas where the underlying bedrock is composed of material that can be slowly dissolved by water. Examples of this type of sedimentary rock include carbonate rocks such as limestone, halite, gypsum, dolomite, and anhydrite. Carbonate rocks may develop karst and other dissolution features due to the effects of circulating groundwater that has been made slightly acidic through the presence of dissolved carbon dioxide (which creates carbonic acid that reacts with the rock, dissolving it). Sinkholes and caverns can form, creating potential hazards (i.e., the land surface could subside or collapse into the underground openings). This may principally occur in areas where cavities filled with water are emptied through groundwater withdrawal or other natural processes, resulting in the cavities being filled with air and reducing support for the overlying rock. Many parts of the Southwest are underlain by karst and soluble carbonate bedrock (see Figure 9.19), especially Arizona’s Colorado Plateau and New Mexico’s Basin and Range. Because karst terrain is very porous and fractures easily, groundwater pollution can also be a serious problem. Contaminants that might otherwise be filtered through underlying sedimentary rock are quickly transported into aquifers by runoff. The hazards of pollution are increased by rampant industrial, agricultural, and residential development over karst features.

See Chapter 4: Topography to learn about the unique way in which sulfuric acid has generated karst in New Mexico.

The Colorado Plateau of northern Arizona contains extensive surface limestone and subsurface gypsum/salt deposits. As these beds dissolve beneath the surface through the movement of groundwater, sinkholes form through the collapse of overlying layers. Karst features such as open caverns also commonly form at the surface. The mountains of southeastern Arizona also contain limestone layers that have dissolved to form caverns such as Colossal Cave near Tuscon—these features are less extensive than those on the Plateau and collapse at the surface is uncommon. In New Mexico, karst is concentrated in the northern Sacramento Mountains and the Guadalupe Mountains, where a large number of impressive caverns (including Carlsbad Cavern) have formed in Permian reef limestone (Figure 9.17). Although karst collapse is less prevalent in New Mexico than in many other parts of the United States, it is still an environmental issue of concern. In Colorado, the highest karst and sinkhole hazards are located in the Roaring Fork and Eagle river valleys, where hundreds to thousands of meters (yards) of subsidence has already occurred via subsurface dissolution and deformation of evaporite rocks. Colorado’s sinkholes also form in arid and easily eroded soils, creating a landform known as “pseudokarst.”

Sinkholes are funnel-shaped depressions in the land surface formed by the dissolution of near-surface rocks or by the collapse of underground channels and caverns (Figures 9.18 and 9.20). Sinkholes can form by several different mechanisms, but all require dissolution of rock beneath the surface (Figure 9.21). Manmade sinkholes can also occur through the collapse of mine shafts and tunnels, or the removal of groundwater and oil (Figure 9.22). Sinkhole formation commonly damages roads, buildings, and utilities, and it is a problem in all four Southwestern states.

Sinkholes may be very small or large enough to swallow even hectares (acres) of land, along with any structures that had been built upon the surface. The early stages of sinkhole development may be indicated by signs of mass wasting such as “pistol-grip”-shaped trees, cracked building foundations, and leaning fence posts. Structural damage from sinkholes can be mitigated, but usually only at significant cost. It is therefore often far more prudent to avoid building in such locations altogether. Evaluating sinkhole risk commonly involves foundation testing by drilling or remote sensing (e.g., measuring electrical resistivity) prior to construction.

Figure 9.17: Carlsbad Cavern in New Mexico is an extensive cave system formed in the Permian limestone of the Capitan Formation. Uniquely, this and many other karst caves of New Mexico were formed through dissolution by sulfuric acid rather than the more common carbonic acid.

Figure 9.18: Hole-in-the-ground sinkhole, Millard County, Utah.

Figure 9.19: Areas of karst in the continental US, associated with carbonate and evaporate rocks. See Key.

Areas of karst in the continental US (Key)

Figure 9.20: Aerial view of large aligned sinkholes in the Permian Kaibab Formation, southeast of Winslow, Arizona.

Figure 9.21: Three mechanisms of sinkhole formation.
A) Dissolution: Rain and surface water percolate through carbonate bedrock, dissolving a hole from the top down.
B) Cover-subsidence: Carbonate bedrock dissolves beneath a permeable overlying layer such as sand. As the sand falls into the hole below, slow downward erosion leads to a depression.
C) Cover-collapse: Carbonate bedrock dissolves beneath an overlying layer made largely of clay. The clay collapses from beneath into the cavity below, abruptly forming a dramatic sinkhole when the surface is breached. This type of sinkhole causes the most catastrophic damage, as it is not easily detected before it forms.

Figure 9.22: This sinkhole near Carlsbad, New Mexico formed after improper solution mining practices created a large cavity beneath the desert. Three days after its collapse, the sinkhole was 200 meters (670 feet) long and 140 meters (450 feet) wide, and there are major concerns that it could expand into the town of Carlsbad.