August 25, 2014

Inside the science of sinkholes

by Bonnie Denmark

Sinkholes aren’t new, but they certainly make news, causing millions of dollars of damage, contaminating water supplies, and even claiming lives. This latest monster under the bed appears often without warning and with potentially catastrophic results, as evident in a sampling of actual headlines:Ground swallows man sleeping in his bed,” “80-foot-wide sinkhole swallows two Florida homes, along with boat and swimming pool,” “Sinkhole swallows man, 80, as he goes for morning newspaper,” “Car plunges headfirst into hole after driveway crumbles, swallowing Long Island woman,” “Sinkhole swallows pricey Corvettes at hallowed museum.”

Where there is one sinkhole, there are apt to be more. And once they are opened, they can continue to expand. But just how do they come about?

Because limestone is soluble, it is eroded by groundwater, leading to sinkholes. (Credit: Wikimedia Commons, US Army Corps of Engineers)

Because limestone is soluble, it is eroded by groundwater, leading to sinkholes. (Credit: Wikimedia Commons, US Army Corps of Engineers)

Sinkholes occur in what is known as “karst terrain,” which covers approximately one fifth of the continental US. Karst terrain is found where porous rock under the surface can be dissolved by groundwater. This soluble bedrock is formed from salt beds, limestone, dolomite, and other carbonates. As underground rock erodes, caverns develop beneath the surface. The holes can increase in size while the ground on top continues to hold together, often giving no indication of an impending cave-in until the hole becomes too large to support the ground above.

Damaging sinkholes are a real concern in many parts of the US, particularly in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Florida often makes sinkhole news because the bedrock there is mostly limestone (calcium carbonate). But sinkhole damage is not confined to the states listed. Earlier this month, record-breaking rains opened up “two dozen medium to large sized sinkholes” on Long Island, NY, as reported by Suffolk County Highway Superintendent Dan Losquadro to CBS-2 News (televised August 15, 2014).

Sinkholes often appear after heavy rains, but droughts and freezes can trigger them as well. Groundwater pressure helps support surface soil; when droughts deplete underground water levels, the surface is more likely to collapse when a drenching rain finally comes, weighing down poorly supported ground material until the surface crumbles.

Sinkhole in West-Central Florida, from Freeze Event of 2010 (Credit: US Geological Survey, Department of the Interior/USGS. Photo by Ann Tihansky)

Sinkhole in West-Central Florida, from Freeze Event of 2010 (Credit: US Geological Survey, Department of the Interior/USGS. Photo by Ann Tihansky.)

Likewise, freezing temperatures, especially when accompanied by groundwater extraction, can lead to sinkholes. More than 110 sinkholes opened during a major freeze event in Florida in 2010. Aurit, Peterson, & Blanford (2013) found “a strong correlation” between sinkholes and sub-freezing temperatures coupled with groundwater pumping. The authors suggest that much sinkhole damage can be circumvented through a water management plan since human activity is often a catalyst. For example, many sinkholes have been traced to pumping groundwater for agricultural purposes. Such was the case in the freeze event, when there was a sudden rush on the groundwater supply as farmers tried to protect crops from cold temperatures. Other human causes of sinkholes include abandoned mines, water main breaks, leaking drain and sewer pipes, drilling of wells, blasting, and industrial runoff.

9.75-meter (32-foot) deep sinkhole in Georgia Tech parking lot, Atlanta, GA, formed as rainwater leaked through pavement and into break in sewer line below. (Credit: Scott Ehardt, Wikimedia Commons)

9.75-meter (32-foot) deep sinkhole in Georgia Tech parking lot, Atlanta, GA, formed as rainwater leaked through pavement and into break in sewer line below. (Credit: Scott Ehardt, Wikimedia Commons)

It is impossible to predict exactly where and when sinkholes will occur. They generally provide no warning, but the following may be clues that a sinkhole is forming (offered by the Lake County, Florida, Department of Public Works):

  • holes or concentric cracks in the ground
  • sagging fence posts or trees
  • newly exposed areas at the bottom of fence posts, trees, and foundations
  • cracks in the foundation of a building
  • other signs of shifting ground, such as a ceiling separating from the wall or improperly closing doors and windows
  • cloudy water pumped from a source that was previously clear
  • standing pools of rainwater in unusual places

Local government offices should know which areas have soluble rock underneath. Although a pricey option, private geoengineering companies can be hired to survey property using ground penetrating radar or other electromagnetic equipment. And if a potential sinkhole is detected, geotechnical engineers may be able inject materials to strengthen the foundation provided that the hole forming is of a manageable size. Knowing what kind of rock underlies your property may be the best place to start. You can access a USGS karst terrain map here that shows which areas of the US are at greatest risk for sinkholes.

 

LEARN MORE

Read more about sinkholes in this informational webpage by the USGS Water Science School.

See a gasp-inducing collection of sinkhole photographs in The World’s Largest Sinkholes, a 2014 photo essay by US News and World Report staff.

Earth’s terrain is in continual flux. For a description of how sedimentary rock forms and is acted upon by erosion, contributing to Earth’s constantly changing landscape, read our module The Rock Cycle.

 

REFERENCES

Aurit, M. D., Peterson, R.O., & Blanford J. I. (2013). A GIS analysis of the relationship between sinkholes, dry-well complaints, and groundwater pumping for frost-freeze protection of winter strawberry production in Florida. PLoS ONE 8(1): e53832.

Sinkhole information. Lake County, Florida, Department of Public Works.

 

Written by

Bonnie Denmark holds an MA in linguistics and teacher certification in English, ESL, and Spanish. She has devoted her professional life to educational and accessibility issues as a computational linguist, multimedia curriculum developer, educator, and writer. She has also worked nationally and internationally as a language instructor, educational technology consultant, and teacher trainer. Bonnie joined the Visionlearning team as a literacy specialist in 2011, assisting the project by developing comprehension aids for science modules and creating other STEM learning materials.

The views expressed above do not necessarily represent those of Visionlearning or our funding agencies.