Geologic Hazards: Sinkholes

In wake of a recent tragedy near Tampa, FL last week, the country reels back and realizes that sinkholes can become deadly.  There are three known deaths in Florida due to sinkholes throughout the state's history, so fatality isn't really associated with this phenomenon.  However, loss of life can and does occur.  What is more, sinkholes are associated with karstic topography, thereby making up 10-15% of the total Earth's surface that are prone to these hazards, as the purple areas on the Circle of Blue map shows. 

Map taken from "Circle of Blue"

The United States Geological Survey (USGS) define sinkholes as, "an area of ground that has no natural external surface drainage--when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface. Sinkholes can vary from a few feet to hundreds of acres and from less than 1 to more than 100 feet deep. Some are shaped like shallow bowls or saucers whereas others have vertical walls; some hold water and form natural ponds. Typically, sinkholes form so slowly that little change is noticeable, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic effect if it occurs in an urban setting."

Map taken from: Northeastern University

On the map from Northeastern University, the green colors (both light and dark shades) represents areas with carbonate rocks underlying the ground.  This is typical of karstic topography, because karstic rock is sediments such as limestone and dolomite embedded into the bedrock.  These dissolve with rain water because the acid in the precipitation reacts with the carbons in those rocks.  This explains the large amount of caves inside karstic areas.  Note Eastern United States and the look at Florida.  The Ozarks of the Central US has also this danger.

Image taken from CBS News

Tropical storms and hurricanes can dump a lot of precipitation on a localized area.  If underneath that area lies a limestone cavern, the top of that cave could collapse, causing these sinkholes.  In June of 2010, one such storm did just that in Guatemala, causing a deep sinkhole.  A photograph from CBS News looks "photo-shopped," but is indeed an accurate picture of what happened there.  A massive hole like that could definitely swallow automobiles and buildings, with damages excessive in terms of money and human lives.  

Urban sprawl is also a culprit in the making of sinkholes.  When urbanized regions become really built up, the land has to support more weight.  The bigger the cavern underground, the less support the ground has.  The ground sinks because it cannot support the weight above the surface.  Because of these urban sprawl and tropical storms along with having a karstic underground, Florida more sinkholes than any other state.

Image taken from USGS

A big concern of sinkholes is flooding.  Often times a new pond that just appears is a sign that a sinkhole is on its way of being formed, because a depression in the land occurs before the collapse.  If someone is building a house on level ground that person may not know that he or she is building upon a future sinkhole.  Some time later, a depression forms and rainwater will naturally collect there, having no other place to drain off to.  This causes more weight upon the unstable surface and a positive feedback is made, where one thing leads to another and then the inevitable occurs.  The ground literally caves in, slowly at first, but more and more things happen that will quicken the process.

Image taken from "About Geology"

Flooding also occurs because of what is happening underneath the surface.  Rainwater dissolves the carbonates, forming caverns underneath the ground.  As a result, there is no place for the water to escape to and gets trapped down there.  Through the process of erosion, water essentially gets blocked in with no drainage to get out.  When a sinkhole forms, a pond may appear as water comes back above the surface due to the sunken ground, called subsidence, forces the water up.

Because depressions are a usual sign of a sinkhole, people naturally uses these geological features as a pit to throw garbage in.  This is also an environmental hazard because it could very easily contaminate the local drinking water supply.  This is why many communities have laws regarding what and where one could dump wastes. This is also a reason why many landfills today in America are heavily regulated.  Illegal dumps can also cause blockage of water, so it cannot drain out.  The unsupported ground has that much more weight and collapses.  Again, positive feedback is in play here.  

The scientists at "How Stuff Works" have this to say in how to detect if a sinkhole may be forming around the house that one is buying.  "Property damage, especially around the foundation, may be a subtle sign that something is wrong. In addition, if vegetation is dying unexpectedly, a sinkhole may have disrupted underlying streams that usually feed the plants. Other signs to look for include polluted or muddied well water, formation of new ponds and trees, or sign posts that appear to be slumping over."

Image taken from "How Stuff Works"

Of course, no one should buy property to live on if the neighborhood looks like swiss cheese!  Often times though, potential geologic hazards do not present themselves as obvious as the above image.  If it did, life for many would be much smoother.  Yes, that is safer, but ultimately, where is the fun in that?  Oh, yeah.  investments and capital interests.  Got it.  Maybe information such as this is important after all!