A blue hole is a depression or sinkhole that is filled with water. There are famous ones at are underwater in oceans and more common ones located inland are associated with karst (cave) regions. The name is derived from the color of the water contained in them. They are formed by the same general process, the erosion of underlying bedrock that results in the collapse or subsidence of the surface.

Sink Holes

Sink holes are common in limestone and other carbonate rock, and salt beds. In the Midwest sink holes are common in karst regions where there is much limestone bedrock present. South of this location, in Kentucky, much of the state is karst. In Hamilton County there are few karst areas. A Ohio Geological Survey karst map identifies 3 general areas of known karst in Hamilton county. These are located at the western end of Winton Lake, North of Taylor creek, and here in Mount Airy Forest. In Mount Airy Forest the different levels of bedrock contain only 25 to 50% limestone, so it is a little unusual to have sinkholes here. But there are and they tend to be small, less than 25 meters in diameter and 3 meters deep. Virtually, none of them hold water other than the one before you.  Below is a map of karst regions in the eastern United States.

As mentioned above sink holes are formed by the erosion of the limestone bedrock.   The typical mechanism for this erosion is slightly acidic rain water. A very weak carbonic acid is created when rain passes through the air and picks up carbon dioxide (CO₂) which dissolves in the rain water.  As the rain passes through the soil it picks up additional CO_2.    The water can then work its way through cracks in the rock.  This acidic water can then dissolve the calcium carbonate (CaCO₃) in the limestone, leaving voids.  As the cracks and voids grow, the faster flowing weak acidic solution can accelerate the erosion process.  Over time the void becomes large enough that any landform above it might collapse or subside forming the sinkhole.  The chemical process is shown below.

Sink holes are often in the news when they are formed by unnatural processes (broken water main washing away soil under a roadbed) and interfere with human activities.

Natural sinkhole processes are classified as collapse, subsidence, solution, and alluvial.  Briefly, this is how the mechanisms vary.

Solution sinkholes - these form in a similar way as subsidence sinkholes except that there is only exposed rock and no overlying sediments. As water infiltrates fractures in the soluble rock, it begins widening the crack. This may form a collection site for groundwater providing that the water cannot leak through the layers of rock below, or haven?t yet eroded through to another fracture becoming subsurface groundwater. The diagram below shows a solution sinkhole, note the thin or non-existent sediment layer on top

Subsidence sinkholes occur in karst areas covered by soils or other unconsolidated materials. They appear to occur very quickly , due to the fast collapse of the overlying materials. The reasoning behind this is that most of the solution process takes place under the surface. Upon formation, a cylindrical hole is created. The hole is then eroded over a period of time into more of a bowl shape. These sinkholes are eroded and widened very easily sue to the presence of soils and unconsolidated materials located on the rims of the hole. Subsidence sinkholes are also prone to become ponds of varied sizes.

Collapse sinkholes form either from sudden mass movements of karst bedrock due to sudden drops in the water table or when the roof of an underground cavern or cave collapses, or both. Unlike solution sinkholes, collapse sinkholes can be angular in shape with vertical walls. Over time, abrasion will erode the walls and create a round or oval-shaped sinkhole. One common reason for a sudden drop in the water table is excessive groundwater pumping. Over time, collapse sinkholes often fill with water.

Alluvial stream sinkholes are created by streams sinking through deposits of alluvium on the surface of the landscape into the underlying soluble karst bedrock. The same natural processes that lead to the formation of subsidence sinkholes can also lead to alluvial stream sinkhole formation. These sinkholes are often prone to filling of water

Blue Holes

As stated above blue holes  are sink holes that contain water.  They are obviously less common than ?dry? sinkholes.  To be filled with water the sinkhole needs to be below the water table level or constantly fed with water.  Some of the more famous ones lie under the ocean.  The one shown below is in Belize and a favorite for divers.

At times the blue hole will be under a river like at http://coord.info/GC20CPJ.

At times the water in a blue hole will come from and underground stream.  That is the case for the blue holes at Cedar Sinkhole near Mammoth Cave http://coord.info/GC2QF82 

or  at Lost River Cave http://coord.info/GC2E3FB. 

These are sometimes called karst windows.  These are sinkholes that developed over existing underground streams/caves.  At times streams will disappear into or emerge from a blue hole.   Blue holes in general are somewhat circular, steep-walled depressions. They are named for the blue hue of the water. The blue holes appear blue from the sun reflecting off microscopic particles of limestone in the water.

Logging Requirements ? Please email the answers to these questions:

1)    Estimate the width, length and depth of the blue hole.

2)    What is the water supply for this hole?

3)    What type of sinkhole is this: collapse, subsidence, solution, or alluvial and why?

4)    What helps give the water in this blue hole its color?

5)    Optional: a photo of yourself at the site.