Watling’s Blue Hole

This tiny lake is a feature on the Bahamanian island of San Salvador.

Watling’s blue hole is a karst feature, produced by the erosion of limestone. Limestone is made of calcium carbonate, a mineral that can dissolve in water, particularly in acidic water. Most rainwater is lightly acidic as it picks up CO2 from the atmospere to make carbonic acid; sulfur in the atmosphere can also create stronger acids and acid rain. When it rains on limestone, that little bit of acidity cause the limestone to dissolve, opening a hole or a gap or a low spot in the limestone where more water begins to pool and flow. As the water pools, it concentrates the acidity and dissolves the limestone more rapidly at that spot. The more water that flows into a spot, the more limestone it dissolves - opening up holes in the ground and widening cracks into caves.

Watling’s blue hole has an interesting water chemistry. On small islands, there is interplay between rainwater and seawater – fresh rainwater falls on islands and pushes the denser seawater out of the way, but also leaves mixed, brackish water in-between. Despite sitting on a rainy tropical island like that, Watling’s blue hole has no fresh water anywhere, not even a tiny lens on top, so this hole must link all the way to the ocean to let salt water flow in constantly to dominate the system.

This hole likely formed when sea levels were lower during the last ice age. It would have sat as an open cave at that time, with water draining into it and flowing out to the ocean. Rising sea levels then brought ocean water into that cave, flooding it and creating this blue hole.

-JBB

Image credit: James St. John https://flic.kr/p/q9spdK

Read more: https://www.bgs.ac.uk/mendips/caveskarst/caveform.htm

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