One of our divers asked about the Roatan ironshore and we just so happen to have an expert answer from Viky Sare, a geologist at Chevron and an avid diver.
Her response follows:
Lonely Planet says, “The jagged black shoreline formation Bay Islanders cal “ironshore” is actually elevated fossil coral reef dating back more than a million years to the Pleistocene epoch. This limestone composition is more commonly known as karst.” That pretty much nails it – but there is more to the story.
“Karst” is a blanket term for all the features that result from solution of a limestone terrain. That solution occurs because limestone slowly dissolves when exposed to fresh water, such as rainwater, which is slightly acidic. Calcite, the main mineral in limestone, does not dissolve in seawater. But karst includes many different landforms – caves (like Carlsbad Caverns), towers as in Vietnam and China, and cenotes (sinkholes), most famous in the Yucatan and Florida, to name a few.
The ironshore karst is a special type, both in terms of its texture and origin. First, texture – it is a black, random spongework of pits, jagged ridges and pointed pinnacles. It is developed in a narrow strip just above the tide line on ancient limestone rocks. If you break off a piece, you will see that the black color is present near the surface and it grades to gray inside. Second, origin – it is being dissolved largely by algae, bacteria and fungi so it is often called “biokarst.” These tiny (microscopic) organisms bore into the rock and dissolve the calcite crystals. They are most dense within a centimeter or so at the surface, causing the black color. Why this results in the characteristic macro-texture is not fully understood, but you always see the two elements together so they must be linked somehow. It is clear that proximity to the marine environment is key. Maybe the splash, spray and mist from the waves breaking provide just enough moisture to sustain the algae.
So, when you travel around the Carribean, look at the rocky shoreline, if it is limestone, biokarst may be well developed. The most famous occurrence is at a place called Hell on Grand Cayman. I have seen it in the San Blas Islands of Panama, the Bahamas and Bonaire. It has also been reported from Morocco, Hawaii and Ireland (in caves). Pretty cool, isn’t it?
A bit of a diversion – just inshore from the ironshore strip, the same limestone is light gray and forms a smoother, lumpy terrain with visible coral heads and marine shells. You can even identify common modern coral species in the rocks; there is a beautiful pillar coral at Cocolobo just to the left of the tables on the way to the pool, check it out. This ancient limestone reef was deposited more than 1.25 million years ago when the sea level was relatively higher. You can find this Pleistocene limestone throughout the Caribbean from the Bahamas to Bonaire. While it is likely that the global climate was warmer during this period (glaciers must have melted); the region’s climate must have been similar to today’s to support the coral growth. Interesting, isn’t it?
Rumor has it that the guys at Bananarama are telling divers that the ironshore around toward Pablo’s Place is lava from a volcano. While there are some visual similarities to lava (black, irregular surface), that is probably not the case. If it was lava, there should be a volcano in the vicinity, usually directly connected to the lava via a flow that follows topography from the source. While there are volcanic rocks in Honduras (you can see a nice volcanic peak on the mainland skyline), I did not see evidence of recent volcanism on Roatan. If it is ironshore, it follows the coast and the source is the sea – it is limestone, not the basalt rock that makes up lava. To prove it – drip weak hydrochloric acid on the rocks – with basalt, nothing happens, on limestone it fizzes as it quickly dissolves the calcite. That is the ultimate defining test. When Will and I come back, maybe we can all go dive over there and check it out ourselves.
Well Marco, I hope this is interesting to you and that you are having a fabulous time in your tropical paradise! Drop me a line from time to time and tell me of your amazing adventures and sightings. Regards to all on the deck.
Cheers – Vickey
So there you have it. Up next, from another brilliant mind and also excellent diver, what in the world is the string of pearls?