Black sand: Origins

The species I study are found in a very specific habitat. Unlike many (dare I say most) marine biologists who work in the tropics, I hardly spend any time at all on coral reefs, mangroves or seagrass beds. I estimate that since starting my PhD, roughly 80% of my dives were done over sand. No coral, no rocks, hardly any distinctive feature at all, just wide expanses of sand. Mind you, not just any kind of sand, the critters I look for seem to be found most often on black sand.

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A black sand beach in Lembeh Strait, Indonesia

For those who have never had the pleasure of diving over black sand or those used to diving in places like Egypt, Maldives or the Great Barrier Reef, the idea of a black sand beach or dive site might seem strange or hard to imagine. When reading the words “tropical beach”, most of us imagine powdery white sand, turquoise water, and a bunch of palm trees added for good measure. But I am happiest starting my dive from a beach as black as my (soul) wetsuit and dive boots. The first time you see a true black sand beach is mesmerising and even a bit alienating, it somehow doesn’t seem right. The water looks grey instead of blue, darker, and less inviting than those beautiful blue lagoons from travel brochures. Until you put your head under water and start looking around…

But why are some beaches black? What is the difference between the powdery white sand of Maldives and the pitch black sands of Lembeh or Hawaii? After the ICRS conference in Honolulu last week I decided to go have a closer look at the origins of my preferred study systems: active volcanoes! There are few places on this world better suited to do this than on Hawaii’s big island. Accompanied by Jamie, a marine scientist specialising in underwater soundscapes, I set off to find out how close to a volcano one needs to get before the hairs on your legs get scorched off 😉

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Lava flow making its way through the forest

Turns out the answer is: not very close at all. In what was the highlight of our trip we flew in an open-doors helicopter over a lake of magma and a lava stream running down a mountain. Even hovering 50m above the lava you can feel the incredible heat emanating from the stream! The Kilauea volcano is one of the most active volcanoes in the world, and has been erupting since 1983. The result is an amazing landscape of solidified lava, covering vast expanses of the island.  During our 3 day stay in the Volcano National Park, we visited lava tubes, hiked over solidified lava lakes, were awestruck by the raw spectacle of watching a volcanic crater at night, and got sunburned ambling across a frozen lava wasteland that was a village until only few year ago. While we were in Hawaii, there were no lava streams entering the ocean, but there were plenty of places where it recently did.

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Fresh lava flow in the ocean

Liquid lava flowing in the ocean is one of the two ways black sand beaches can be formed. The lava is so hot that when it runs into the cold water, it causes explosions that shatter tiny fragments of volcanic sand/glass around. This process can happen so fast that a black sand beach can be formed overnight! The size of these fragments can be from small boulders to actual fine sand, dependent on the temperature of the lava and the water. The dark colour of the sand is caused by the mixture of minerals inside the lava, the minerals giving the darker colours are mostly iron oxides such as magnetite.

The other way black sand beaches are formed, is through erosion of black volcanic rocks by rivers, which then carry the black grains of sand down to the ocean. Depending on which other types of rock the rivers flow over other minerals will be added to the mix, resulting in sand colours that can range from pitch black to dark brown or even beige or green. For any geology-inclined readers, this site has got all the details you could possibly want to know about black sand.

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Black sand + dive boots

So to summarise: black sand ecosystems are formed by some of the most epic geological processes in the world, magma from the centre of the earth erupting and shaping the world as we know it. The powdery white sand beaches of Maldives on the other hand, are mostly formed by the build-up of whole lot of parrotfish poop***. An interesting process for sure, but give me an epic black sand beach over a pile of fish poop any day 😉

 

***Technically speaking parrotfish poop, eroded corals, shells and bits of other stuff, but I’m taking some freedom here 😉

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