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Geo-Joint: Parrotfish and Groundwater

Posted on January 06 2016


Ah, Hawaii, that endless font of fascinating geographical oddments. For all the damage that thousands of years of invading species and tourists have done to that paradise, it still manages to be a wonderous locale. The lava that pours up from the depths builds the basis of the islands, but the erosion of rain and waves grinds the lava into sand and soil. The soil is often quite red due to the iron content of lava, and the beaches can be an exotic black color. One of the most famous, the Black Sand Beach at Kaimu near Kalapana on the Big Island is sadly gone, covered by Pele’s relentless outpourings. But many beaches in Hawaii are off-white or tan, similar to those on the mainland. Unlike the mainland though, those colors don’t come from quartz and feldspar grains, but from the disintegration of coral. The pounding surf of the islands provides plenty of power to break up dead coral and render those calcite skeletons into sand, but there is another factor at work creating the stuffs of Hawaiian beaches.

The parrotfish, called ulu in Hawaii, is a multi-hued resident of the coral reef that can grow to four feet in length while eating rocks and coral. If that sounds like hard work, it is, but the parrotfish has a prodigious set of teeth at the front of its mouth, and more grinding teeth in its throat. It’s not really interested in those hard elements it’s eating, but is actually trying to ingest the algae that live on them. The reefs, though they lose a chunk of their coral with each bite, are in better shape thanks to the parrotfish, for without them the algae would take over and suffocate the coral. And the the parrotfish does another good turn – it poops out ground up coral and rock as sand, which builds beaches. You might think it doesn’t amount to much, but it’s calculated that each fish provides as much as 800 pounds of sand per year – one Australian species can produce a ton! Other animals like urchins and worms contribute similarly to the sand supply, but parrotfish do the most. Just about all the sand at those expensive tropical resorts has been eaten, and excreted, by parrotfish.

While the parrotfish are busily working their, um, butts off to make more ground for Hawaii, another surprising process is taking it away. We know that Hawaii is fed by the hotspot magma source and that as the tectonic plate moves northwest, new ones are formed, and the older ones, no longer fed, erode away. That erosion is not surprising given the torrential rain the islands receive, and the active surf chewing at their edges. But studies have shown that the rainwater, beyond it’s physical erosion is also chemically dissolving the hardened lava once it soaks in and becomes groundwater. In fact, chemical erosion does more to wear down the islands than the surface-flowing rainwater. Oahu, while now off the hotspot, will see its mountains grow due to tectonic uplift for another 1.5 million years, but then begin to subside as the dissolving action deflates it to a profile like those of the far-flung northwest Hawaiian chain. In other words, flat. By then, Honolulu’s setting won’t be as picturesque, but don’t worry – subsurface Loihi, now feeding off the hotspot, will probably have built itself into a new and dazzling tropical island paradise.

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