Warm Geothermal Springs on Puako Reef

by B. N. Sullivan

The Big Island of Hawaii, where we live, is volcanically active. Kilauea Volcano, in the southern part of the island, has been erupting continuously since 1983, sending lava down its flank and into the ocean. Mauna Loa, another active volcano on the island, erupted most recently in 1984, and could erupt again at any time.

The coastline at Puako, where we dive most often, was formed by an old lava flow from Mauna Loa. The layers of hardened lava flows run well beyond the shoreline, extending seaward below the surface of the ocean to form the volcano's submarine flank. The old lava flows also form the substrate for the wonderful coral reef that fringes that particular section of the western coast of the Big Island.

Molten rock -- called magma -- moves along constantly underground beneath the volcanoes, and far below the reef and the sea floor. Here and there, cracks have formed in the substrate beneath the reef. When that happens, sea water seeps down and mixes with hotter fluids closer to the magma. That warmed sea water then rises back up toward the surface.

About 15 years ago, we discovered a couple of spots on the reef at Puako where this geothermally heated water rises up through chimney-like vents in the reef, creating warm springs. The photos on this page show one of those vents. I know it doesn't look very impressive, especially in the first in the photo, but we find these sources of volcanically heated water on the reef quite fascinating. [Click on any of the photos for a larger view.]

We first noticed this one, and another about 50 meters away, by their appearance. These vents are relatively small, but they are eye-catching to divers. They are situated in a broad coral garden dominated by a certain species of finger coral (Porites compresa). In fact, in that area, that species of coral is so dense that it uniformly and completely covers the substrate for acres.

Amidst that large stand of finger coral a couple of spots stand out, because they look different. There are deep holes, literally, in that dense forest of finger coral, and the holes are surrounded by a crust of very different looking corals.

It was the unexpected interruption of the field of finger coral that first caught our eye. We swam over to have a better look at these anomalies, and that's when we discovered that the crusty looking corals actually were arrayed around holes in the reef. When we hovered directly over the holes to peer inside, we felt the warm water emanating from below. We understood right away that these holes were venting geothermally heated water. It was an exciting find.

On one occasion, we showed the vents to a marine biologist friend who was visiting and diving with us. When she returned to Honolulu, she in turn told some oceanographer friends about these geothermal vents in Puako Reef. The news traveled through the grapevine, I guess, because we then got a call from someone at NOAA who asked if we would be willing to serve as dive guides to some researchers who wanted to have a look at the vents the next time they visited the Big Island.

We agreed, of course, and a party of three divers arrived in Puako several weeks later. We led them to the vents, where they took some photos, and some water samples. Once we were back on shore, they said these vents probably were not very important. In any case, they did say that they would let us know what was in the water samples, but then we never heard from them again.

We concluded from the silence that, indeed, 'our' little geothermal vents were not scientifically important. Still, we always have enjoyed this little underwater curiosity on our reef.

In this final photo, at left, our friend Dan is poking his head and shoulders into the larger of the two vents. I had asked him to pose for a photo near the mouth of the vent, to provide a reference for the size of the opening. Dan decided spontaneously that this posture would demonstrate the size better than simply posing beside it! (Thanks, Dan.)

In fact, the photo also demonstrates something besides the size of the vent opening. It should tell you that the water emitting from the vent is warm, but not hot, else Dan would not be able to actually enter the space like that! Using the temperature indicators on our dive computers, we have measured the temperature of the water coming out of the vents on many occasions. It varies, but always is in a range between 27-29 C (about 81-83 F). In other words, it is always several degrees warmer than the ambient water temperature, which typically ranges from about 24 C (75 F) in late winter to 27 C (80 F) in late summer.

Some of the reef critters seem to have discovered these vents. On numerous occasions we have spotted sea turtles hanging out around the edges of the vents. We also have seen any of several species of crab, and even the odd slipper lobster, snuggled down into the hole, clinging to the stony side of the vent, and at times we have seen clusters of sea urchins ganged up along the rim.

We, too, have taken advantage of this relative warmth, especially after long, deep dives. Making our way to our exit point, we can choose a route via one of the vents, and take turns hovering in the warm water for a few minutes. Ahhh.