How Did the Spouting Horn on Kauai Come to Exist?

The spouting horn is a remnant of activity that has occurred intermittently between 500,000 and 3.6 million years ago.

While vacationing in Hawaii, I enjoyed seeing the spouting horn on Kauai. Would you tell me how this natural wonder came to exist?

Thomas Rosendahl
Westchester, Calif.

If you’ve stood on the natural ledge of black lava at Lawai Beach in Koloa in the southern portion of the island of Kauai, watching as waves crash against the rocks, are channeled underground, and finally spewed upward, you’ve seen a natural phenomenon that comes courtesy of ancient volcanic activity.

The Hawaiian Islands are the tips of part of a large chain of volcanoes, and the spouting horn is a remnant of activity that has occurred intermittently between 500,000 and 3.6 million years ago, according to Rick Hazlett, co-author of Roadside Geology of Hawaii. The horn is a tube, formed by lava flow, that extends into the sea, with its mouth on the rocky shore.

The action of the waves causes the spouting, which, depending on the intensity of the surf, generates spumes that the wind can reportedly blow as high as 60 feet. Rainbows often form in the spray of this miniature, oceanic Old Faithful. Some visitors have been known to pass time wagering on the height of the upcoming spray.

Lava tubes such as the spouting horn’s form when a moving stream of lava “roofs” over. This happens because of extreme heat loss to the atmosphere from the top of the flowing lava. An upper cooled “skin” is formed that, under the right circumstances, becomes the roof of an enclosed tube, according to Bruce Houghton, professor of volcanology at the University of Hawaii.

The lava under the roof is then insulated against major heat loss and so flows between the roof and the floor of the channel, slowly forming a thicker enclosing crust.

Generally, at the end of the eruption, the channel stays full of molten lava, which then slowly cools and solidifies, says Houghton. Much more rarely, the molten interior will drain and leave an open tube such as the spouting horn.

Wave action forces water into the cavity through the partially submerged opening, compressing the air already in the space. The pressurized mixture of air and water is forced out of the opening on top as a jet.

“This sort of cavity geometry is not all that common in nature,” says Houghton. “Most cracks are too thin and narrow to store an appreciable volume of air or water, and most lava tubes are not hollow.” Also, the lower opening to the system needs to be positioned exactly right with respect to modern sea level-if it were too deep below “wave base” there would be no spouting.

The hole on top through which spray passes was probably caused by erosion from waves at a time when the sea level was higher than it is now.

The spouting horn is also distinctive for the moaning sound that occurs before and sometimes after each spouting, reports Hazlett. The sound is caused by air driven under pressure through fractures in the roof of the lava tube by the water surging inside it.

Local tradition says that the horn shot higher in Hawaii’s plantation-owning past, but that when the salt spray damaged the nearby cane fields, the owners supposedly had the opening of the spout enlarged so that the spray wouldn’t carry as far. Today, a weak surf sometimes makes the spray less than sensational.