VOLCANO WATCH – USGS Hawaiian Volcano Observatory

Do you, like most people, think that eruptions in Hawai`i are confined to the Island of Hawai`i? If so, you may be surprised to learn that Haleakala volcano on Maui has erupted at least once in the past 500 years and will probably erupt again.

While most eruptions in the Hawaiian Islands over the past several hundred years have occurred on the Islands of Maui and Hawai`i—not all of them have.

Submarine eruptions west of Maui have been reported three times in the past 60 years. The first was in August–September 1955, near Necker Island (650 km or 410 mi northwest of Honolulu). The most recent was in early 2011, when a sea captain reported “steam rising up violently from the sea surface” in the vicinity of Milwaukee Bank, more than 3,000 km (almost 2,000 mi) northwest of Honolulu. No samples were obtained, so volcanic activity could not be confirmed. Neither location was expected to host volcanic activity ever again.

The most intriguing possible submarine eruption was reported much closer to O`ahu. On May 22, 1956, while flying low over the Kaua`i Channel, the pilot of a military transport reported “a square mile of boiling water with sulphur and ashes on the waves.” His crew reported smelling sulfur fumes hanging over the area; however, subsequent flights over the same area reported only yellow streaks in the water. By May 24, all signs of an eruption seemed to be dissipating.

This event was unique, because some pumice washed up on O`ahu beaches on May 28. A few samples were sent to Gordon Macdonald (the Hawaiian Volcano Observatory’s Scientist-in-Charge at the time) for identification. He described the fragments as “dark brown to black basaltic pumice…fresh and uneroded, and tests revealed that they would float for only a few hours.” Based on his examination of the pumice, Macdonald surmised, “A definite assertion is not possible, but the evidence appears to me to indicate a submarine eruption.”

The events of May 22 were most likely located along a submarine ridge extending northwest from Ka`ena Point, O`ahu. In recent years, University of Hawai`i and USGS scientists have explored this region in detail and found no evidence of recent eruptions on the ocean floor. In a summary of these findings, John Sinton, a long-time geologist with the University of Hawai`i, says that the Ka`ena Ridge “is now known to be a precursor volcano to the island of O‘ahu. It mainly was active from more than 4.5 to about 3.5 million years ago and is no longer considered to be active. There is a field of younger lavas that mostly lie in deep water between O‘ahu and Kaua‘i and drape the southern side of Ka‘ena Ridge. Some of these have been dated, yielding ages as young as about 340,000 years.”

So, what about the possible submarine eruption in 1956? Pumice washing up on Hawaiian beaches was fairly common in the 1950s, and much of it was recently identified as originating from a 1952 eruption in the San Benedicto Islands off the west coast of Mexico. Could the pumice collected on O`ahu in 1956 have originated from that eruption?

To be sure of its origin, we need an actual sample of the 1956 pumice. Unfortunately, all we have is Macdonald’s description of it being “brown to black.” But the 1952 San Benedicto pumice was described as “greenish brown.” Another difference is that the 1952 pumice had no crystals, while the 1956 samples had crystals of olivine and labradorite—minerals that are common to Hawaiian eruptions. From this information alone, we can conclude that the 1956 pumice was not from Mexico and was more likely erupted from a Hawaiian volcano.

The certainty of a Kaua`i Channel submarine volcanic eruption in 1956 will elude us unless we can retrieve a sample of recent lava from the sea floor—or unless another eruption occurs in the channel. Until then, keep your eyes peeled for signs of a submarine eruption the next time you fly to Kaua`i!