# Volcano Watch — Steamer passenger recorded unusual eruption

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In February 1877, an unusual eruption occurred on Mauna Loa Volcano. Part of the eruption was submarine, and the eruptive fissure was neither at the summit nor along one of Mauna Loa's well-defined rift zones.

In February 1877, an unusual eruption occurred on Mauna Loa Volcano. Part of the eruption was submarine, and the eruptive fissure was neither at the summit nor along one of Mauna Loa's well-defined rift zones. It lasted only six hours on February 14 and occurred in Mokuaweoweo Crater, much like many other Mauna Loa eruptions. However, on February 24, the eruption migrated 22 miles from the summit to a series of fissures located offshore from Kealakekua Bay. At the time, only the shallowest of three eruptive fissures made itself known by the occurrence of steam and floating lava blocks within the bay.

The interisland steamer Kīlauea fortuitously arrived in Kealakekua Bay on the morning of the 24th, and a passenger wrote a detailed description of the submarine eruption in the Hawaiian Gazette, published in Honolulu on February 28th. The eruption was first observed about 3:00 a.m., when red, green, and blue lights appeared about one mile offshore. Daylight revealed steam and lava blocks on the ocean's surface along a west-northwest-trending line that extended one mile offshore, where the water's depth is about 120 to 360 feet. The surface of the ocean in the most active area appeared agitated, as though boiling, and many blocks of solid lava as large as two feet across rose from below. The lava blocks floated readily while hot and emitted sulfurous gases (probably mostly sulfur dioxide) but sank after cooling. Numerous fish were killed by the activity. The locations of the three submarine fissures are shown on the detailed map.

A strong earthquake, felt during the night of the eruption, may have signaled movement along the Kealakekua fault that allowed magma to enter fractures and erupt on the sea floor. The two deeper eruptive fissures were discovered during submersible dives using the U.S. Navy's Sea Cliff in 1975. The shallowest of the three ventareas was explored using the NOAA submersible Makalii, operated by the Hawaii Undersea Research Laboratory, in 1983. Laboratory analysis of the lavas recovered during these submersible dives reveals that the lavas were indeed submarine-erupted, as the chilled glass still contains dissolved gases that are lost to the atmosphere when lava erupts subaerially.

Eruptions on Mauna Loa Volcano occur not only at the summit and along two main rift zones that trend towards the southwest and the northeast, but also from radial vents on the north and northwest flanks of the volcano. The 1877 eruption in Kealakakua Bay was such an eruption, as was the 1859 Mauna Loa eruption that sent lava to the Kona coast north of Kiholo Bay through the saddle between Hualālai and Mauna Kea Volcanoes. The accompanying map shows the location of known radial vents on Mauna Loa Volcano, including those in Kealakekua Bay. By contrast, all known eruptive vents on Kīlauea Volcano, including the currently active eruptive vent on the East Rift Zone, occur in the summit area and along the two well-defined rift zones that extend towards the east and the southwest.

The area of vents, including the summit and rift zones of the active volcanoes Mauna Loa and Kīlauea, is defined as lava-flow hazard zone 1. The north and northwest flanks of Mauna Loa are included in hazard zones 2 and 3 because of their low areal coverage by flows in historic times and because the eruptions from radial vents on Mauna Loa occur only occasionally. However, this area, where eruptions have occurred along radial fissures in the past, could be subject to ground cracks, subsidence, and eruptive fissures that are otherwise restricted to hazard zone 1.

### Volcano Activity Update

The 10-year-long eruption at Kīlauea Volcano started up again during the morning on February 16, after a period of inactivity lasting about 10 days. The eruption, which is once again at the episode 51 vents on the west flank of Puu Oo, has been passive, with only small volumes of pasty lava flowing through the pre-existing tubes. By Thursday afternoon, the lava had advanced to the 1,900-foot level, where sluggish pahoehoe flows broke out of the tube system. A small lava pond once again occupies the Puu Oo Crater and produces a glow at night. The ground-vibration recorded near the vent (tremor) is of a much smaller amplitude than we have seen before during an eruptive period, and is consistent with our observations of the small volume of the flows and the passive nature of the eruption.