Volcano Watch — Lava explodes into the sea at Kamoamoa

Lava erupted from the episode 51 vents has been entering the sea at Kamoamoa since November 8. On Tuesday, a new phenomenon was observed. For the first time, large steam explosions and jetting of steam and lava occurred and formed a small cone about 25 feet tall and 25 feet inland from the advancing flow front. Although small explosions are common when lava enters the sea, during the entire 10-year duration of this eruption, such large explosions had been observed only once before, when lava was entering the sea near Kalapana nearly five years ago.

The cones formed by such explosive interaction of lava and seawater are called littoral cones; there are around 50 along the coastlines of Kīlauea and Mauna Loa Volcanoes, but there are none on any of the older volcanoes in Hawai`i. The map shows the approximate locations of the known littoral cones on Hawai`i. Their ephemeral nature is due to their being constructed mainly of unconsolidated sand and their proximity to the ocean and the erosive action of the waves.

These cones look like eruptive vents, and their activity is similar to that observed at such vents, particularly early in an eruptive episode, when high fountains commonly occur. However, littoral cones occur far from the eruptive vents where lava flows - either confined within a lava tube or channel - enter the sea.

The activity of Tuesday was characterized by lava fountains as high as 300 feet and exploding bubbles of lava as large as 50-75 feet in diameter. These lava bubbles produce paper-thin glass sheets, called "limu o Pele" (Pele's seaweed), that can still be found to the west of the newly formed lava delta at Kamoamoa. The main activity occurred during the evening hours and produced a spectacular show, seen clearly even from the Chain of Craters Road nearly one-half mile away. By about 9:00 p.m., the large explosive activity was over, though many visitors stayed the night, hoping for additional activity.

Most large littoral cones are composed mainly of small air-borne particles of glass, called "tephra," that accumulate in arcuate deposits behind the point where the lava enters the sea. These cones are usually associated with large `a`a flows rather than tube-fed pahoehoe flows, like those now entering the sea at Kamoamoa. The cones that formed Tuesday evening consist mostly of long ribbons of lava that stuck together to form the cone. These ribbons were still molten when they hit the ground and were not thrown far from the vent.

Of the many older littoral cones around Hawai`i, only Pu`u Mo`o on the flank of Kīlauea near the Great Crack is composed mainly of lava ribbons. On Tuesday, the tephra that formed and the impressive plume of laze (lava haze consisting of steam and hydrochloric acid) were blown away from shore by strong trade winds. Had the winds been onshore or lighter trades, the cones formed would have consisted of alternating layers of tephra and ribbon bombs.

Some of these older cones are truly impressive in scale and must have formed from high fountains over several weeks of continuous lava entry into the sea. For example, the largest such cone is Pu`u Hou, located on the coast of South Point. It formed when `a`a lava from the 1868 Mauna Loa flowed into the sea. The main `a`a channel is located between two cones, the 240-foot-high Pu`u Hou (new hill) to the southeast and another hill 118 feet high to the northwest. Photographs taken in 1924 show the cones to have been much larger; erosion by the waves has removed a significant part of the original cones.

In historic times, only three littoral cones have been formed, including the 1868 cones described above. The 1840 flows from Kīlauea Volcano entered the sea about five miles west of Cape Kumukahi and formed large cones of tephra. Only small remnants of these once-large cones remain today; most having been removed by wave erosion. The only other historic cone formed when the 1919 Mauna Loa flow entered the sea about 25 miles northwest of South Point. Like the other historic cones, most of this tephra and cinder cone have been removed by wave erosion.

Judging by the short life spans of the historically produced cones, special conditions must occur that allow some of the older and smaller cones to be preserved. Examination of the approximately 28 small littoral cones along the coast northwest of the 1868 Pu`u Hou cones suggests that the flows that produced them progress farther seaward, surround the cones, and are more resistant to erosion by the waves.

It is not certain that the newly formed littoral cone will be preserved. The lava bench on which the cone is built is unstable and may slump into the ocean, carrying the cone with it. If the eruption continues, subsequent lava flows could also bury it. On the other hand, it seems likely that more small cones may form as lava continues to pour into the ocean and the tube system consolidates to one or two main lava entry points.