Volcano Watch — Littoral cones

A recent visitor to the Hawaiian Volcano Observatory was involved with the removal of abandoned vehicles from the area surrounding the Sand Hill cone in coastal Puna. He was puzzled by the location of an apparent volcanic vent so far away from the rift zone of Kīlauea.

Nearly everyone who sees a volcanic cone in Hawaii assumes that it is the product of high lava-fountaining from a vent. While this is true in most cases, some cones, such as Sand Hill in Nanawale, are formed by another process that does not entail a vent source of lava. This alternative cone building process is the result of the explosive interaction of a molten lava flow with water. Cones formed in this manner are usually found along the sea coast, or what was once the sea coast, and are, thereby, called littoral cones.

When a lava flow enters the ocean, water is rapidly heated and vaporized, resulting in violent steam explosions. `A`a flows, with more surface area than pahoehoe flows, heat the water faster and produce larger explosions. The molten rock is tossed skyward by the steam explosions and shatters into sand-sized particles when it cools. If this process is repeated in the same location over a long period of time, the tossed-up sand accumulates and builds a cone. The Sand Hill at Nanawale is the product of a flow that entered the ocean in lower Puna during the massive 1840 eruption of Kīlauea Volcano.

Pu`u Hou, the largest littoral cone on the island, was formed when flows from the 1868 eruption of Mauna Loa entered the ocean west of Ka La`e (South Point). The cone is nearly 80 meters high. Littoral cones generally do not last very long because of the erosive action of the ocean. Pu`u Mahana, a littoral cone located east of Ka La`e, has been eroding for some time. The flow that formed Pu`u Mahana contained abundant olivine, and this heavy, green mineral is now being deposited along the beach as the cone erodes and waves carry away the lighter glass particles. This is the source of the "green sand" beach of Ka`u.

We have observed the formation and demise of at least eleven littoral cones in the Kamoamoa area during the current Pu`u `O`o eruption. None of the cones were higher than 10 meters, and most of them were destroyed by wave activity or by the slumping of the newly-formed land surface into deeper water. Broad, black sand beaches have been formed and subsequently covered by later flows. The longshore current that flows from Cape Kumukahi to Ka La`e transports sand from the present ocean entries to the base of the high cliffs along the Chain of Craters road.

Sand of another color (white) and other interesting products of the deep scientific hole drilled near Radio Bay in Hilo will be the topic of a talk by Dr. Don Thomas, Director of the University of Hawaii at Hilo's Center for the Study of Active Volcanoes. The talk is part of the National Park Service's "After Dark in the Park" program on Tuesday evening, August 29, at 7:00 p.m. at the Kīlauea Visitor Center auditorium.

Volcano Activity Update

Current eruptive activity paused for two-and-a-half days, but flow activity resumed at 10:00 a.m. on Friday with a pahoehoe breakout near the 2250-foot elevation. During the pause in activity, the summit of Kīlauea inflated, and the number of microearthquakes increased.

A shallow earthquake located four miles southeast of Kīlauea summit was felt by Volcano residents on Thursday night at 7:23 p.m.. The earthquake had a magnitude of 3.2 on the Richter scale.