March 15, 1996

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Lava Ocean Entry and Bench Collapse

The 13-year-old East Rift Zone eruption of Kilauea Volcano has returned to the steady-state condition that existed prior to the dramatic eruptive surge on February 1st. The lava pond within Pu`u `O`o Crater, which sloshed up to within 130 feet of the rim on February 1st, is actively churning at the 300 foot depth, and about 75,000 gallons per minute of new lava is erupting from a vent beneath the west flank of Puu `O`o.

This lava is flowing through a 6.8 mile-long tube as it continues to feed breakouts on the coastal plain on its way to the ocean. These intermittently advancing pahoehoe flows have resurfaced approximately 370 acres of the eastern part of the flow field since Valentine's Day. However, these flows account for only a small portion of the erupted volume. Most of the new lava is pouring into the Pacific at Kamokuna, inside Hawaii Volcanoes National Park.

[to save an image of the map, click on the map]

The ocean entry at Kamokuna has been active since early September, but lava from this eruption had reached that point on the coast before. The old sea cliff was initially covered by lava in August 1994, and then again in January 1995. During most of that interval, lava was spreading across the breadth of the flow field, plumes rose from ocean entries spanning 2.5 miles of coastline, and National Park visitors had a great view from the end of Chain of Craters Road.

The tube feeding the western flows was abandoned after the August 1995 pause, and, since then, activity has been limited to the eastern portion of the flow field. Over the last six months, the eastern tube has matured into a major pipeline to Kamokuna that has survived two 24-hour eruptive pauses in November and December and a 10-day pause in early February. This situation has provided an opportunity to document processes associated with a long-lived, well-established ocean entry.

We refer to the leading edge of a recently formed lava delta as the lava "bench." The bench is underlain by loosely consolidated submarine debris made up of black sand, pumaceous lava fragments, and dense lava blocks formed or incorporated during the contact between hot, flowing lava and the ocean. Catastrophic bench collapses are caused by the combined factors of a steep offshore slope, weight-loading by the new lava flows on the bench, fractures in the lava tube that allow seawater to enter, and/or high surf conditions.

Bench collapses of variable magnitude at Kamokuna have increased in frequency over the past two months and are now occurring about once every two weeks. After each collapse, a severed lava tube or incandescent fault scarp is exposed to the surf, and violent hydrovolcanic explosions ensue. The types of explosive events observed at Kamokuna include sudden rock blasts, sustained and powerful steam-and-cinder jets, lava fountains, and spectacular "bubble-bursts."

The largest of the recent bench collapses occurred on January 31 and claimed nearly four acres of new land. A rock blast associated with this event sent hot blocks nearly as large as a bale of hay over 900 feet inland. The collapse triggered a small tsunami that swept breadbox-size chunks of rock more than 100 feet onto the shore.

Among the more spectacular pyrotechnic displays at Kamokuna was that following a major bench collapse on January 20th. Lava bubbles up to 70 feet in diameter repeatedly formed and burst on the inland side of the entry. Steam jets laden with hot lava fragments rose in a 500-foot-high rooster-tail, and lava fountains reached a height of 300 feet. The spatter and lava fragments from this fountain piled up to form a littoral (on or near the shore) cone 20 feet high and 30 feet in diameter. Twelve such littoral cones have formed on the Kamokuna bench since last October, and all have been claimed by the sea in subsequent bench collapses.

As we have stated in many previous Volcano Watch columns, the place where lava enters the ocean is a very dangerous one, indeed. Bench collapses and the ensuing hydrovolcanic explosions occur without warning. The bench collapse in April 1993 took the life of one visitor, and the resulting rock blast injured several others. In addition to the dangers at the ocean entry, there are many life-threatening hazards associated with hiking near active lava flows. All visitors are required by law to stay within boundaries defined by National Park rangers. The activity at the Kamokuna entry can be safely viewed from within the National Park at the end of Chain of Craters Road.

Volcano Update

The current eruption of Kilauea continues unabated, with flows entering the ocean at Kamokuna. There were two felt earthquakes this week. The first, a magnitude 4.0 located at a depth of 20 miles below the west side of Mauna Kea's summit, occurred on March 8 at 8:15 pm. The second, a magnitude 4.0 on March 14 at 6:59 am, occurred 13 miles due south of the southernmost point of Maui. No damage was reported. If you feel an earthquake, please call the U.S. Geological Survey's Hawaiian Volcano Observatory (967-7328) and give us your location and a brief desciption of what you felt.