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Volcanowatch

August 9, 2001

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


Lava bench is no boardwalk

area of East Kupapa`u bench and barrier area
East Kupapa`u bench and safety barrier areas. | large |

What's the difference between a bench and a boardwalk? Both offer a view of the sea, but while the latter is a pleasant place for a stroll, a walk on the lava bench can kill you.

A lava bench is created where lava flows over a seacliff and enters the ocean to form new land. The bench is typically down-stepped by several meters (yards) below the level of the former seacliff. Over time, the old seacliff may be buried by lava flows draping over it. This "lava drapery" can form a ramp that makes it easy to descend to the bench. If you want to live, resist this temptation.

Why is an active lava bench so dangerous? When lava enters the ocean, most of it quickly chills and shatters into small particles of black sand and loose blocks. The bench builds outward over a steep, unstable slope composed of this rubble. Frequent landslides and slumps on the submarine slope can cause all or part of the bench to collapse. These collapses can remove acres of new land, plus slices of the old seacliff, in seconds, with little or no warning.

During a collapse, the hot interior of the bench is exposed to the surf, triggering violent steam explosions that can shower a broad area with bits of molten lava and hurl dense rocks as big as a bale of hay 100 m (300 ft) inland. During one partial bench collapse witnessed by our staff, the bench was engulfed in acidic steam clouds that billowed back from the explosions at the water's edge. Anyone caught on the bench would have been trapped in a white-out while explosion debris rained down on their heads.

Here's a clue to how unstable benches really are: lava from the current eruption of Kilauea has been flowing into the sea most of the time since 1987, yet only 206 hectares (510 acres) of new land has been added to the island. And most of that was created in a few days time when the shallow bay of Kaimu was filled. The steep drop-off along most of Kilauea's southern coast has resulted in nearly all of the would-be new land ending up beneath the waves.

The bench doesn't even have to collapse to kill you. The current bench at East Kupapau is about 600 m (2000 ft) long and extends over 100 m (300 ft) seaward. A large billowing steam plume marks the point where lava is discharged from the tube into the water. An older portion of the bench is covered in black sand, luring people over the seacliff like lemmings. What they don't realize is that it takes only one large wave washing over the nearby active flows to envelop this beach in scalding water and acidic steam. And, speaking of collapse, the current bench is cut by a large crack-usually a sign that part of it is getting ready to slide into the sea.

The seacliff that separates the new land from the old can be obscured over time by the new lava flows that drape it. Eventually it becomes difficult to recognize the boundary where the bench begins, so heed all posted signs. The best place from which to view active benches is from the top of the seacliff well to either side of the bench. This usually offers the best viewing without the dangers of being directly above the bench.

Island residents should be more aware than visitors of the dangers. But, as the saying goes, familiarity breeds contempt. Three of the four people killed at the bench lived here and the fourth was with a resident. It is so easy to avoid becoming part of geologic history-JUST STAY OFF THE BENCH!

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in two separate areas. Small surface flows, primarily ooze-outs from inflated areas in the coastal flats, are occasionally observed. Lava continued to enter the ocean in the area east of Kupapa`u throughout the week. The public is reminded that the ocean entry areas are extremely hazardous, as detailed in the above article. Swimming in the area can be a blistering or even deadly experience.

There were no earthquakes reported felt during the week ending on August 9, 2001 skip past bottom navigational bar


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Updated: August 24, 2001 (pnf)