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Volcanowatch

December 2, 2004

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


To be or not to be a lava bench or delta

What's in a name? In geology, plenty. Some of the biggest controversies erupt over terminology. For example, HVO scientists like to argue over terms like "lava delta" and "lava bench." Careful readers of Volcano Watch and the accompanying Eruption Update have probably noted that the two terms sometimes appear to be used synonymously and sometimes not. What's up with that?

"Lava delta" was first used in Hawai`i to describe the features formed by Mauna Ulu lava flows that entered the ocean between 1969 and 1971. The term described lava platforms that were built directly out into the ocean on piles of black sand and debris deposited on the ocean floor. Only a handful of lava deltas were built during that eruption, each active for less than 2 months, infrequently observed because of their remote location.

A lava delta is the part of a lava flow that builds out from the pre-eruption coastline. Lava deltas are usually described as broad, fan-shaped parts of the newly formed coastline. Active deltas form with the addition of new lava. Deltas cut off from their lava supply become inactive.

During the nearly 22 years of the ongoing Pu`u `O`o eruption, we have observed about 100 ocean entries and the formation of dozens of lava deltas. Some individual ocean entries were active for more than 15 months.

Many of the lava deltas that formed during the current eruption did something not observed during Mauna Ulu time: the leading edges of lava deltas collapsed catastrophically into the sea, producing violent explosions.

We observed that lava deltas were built incrementally by flows advancing along newly formed beaches parallel to the coastline, not advancing directly out into the ocean like river deltas. These constructional subunits of lava deltas were recognized as extremely hazardous, and a new term, "lava bench," was coined to describe them.

Lava benchesare the unstable margin or leading edge of a lava delta. Lava benches often form at the base of older sea cliffs or cliffs formed from the collapse of the former leading edge of the lava delta into the sea. As with lava deltas, active lava benches are defined as currently being supplied with lava.

Was this new term really necessary? Generally, most HVO and National Park Service personnel have found it a useful way to describe the really hazardous parts of lava deltas.

"Stay off the lava delta" is too broad a warning, as most of the delta is relatively safe. "Stay off the bench" is a warning that HVO and the National Park Service (NPS) commonly chant. How far back from the leading edge of the lava delta does the bench extend? Even the best definitions can't make that clear.

The "unstable" margin may extend well inland of the active lava entries and can only be identified after it collapses, not before. When a bench is built out from a sea cliff, it is easy to say that the hazardous region extends back to the cliff. It becomes less clear when additional lava flows drape the cliffs and make an inviting but exceedingly dangerous path to the ocean entries.

Rest assured that benches, deltas, and hazardous areas associated with them are readily recognized by experienced HVO personnel. The difficulty lies in trying to tell visitors how to recognize the features on their own. In recent years, the NPS has resorted to placing warning signs and roping off the high-hazard areas of lava deltas, where practical, to make the hazardous areas as clear as possible.

We all agree on what's hazardous, and we all agree on the importance of conveying that hazard to the public. But the scientific controversy goes on about the best way to describe these geologic structures. Regardless of the terminology, the leading edges of active lava deltas and lava benches are highly hazardous, and anyone who values life should stay off of them.

Activity update

Eruptive activity at Pu`u `O`o continues. The PKK flow continues to host scattered breakouts from near the top of Pulama pali to the coastal plain. The ocean entry at Lae`apuki has been inactive since November 27 or 28. As of December 1, there were active surface flows on the coastal plain, along the western margin of the flow that was feeding the entry. These flows were located about 400 m (1,300 ft) inland of the sea cliff, 3.6 km (2.2 mi) from the end of the pavement on Chain of Craters Road in Hawai`i Volcanoes National Park.

Expect a 2-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs. The eruptive activity in the crater of Pu`u `O`o remains weak, with several spatter cones glowing but not doing much else.

No earthquakes were reported felt on the island during the week ending December 1.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity has decreased over the past three weeks after being high for several weeks in early November and generally elevated since July. Thirty earthquakes were recorded beneath the summit area during the week ending December 1. Nearly all are 40 km (23 miles) or deeper and are the long-period type with magnitudes less than 3.

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Updated: December 13, 2004 (pnf)