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August 12, 1999

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


Beware Kilauea's new land - not exactly terra firma?

Kilauea Volcano has been erupting nearly continuously for the past 6,068 days since activity started on January 3, 1983. Lava first entered the ocean on November 28, 1986 and except for brief pauses in eruptive activity, has been constantly flowing into the sea.

For all the years that lava has been flowing into the sea, only a meager 206 hectares (510 acres) of new land has been added to the island. Most of this new land was created when the shallow bay of Kaimu was filled. The difficulty in forming new land is due to the sharp drop-off of the ocean floor offshore.

The topography of the ocean floor off the southeast coast of the island where lava is flowing into the sea is very steep. Within 1,000 m (3,300 ft) of shore, the bottom drops down 300 m (1,000 ft), and this precipitous slope is maintained down to a depth of nearly 3,500 meters (11,500 ft).

When lava enters the ocean, most of it quickly chills and shatters into small particles. These particles form layers of deposits and are called hyaloclastites. The hyaloclastites settle on the steep slopes and occasionally tumble down into the deep basins offshore.

Lava entering the ocean is often able to gain a foothold on the steep slope, and subsequent flows can build a small peninsula or bench from shore. The bench appears to be a solid piece of land, but this is deceiving. This bit of new land is built upon a poor foundation.

The hyaloclastic layer is essentially a pile of sand. Wave action can erode the sand from beneath the bench, and without any basal support, the bench will collapse and slide down the steep slope. This process is occurring wherever new land has formed.

The rate of erosion may be slow. It is not unusual for shore fishermen on the 1960 Kapoho lava flows to find their favorite fishing spot gone after years of fishing from that site. The rate of erosion may be rapid as implied by the sparse area of new land created during this long-lived eruption.

The present bench at Kamokuna is large and has extended more than 200 m (655 ft) from shore. The bench is now calving small blocks, and eventually, the entire bench will collapse. Bench collapses are dangerous. One fatality and numerous injuries are attributed to earlier bench collapses.

Hawaii Volcanoes National Park has posted signs near the ocean entry area to warn visitors from going onto the unstable bench. It is difficult to recognize the boundary where the bench begins, so let the posted signs be your guide. Stay back of these signs and stay alive.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes from the vent to the sea. Short surface flows emanating from breakouts of the tube system were observed in the upper coastal flats. Lava is entering the ocean near Kamokuna and enlarging the bench. Occasional phreatic activity was observed at the entry. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

Three earthquakes were reported felt during the week ending on August 12. A resident of Pahala felt an earthquake 43 minutes after midnight on the morning of August 9. The magnitude-3.3 earthquake was located 13 km (8 mi) west of Pahala at a depth of 7.8 km (4.7 mi). Two shallow earthquakes located near Pu`ulena Crater were reported felt by a resident of Leilani Estates on August 12. The earthquakes occurred a minute apart at 10:51 a.m. and 10:52 a.m. with magnitudes of 2.1 and 1.7, respectively.

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Updated: 2 Sep 1999