Volcano Watch — Dangerous ledge collapses becoming more common

Lava from the 11-year long eruption along Kīlauea's East Rift Zone continues to pour into the ocean near Kamoamoa inside Hawai‘i Volcanoes National Park. Nearly all the lava erupted during the past year has flowed from the erupting vents adjacent to the Pu‘u ‘O‘o Cone through an underground tube system to the ocean. At times, new surface flows break out of the tube system and advance over the pali, as occurred this past week. The occurrence of such breakouts may be related to increased eruption rates which exceed the carrying capacity of the tube. These surface flows have generally occurred on top of flows erupted earlier in this eruption. Lava travelling in the tube to the ocean has a different fate.

New land is formed where the lava enters the ocean. The processes that add new land to the island are complex, and much of the land that forms is quickly destroyed, mainly by sudden collapse of ledges of new lava along the coast into the sea. It is these collapses that make the area so dangerous.

The figure shows an aerial view of a collapse that took place this past Monday evening, when a slice of land about 30 meters wide and 200 meters long slid into the ocean. Since Monday, most of the area removed has been rebuilt by new lava. The figure also shows small slices, or ledges, of land formed at different times that have been added to the coast. In each case, more land formed and then part, or all, that had formed slid away, only to be followed by the addition of another ledge. Only a small fraction of the land formed survives and is permanently added to the island. Most of the slumps remove all or part of the newest ledge. However, in January, a large collapse removed even more than the newest ledge and cut back into much older lava.

We have been evaluating any precursors that might indicate when these collapse events will take place, but so far the best indicator is the size of the newest ledge. When the ledge grows large, collapse is likely within a matter of days. We have not been able to predict these events any better than that. Needless to say, a lack of precise prediction makes the area extremely hazardous, as was so evident last April, when a collapse occurred with a Park visitor on the slice that slid away. His body has not been recovered.

The reason why the ledges are unstable is that they are built on black sand produced by steam explosions at the coast. Much of the lava that enters the ocean is shattered into sand and gravel-sized glass fragments that form black sand beaches. However, most of this sand is carried down the steep slope offshore. The island grows as flows extend outward on top of this weak foundation. It is not surprising that the lava ledges slide away when one considers what is beneath the seemingly solid lava.

The main lava delta has now extended across the shallow area that was originally offshore from Kamoamoa, which makes the area far more unstable than it was even a few months ago. In order to build new land, the wedge of sand must build up from deep water before the lava can create a veneer on top. Because of this greater slope immediately offshore, the ledge collapses are occurring more frequently now than in the past.

In addition to the danger from instability, when the ledges slide into the water, large volumes of incandescent rock in the ledge react violently with seawater. Steam explosions have thrown glowing, angular blocks of lava hundreds of feet inland from the coast. During the collapse last April that killed the one visitor, 14 additional people were injured as they were showered by blocks thrown inland during the ensuing steam explosion. If these visitors had actually been hit by the ballistic blocks, their injuries would have been far more severe than the lacerations and sprains they received.

Many visitors foolishly go beyond the signs delineating safe areas that are posted by the Park Service. In the evening, it is not uncommon for 20-25 visitors to be out on the newest ledge "getting a closer look." During the past weeks, we have observed several visitors standing on the ledge that slid away during the collapse. The staff here at the Hawaiian Volcano Observatory does not go out on these ledges because we think the area is too dangerous. When we are working near the coast, our staff wear hardhats in case there are steam explosions that throw blocks into the air, or in case the spatter (molten blobs of lava) generated at the coast increases in height unexpectedly. Visitors who go into dangerous, closed areas jeopardize their lives.