Our Beautiful World


WHEN LAVA ENTERS THE SEA


Collapse of active lava deltas

Studies have shown that a lava delta is most prone to collapsing into the ocean when its leading edge moves beyond the shallow waters of the coastline and encounters the steep offshore submarine slope, which typically ranges from about 20 to 45 degrees. As the advancing delta approached these steeper slopes, seaward growth of the delta slow.

1) Cracks typically form along leading edge of delta
The instability of a lava delta becomes rapidly apparent when a series of cracks and an ocean-facing scarp form near its leading edge. The cracks may extend more than 100 m parallel to the new shoreline and several tens of meters inland. The area seaward of these cracks and scarps is called a lava bench, the most dangerous part of a growing lava delta. Visitors are advised to
avoid lava benches, which are prone to collapsing and frequently hit by explosive debris and swept by scalding waves.


This ocean-facing scarp and lava bench was formed when the leading edge of the delta subsided
about one meter into the sea. Note the prominent crack in the lower left on the delta side of the
scarp -- such cracks often but not always develop just before the delta either subsides or collapses into the sea. The spatter and tephra deposits on the bench (lower right) are from small explosions
at the edge of the bench.

2) Collapse of Delta into ocean


Sketch of a collapsing lava delta.

The leading edge of a growing lava delta collapses when the loose debris underneath slides down the submarine slope. The failure occurs because the loose fragmented debris underneath can no longer support the delta's growing mass or a deeper submarine landslide undercuts the delta. During collapses, the lava tube(s) either become cracked or completely severed. This usually results in the sudden explosive mixing of seawater and lava.

Frequent collapses jeopardise visitors
Most of the lava deltas that have grown since 1986 on the southeast coast of Kilauea collapsed repeatedly. As a lava delta matures and its leading edge collapses and then advances seaward again, the former shoreline and sea cliff become increasingly difficult to identify.


In the sketch, the leading edge of the delta collapses into the sea;
note that the former sea cliff is buried by the delta.

With the addition of new flows, the leading edge of the delta grows seaward again. In many cases, what appears to be a former sea cliff is actually the headwall scarp of an earlier collapse event that cut back into the middle of the delta; the sea cliff is still buried by the delta. The entire area is unstable and may collapse into the sea!


MAN SWEPT INTO OCEAN DURING COLLAPSE OF A LAVA BENCH.

On April 19, 1993, several visitors to Hawai'i Volcanoes National Park walked into an area along the shoreline of Kilauea Volcano that was clearly identified as unsafe for viewing lava entering the sea. Four people were caught close to the edge of a growing lava delta when its leading edge collapsed. Three managed to run to stable ground, but one person fell into the ocean and was never found. Other visitors suffered minor injuries as seawater suddenly came in contact with hot rock and the resulting explosions showered them with incandescent rocks. Several people were also injured from falls as they fled in panic.

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