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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!
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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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