Our Beautiful World


Explosions at the Edge of an Active Lava Delta

When lava pours into the ocean at high rates from a lava-tube entry, beautiful and spectacular explosions called tephra jets commonly occur. With temperatures higher than 1,100 degrees C, lava can instantly transform seawater into steam, causing explosions that blasts hot rocks, water, and molten lava fragments into the air. In general, the more intense the incoming waves, the more energetic the tephra jets. The incoming waves disrupt the lava exiting the tube and increases the surface area of the molten stream that is exposed to seawater by more than 10 to 20 times.

The most violent and dangerous steam-driven explosions, however, occur when the leading edge of a growing lava delta suddenly subsides or collapses into the sea. The resulting disruption of the lava tube system within the delta forces seawater to mix with lava and hot rock surrounding the tube system in a confined environment.

For people standing on the delta or its leading edge, these sudden steam-driven
explosions can be fatal.

Types of Explosions

Based on observations during the growth of several lava deltas along Kilauea Volcano's southeast coast between 1992 and 1994, four general types of explosive interactions between lava and seawater have been identified;

1) Tephra jets and blasts typically occur when part of a lava delta completely
collapses into the sea.

2) Bubble bursts and littoral lava fountains may occur when the leading edge of a delta
subsides but doesn't slide away completely, which thereby allows seawater to infiltrate
the lava tube system.

Since a growing delta may collapse or subside at any time and the intensity of any one type of explosion may change suddenly, a growing lava delta is hazardous and should only be viewed from behind the former sea cliffs.

Complete collapse of lava delta triggers large tephra jets and blasts

A complete collapse of a delta's leading edge will sever lava tubes within the delta so that an
open stream of lava pours into the sea and ***extreme***ly hot rocks adjacent to the lava tube
system are exposed to relatively cold seawater. When lava and seawater mix in such an
"open" environment, two types of explosions may be generated: tephra jets and blasts.

Tephra jets

A tephra jet is by far the most likely type of explosion a visitor is likely to witness after
a bench collapse has severed an active lava tube. Waves rushing against the open stream
of lava "explode" in a cloud of steam, hot water, molten spatter, and tephra fragments.
Where the spatter and tephra fall onto the delta's leading edge, a semicircular littoral
cone a few meters tall usually forms.

Visitors often walk on the sides of active littoral cones, unaware that the intensity
of explosions can increase at any time or that the cone could collapse into the sea
without warning.

Tephra blasts

Collapse of a growing lava delta exposes ***extreme***ly hot newly solidifed lava flows within the delta to seawater, triggering a type of steam explosion we call a blast. The explosive blast shatters the solidified lava flows of the delta into fragments and hurls them as far as 200 m inland! A collapse event on April 19, 1993, swept a man into the ocean and generated a relatively large blast that showered lava rocks 25 to 110 cm in diameter over an area equal
to about 3 football fields.


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