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Explosions at the Edge of an Active Lava Delta

Lava pours directly from a severed lava tube into sea Wave splashing against lava triggers tephra-jet explosion Tephra-jet explosion seconds later

When lava pours into the ocean at high rates from a lava-tube entry, beautiful and spectacular explosions called tephra jets (above) commonly occur. With temperatures higher than 1,100 degrees Celsius, lava can instantly transform seawater to 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 disrupts 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 it's leading edge, these sudden steam-driven explosions can be fatal.

Typical active lava delta before it collapses or subsides

Sketch of an active lava delta and lava entering the sea
Sketch by J. Johnson, modified from Mattox and Mangan, 1997.

A lava delta grows seaward as lava enters the sea and builds a foundation of loose lava fragments on the submarine slope. The platform of debris is subsequently capped by pahoehoe lava flows. In this sketch, the delta's leading edge has reached the steeper submarine slope and has started to subside, forming a lava bench. Lava tubes at the edge of the delta can reside below or at sea level, due to the continuous and sometimes abrupt subsidence of the delta and bench. As long as the tubes remain intact and water does not gain ready access to the tube system, lava entering the sea will not generate significant explosions.

Lava bench at leading edge of lava delta On this lava bench (area below and to right of headwall formed by an earlier bench collapse), lava travels through tubes before pouring into the sea (steam plumes in the distance).

 

Types of explosions

Based on observations during the growth of several lava deltas along Kilauea Volcano's southeast coast between 1992 and 1994, we've identified four general types of explosive interactions between lava and seawater:
Tephra jets and blasts typically occur when part of a lava delta completely collapses into the sea.

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 extremely hot rocks adjacent to the 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

Sketch of a collapsing lava delta
Sketch by J. Johnson, 2000

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.

Tephra jet explosion and littoral cone on active lava delta Tephra jet explosion at night on lava delta
Vigorous tephra jet after delta collapse 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.

Blasts

Sketch of a collapsing lava delta
Sketch by J. Johnson, 2000

Collapse of a growing lava delta exposes extremely hot newly solidified 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 sea and generated a relatively large blast that showered lava rocks 25 to 110 cm (10 to 45 in) in diameter over an area equal to about 3 football fields.

Boulder ejected by blast onto delta This boulder-sized piece of solidified lava was hurled more than 10 m inland by an explosive blast. Visitors standing anywhere near the edge of a delta to get a close view of lava pouring into the ocean can be hit by such "flying" rocks.

 

Partial collapse of lava delta
triggers littoral lava fountains & bubble bursts

A partial collapse of a delta's leading edge may significantly fracture the solidified rock around the active lava tubes, which allows seawater to seep into the tube system. When seawater and lava mix within the confines of a lava tube, pressure may build to cause explosions that blast a hole through the roof of the tube. Two types of steam-driven explosions may be generated in such a "confined" environment: littoral lava fountains and bubble bursts.

Littoral Lava Fountains

Sketch of a littoral lava fountain, oblique view Top: aerial view of littoral lava fountain on an active lava bench.

Bottom: cross-section of a littoral lava fountain and lava tube beneath.

Sketch of a littoral lava fountain
Sketches by J. Johnson, modified from Mattox and Mangan, 1997.

Spectacular and rare, this type of lava-seawater explosion produces fountains of molten lava and steam that reach heights of more than 100 m. The explosions of molten spatter, bombs, and smaller tephra fragments quickly build a circular cone on the subsided lava delta, sometimes in a matter of minutes. Originating from deeper within the subsided delta and closer to the shoreline than bubble bursts, littoral lava fountains are much more energetic and dangerous.

Littoral lava fountain Littoral lava fountain
COPYRIGHTED images used with permission, G. Brad Lewis.

Bubble Bursts

Sketch of a littoral lava-bubble burst
Sketch by J. Johnson, modified from Mattox and Mangan, 1997.

Bubble bursts are characterized by sporadic bursts of molten, dome-shaped lava sheets emanating from a circular rupture in the roof of a tube a few meters inland from the shoreline. Individual bubbles can reach diameters of 10 m in less than 2 seconds before they burst. The bubble fragments continue on their radial trajectories for up to 10 m more before falling to the ground. At the end of a burst, a pool of lava that remains in the roof of the lava tube gradually drains away. These bursts are frequently accompanied by a loud boom that shakes the entire delta.

Lava bubble burst and littoral cone on active delta, Kilauea Volcano Lava bubble burst and littoral cone on active delta, Kilauea Volcano

 

Hazardous activity near active lava deltas

References

Mattox, T.N, 1993, Where lava meets the sea: Kilauea Volcano, Hawai`i: U.S. Geological Survey Earthquakes and Volcanoes, v. 24, n. 4 p. 160-177.

Mattox, T.N, and Mangan, M.T., 1997, Littoral hydrovolcanic explosions: a case study of lava-- seawater interaction at Kilauea Volcano: Journal of Volcanology and Geothermal Research, v. 75, p. 1-17.


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Updated : 2 June 2000 (SRB)