When Lava Enters the Sea: Growth & Collapse of Lava Deltas

Photograph by C. Heliker on November 12, 1992.

Only four days old, this fan-shaped lava delta (center of image) extends about 200 m into the ocean on the southeast coast of Kilauea Volcano, Hawai`i. The diffuse plume of white steam at the leading edge of the delta indicates that several small lava flows are pouring into the ocean. Countless deltas have formed along this coastline during Kilauea's eruptive history.

When pahoehoe lava enters the ocean for extended periods of time, new land is created in the form of a fan-shaped platform known as a lava delta. Lava pouring into the ocean from either surface flows or lava tubes cools rapidly, usually shattering into sand- to block-size fragments. These fragments accumulate along the submarine slope to form a loose foundation that will eventually support overlying lava flows that build the delta above sea level.

When the lava fragments accumulate on a relatively steep submarine slope, the leading edge of a lava delta will collapse frequently to form a series of submarine landslides. These collapses can sweep unwary visitors into the sea and trigger strong explosions. A lava delta will grow both laterally and seaward until a collapse occurs or the supply of lava to the ocean is interrupted.

Typical growth of a lava delta at Kilauea Volcano

Pahoehoe lava entering the ocean at a new location either oozes across a cobble or black sand beach or spills over a sea cliff, typically 1 to 5 m tall. As waves splash over the advancing lava, the surface of the molten stream cools quickly and shatters into small, glassy fragments. Because the seawater does not become trapped or confined within the lava, explosions rarely occur. For this reason, the first few hours of a new ocean entry are the safest time to view up close lava entering the sea.

2. Lava fragments tumble down submarine slope

Lava reaching the ocean is cooled so rapidly by the incoming waves that it "freezes" to form to a black glass and shatters into sand- to block-sized fragments that accumulate along the submarine slope just offshore.

Sketch by J. Johnson, 2000

These loose lava fragments form the unstable foundation for all subsequent lava flows and loose debris. The volcanic debris along the submarine slope consists chiefly of angular pieces of glassy lava produced by rapid cooling and blocks of large cylindrical lava tongues and pillows from stubby submarine lava flows. Also, during a recent series of deep-water scuba dives, scientists observed small channeled lava flows as wide as 1.5 m moving down the leading slope of the submarine delta; highly fractured, these intact flows apparently interfinger with the loose fragments.

3. Lava flows build lava delta across loose debris

Sketch by J. Johnson, 2000

As the loose debris builds a foundation forward and upward, small lava flows spread atop the debris to form a lava delta above sea level that may extend tens to hundreds of meters beyond the old shoreline. At the same time, the entire delta can slowly sink as the submarine debris pile shifts under the weight of the overlying lava flows; recent studies of several growing lava deltas showed that they subsided several centimeters per month. This new land is extremely unstable!

Sketch by J. Johnson, 2000

4. Lava tube system develops in delta

Photograph by T. Mattox on June 2, 1993

Skylight in photograph above shows lava flowing through a tube within an active delta. As the lava-flow field on the delta matures, a lava tube system usually develops and lava may enter the ocean from one or more tubes. The exit points of these lava tubes often reside at, or below, sea level and are usually marked by a single, vigorous steam plume.

Collapse of active lava deltas

Our 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 approaches these steeper slopes, seaward growth of the delta slows.

1. Cracks typically form along leading edge of delta

The instability of a lava delta becomes readily 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 a called a lava bench, the most dangerous part of a growing lava delta. Visitors are advised to avoid lava benches, which are prone to collapse and frequently hit by explosive debris and swept by scalding waves.

Curving scarp 1 m tall marks the edge of a lava bench that subsided into the sea. Scarps and cracks on a delta's leading edge identify areas that are likely to collapse.

2. Collapse of delta into ocean

Sketch by J. Johnson, modified from Mattox and Mangan, 1997.

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

Sketch by J. Johnson, 2000

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!

Sketch by J. Johnson, 2000

Photograph by L. Keszthelyi on August 18, 1996.

Lava spreads across a new lava bench a few days after an earlier bench collapsed into the sea to from the 5-meter tall scarp visible at left. At first glance, this scarp may appear to be the former sea cliff. However, the absence of vegetation along the edge and the deep curved "bite" of the scarp indicate that it cuts through the active lava delta. This area is not safe for viewing the ocean entry because the bench can collapse again, removing even more of the delta and triggering explosions that can hurl debris onto the delta.

Hazardous activity near active lava deltas

• Explosions at lava entry points
• Waves send scalding water onto new land

References

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.

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

Moore, J.G., Phillips, R.L., Grigg, R.W., Peterson, D.W., and Swanson, D.A., 1973, Flow of lava into the sea, 1969-1971, Kilauea Volcano, Hawaii: Geological Society of America Bulletin, v. 84, n. 2, p. 537-546.