November 29, 2001
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
What happens when lava flows into the sea?
Lava from Kilauea continues to flow into the sea at East Kupapa`u and Kamoamoa. The steam plumes that usually rise from these ocean-entry points do not surprise visitors; it seems reasonable that incandescent lava at a temperature of about 1,150 degrees Celsius (2,100 degrees F) would cause seawater to boil. Sometimes, however, streams of lava pour into the sea with little or no steam, as if the lava were not hot at all. How can this be?
From experiments and from observations in Hawai`i and at other volcanoes, scientists have found that the outcome of lava-water interaction is determined by the way the interaction occurs. In order for a volume of seawater to boil, lava must heat it to at least 100.64 degrees Celsius (213.15 degrees F) at the sea surface, and to even higher temperatures below the surface. Obviously, there's a lot more water than incandescent lava, so if the heat is transferred slowly from lava to seawater, heated water mixes with cooler water, and the boiling point is not reached.
Circumstances that enhance the rate at which heat is transferred to near shore water will favor steam formation. Since heat is exchanged at the interface between lava and seawater, processes that increase the surface area of lava exposed to seawater increase steam formation. High lava-flow rates produce more heated surface area. When an active bench collapses, large surfaces of hot material are suddenly exposed to seawater.
Processes that subdivide lava into smaller pieces can dramatically increase the surface area. For example, a mass of lava the size of a basketball broken into fragments 1 mm (0.04 inches) across increases in surface area by 250 times.
Circumstances that reduce or prevent mixing of heated water with cooler water also enhance steam production.
Thin streams of lava that enter the sea quiescently do so because they are not delivering heat to the near-shore waters rapidly enough to overcome the cooling effects of mixing.
The persistent steam plumes at the East Kupapa`u and Kamoamoa entries form where lava tubes empty at the bench margins. Some of this lava solidifies and extends the bench, and some flows onto black sand beaches. When the surf splashes onto hot surfaces, the thin layer of water is quickly heated to the boiling point, contributing to the steam plume. The surf disrupts some of the molten lava, breaking it into smaller blobs. Quenched by water, the blobs are broken into even smaller pieces. The resultant increase in surface area promotes rapid heating of water. The volume of water available for mixing in the surf zone is less than that available in deeper water. Consequently, heat is delivered to the surf zone rapidly enough to generate steam.
Although relatively benign plumes are the norm where lava meets the sea, explosions of varying magnitude sometimes occur. The most violent explosions take place when part of an active bench collapses into deeper water. The collapse suddenly exposes large surface areas of hot lava to seawater; old and new lava, both solid and liquid, are usually involved.
Large volumes of water suddenly flash to steam, which propels lava fragments in all directions; large fragments have been known to travel hundreds of meters (yards) inland. Lava tubes feeding the edge of the bench can be flooded by seawater, sending jets of ash and steam skyward.
Bench collapses and the resultant explosions can be lethal. To avoid becoming a fatality, heed all of the warnings and signs limiting access to benches.
Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. Surface flows are observed above the pali and at various places along the trace of the Kamoamoa and Kupapa`u tube systems. Lava continues to enter the ocean in two locations: Kamoamoa and the area east of Kupapa`u.
As mentioned in the article above, the benches of the ocean entries are very hazardous, with possible collapses of the unstable new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.
Updated: December 5, 2001 (pnf)