Our Beautiful World

WHEN LAVA ENTERS THE SEA


Growth and Collapse of Lava Deltas

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 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

1) Pahoehoe lava first enters ocean quietly
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 a 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.


A simple sketch by J. Johnson, of lava fragments tumbling down a slope.

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 a 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 ***extreme***ly unstable!


Sketch by J. Johnson, 2000

4) Lava tube system develops in delta


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 laa 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.

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