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November 12, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Rocks float briefly where lava meets the sea

Lava and the surf--two powerful forces seeking supremacy over each other. One consequence of this battle is "floating rocks" seen near the ocean entry, where lava from Kilauea's ongoing eruption reaches the south shore of the Big Island.

Floating rocks result when surf rolls across and tears molten fragments away from actively spreading lava or when ocean water enters a shallow lava tube and blasts lava bombs into the water. These fragments may stay suspended on the sea surface for several seconds before sinking from sight.

A random chunk of cold lava tossed into water will sink immediately. So why can lava from the surf zone float?

In general, floating occurs when an object is less dense than the liquid that suspends it. Few solid Earth materials are less dense than water. However, if gases, such as air or steam, are incorporated into denser materials, they can create a froth, or foam, of lower density. An additional requirement for floating is that the pockets of air be isolated or impermeable. If too permeable, water can displace the gas and fill the voids, thus destroying the favorable density contrast.

Each of us has seen driftwood, which can float because air is incorporated among the woody cells. Boogie boards and other styrofoam products float, not because they're "styro," but because they're foam. Air has been incorporated during manufacturing to create the buoyancy of these products. Pumice, a naturally occurring Earth material, will float if its bubble-like voids are so tightly bounded that water can't penetrate and fill them.

Let's return to the ocean entry. As the surf and hot lava make contact, the lava is cooled quickly. Experimentally, we can mimic this process by plucking melt from a flow and submerging it in a bucket of water. In this instance, the blob of melt literally puffs up. It expands abruptly as water enters the melt, perhaps along cracks and microfractures, while simultaneously expanding to steam. The liquid water expands to roughly 22 times its initial volume during conversion to steam at the Earth's surface. By creating gas pockets, the expansion leads to the lower density that allows the cooled melt, now rock, to float on the ocean surface.

Nothing floats forever. For a Big Island shoreline floater, 30 seconds is a long life, and one minute makes a world champion. Sinking results as the steam cools and liquid water penetrates the gas pockets. At first, the floating fragments of quenched lava remain so hot that the voids are pressurized against sea water. As cooling progresses, however, the steam condenses, the pressure is reduced, and liquid is able to infiltrate relentlessly. In the time it takes to unpack a camera and photograph, the fragment, a gently bobbing floater, slips beneath the sea and sinks like the rock it has become.

Eruption and Earthquake Update

There was a short pause in the eruptive activity of Kilauea Volcano. The pause started around 6:00 a.m. on November 7 and ended about 28 hours later. Lava drained from the tube system and stopped flowing into the sea. The eruption resumed at the Pu`u `O`o vent, and lava reoccupied the network of tubes from the vent to the seacoast. Several short-lived surface flows originated from breakouts of the tube system midway down the pali and in the coastal flats. Lava is again entering the ocean along a section near Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam clouds are highly acidic and laced with glass particles.

There were four earthquakes reported felt during the past week. A resident of Pahala felt three earthquakes between 1:00 p.m. and 1:30 p.m. on Saturday, November 7. The first two earthquakes originated 2 km (1.2 mi) east of Pahala at a depth of 4.5 km (2.7 mi), and the third earthquake originated 4 km (2.4 mi) southeast of Pahala at a depth of 8.6 km (5.2 mi). The three earthquakes had magnitudes of 2.8, 2.7, and 2.6, respectively. Early Monday morning at 4:58 a.m., a magnitude-3.0 earthquake was felt in Volcano. The epicenter of the earthquake was 3.7 km (2 mi) west of the summit of Kilauea Volcano at a depth of 6 km (3.6 mi).

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Updated: 23 Nov 1998