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June 4, 1998

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

Just how hot is that ocean at the lava entry?

Billowing clouds of steam rising from two discrete locations along the Kamokuna-Waha`ula coastline are often the only reminders we have of Kilauea Volcano's near-constant effusion of lava into the sea. Surface flows and lava from truncated lava tubes are quickly quenched as they enter the water, shattering to glassy sand and rubble, producing acidic, particle-laden steam clouds. More commonly, lava enters the sea via near-surface tubes encased within the bench, building at the ocean edge. The basalt forms elongate lava tongues and channelized flows that extend up to 70 m (230 ft) down the steep submarine slope. The temperature of the lava in the tubes is about 1250 degrees C (2,200 degrees F).

With the near-constant water-lava contact and a dense, rapidly evolving steam cloud, does the water near the lava entry boil like water in a steam kettle? Surprisingly not. Water in contact with surface and near-surface flows flashes to steam and quickly rises; steam produced by submarine flows is quickly quenched by sea water. Although the temperature of water immediately adjacent to the submarine lava reaches 88 degrees C (190 degrees F), it degrades quickly to 27 degrees C (81 degrees F), only slightly above the ambient ocean temperature, within a few inches of the contact.

This is not to say that the water isn't hot. Temperatures can be unpredictable. A thin layer of surface water near the entry ranges from 38 - 68 degrees C (100 - 155 degrees F). For perspective, 42 degrees C (108 degrees F) bathtub water is too hot for most people to climb into, scald-free faucet water is calibrated to 46 degrees C (115 degrees F), and coffee between 60 and 65.6 degrees C (140 and 150 degrees F) has been known to scald the skin.

Now, you don't need a boat and a thermometer to get a general idea of thermal differences. The changes in water temperature can be interpreted from a distance by the variably tinted sub-concentric circles that expand away from the entry. Several sharp color and temperature gradients exist between the hydrothermal plume and the surrounding ocean. The plume, which is generally less than 3 m (10 ft) thick and as much as 2 km (1.2 mi) from shore, increases in size when the volume of lava entering the sea increases.

The concentric patterns comprise several distinct temperature regimes. The hottest water is yellowish-brown. This water is heated at the entry point and travels offshore in a horseshoe pattern until it reaches the longshore currents. Temperatures range from as high as 68 degrees C (155 degrees F) near the lava contact to 35 degrees C (95 degrees F) 70-100 m (230-330 ft) offshore. Where water is trapped in pockets along the bench, temperatures are as high as 88 degrees C (190 degrees F). The brown color results from high concentrations of suspended glass fragments and occasional gelatinous zooplankton. Large, temporarily buoyant steaming rocks are also seen in this zone. The temperature of the pale-green surface sea water surrounding the plume is elevated only 1.1 - 2.8 degrees C (2 - 5 degrees F) above that of ambient sea water.

Enclosed within the discolored plume is a patch of deep-blue-to-black curiously placid water. This upwelling is the coolest water near the entry and forms when heated submarine water quickly rises, mixing with cooler sea water on its way to the surface. Infrared video of the plume shows a cyclic pattern of upwelling, expansion and quiescence. The video shows a centralized concentrated warm spot that rises and spreads as concentric circles away from the spot. The cycle takes 4-6 seconds. Exactly why this upwelling is so centralized is not clear.

Keep in mind, water temperatures at the bench are unpredictable. Any passing fancies about swimming near the entry could get you into a whole lot of hot water. The bench remains unstable and visitors to the entry are advised to stay at least 400 m (1/4 mi) inland.

Eruption and Earthquake Update

The east rift zone eruption of Kilauea Volcano from the Pu`u `O`o vent continued unabated during the past week. The lava flows through a network of tubes to the seacoast and enters the ocean at two locations-Waha`ula and Kamokuna. The public is again reminded that these two areas are extremely dangerous, and the National Park Service has restricted access to them because of frequent explosions accompanying collapses of the growing lava delta.

A resident of Leilani Estates reported feeling an earthquake at 7:53 p.m. on Thursday, June 3. The magnitude 2.9 earthquake was located 1.5 km (1 mi) east of Puulena crater at a shallow depth.

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Updated: 4 June 1998