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What happens to lava flows after they enter the ocean?

Updated: 
August 30, 2015 - 1:30am

Does lava continue to flow exactly as it did on land or does it behave differently after it enters the ocean?

The availability of high-resolution bathymetry off the coasts of volcanic islands like Hawaii allows us to get a peek at flows that have continued to advance under water. Geologists studying recent lava flows in the Azores, a volcanic island chain 850 miles west of Portugal, could easily distinguish the underwater extent of lava flows that had originated on land. They found that the flows behaved differently underwater, primarily because of rapid cooling by water and by buoyancy of the advancing flows.

Water can cool the surface of a lava flow more efficiently than can air, so lava flowing in water develops a solidified skin very rapidly. However, when the crust reaches moderate thickness, it insulates the lava flow interior just as well as it does in air. This results in flows stalling after advancing short distances below the surf zone, pressurizing (or inflating, like pahoehoe flows), and advancing farther through multiple breakouts. The most common form was dubbed “dendritic,” because multiple breakouts occurred along a broad flow front, several of which branched again.

Lava flows also become buoyant underwater. The flows don’t float because their density is still greater than the density of seawater, but they flow more slowly. This is because upward buoyancy forces partly counteract the downslope pull by gravitational forces.

The combination of buoyancy and enhanced cooling slows lava flows moving offshore along the sea bed, thereby causing them to pressurize and thicken.

High-resolution bathymetry is also available for several offshore areas of Hawaii Island, and we are looking for these same effects on lava flows that entered the ocean north of Kailua-Kona. The Huehue and Kaupulehu lava flows from Hualalai volcano entered the ocean along this coastline, as did the pahoehoe and aa branches of the 1859 Mauna Loa lava flow. Despite the fact that these flows are tens of miles long on land, their submarine lengths are less than 3.8 miles.

Interpretations from recent lava flows in the Azores seem to also be true in Hawaii. For example, the 1859 Mauna Loa lava flow advanced more than 31 miles to the sea in eight days, based on eyewitness accounts; however, the flow appears to have advanced only about 1.2 miles offshore even though it remained active for months.

In the South Kona District, some high resolution bathymetry exists but coverage is spotty so we rely on other evidence for how far recent flows advanced underwater. Just like on land, the slope of the ground over which lava moves affects its speed, with lava flowing faster over steeper slopes. Offshore slopes along the northwest coast of Hawaii Island are 164 to 328 feet deep at 0.6 mile from the coast. Much steeper topographies are encountered south of Hookena; depths there are around 1,640 feet at a distance of 0.6 mile from the coast.

In 1919 and 1950, Mauna Loa lava flows in South Kona rushed downslope about 12 miles to the ocean and continued to flow into the ocean for weeks. While the ocean entries were active, steam was observed rising from the ocean surface 0.5 to 3.5 miles offshore, with many fish killed in the vicinity. Notably, several of the fish were varieties never seen before. Later study by ichthyologists confirmed that these deep-sea creatures probably came from depths of about 3,300 feet, suggesting that the flow may have advanced 1.2 to 2.4 miles offshore in both cases to reach those depths.

The slowing of lava flows as they enter the ocean may help explain some aspects of lava delta development and, more broadly, volcanic island development. When lava next enters the ocean in Hawaii, we may be able to use this information to better assess the extent of any hazards the lava delta and underwater lava flow pose to visitors and near-shore boat traffic.

Kilauea activity update

Kilauea’s summit lava lake level, which fluctuates in response to summit inflation and deflation, varied this past week between about 148 and 190 feet below the vent rim within Halemaumau Crater.

Kilauea’s East Rift Zone lava continues to feed widespread breakouts northeast and east of Puu Oo. All active lava remains within about five miles of Puu Oo. The most distant breakouts are evident by the smoke plumes produced by burning vegetation along the edge of the flow field.

One earthquake was reported felt on Hawaii Island in the past week. On Thursday at 4:39 a.m., a magnitude-3.9 earthquake occurred 8.9 miles south of Volcano at depth of 5.8 miles.

Visit the Hawaiian Volcano Observatory website at hvo.wr.usgs.gov for Kilauea daily eruption updates and other volcano status reports, current volcano photos and recent earthquakes; call 967-8862 for a Kilauea summary update; email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.

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