Volcano Watch — What happens to lava flows after they enter the ocean?

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

What happens to lava flows after they enter the ocean?...

Two of three lava flows that raced down the southwest slope of Mauna Loa on June 2, 1950, are shown here. As lava entered the ocean, steaming water marked its offshore course. A semicircular area of hot, turbulent water killed many fish that were seen drifting on currents or washing up on shore over the next few days. Photo courtesy of U.S. Air Force.

(Public domain.)

The availability of high-resolution bathymetry off the coasts of volcanic islands like Hawai‘i 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 1,360 km (850 mi) 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 due to 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 pāhoehoe 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 the Island of Hawai‘i, and we are looking for these same effects on lava flows that entered the ocean north of Kailua-Kona on the west side of Hawai‘i. The HUEHUE and Ka‘ūPULEHU lava flows from Hualālai volcano entered the ocean along this coastline, as did the pāhoehoe and ‘a‘ā branches of the 1859 Mauna Loa lava flow. Despite the fact that these flows are tens of kilometers long on land, their submarine lengths are less than 6 km (3.8 mi).

Interpretations from recent lava flows in the Azores seem to also be true in Hawai‘i. For example, the 1859 Mauna Loa lava flow advanced over 50 km (31 mi) to the sea in 8 days, based on eyewitness accounts; however, the flow appears to have advanced only about 2 km (1.2 mi) 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 the Island of Hawai‘i are 50–100 m (164–328 ft) deep at 1 km (0.6 mi) from the coast. Much steeper topographies are encountered south of Ho‘okena in South Kona; depths there are around 500 m (1,640 ft) at a distance of 1 km (0.6 mi) from the coast.

In 1919 and 1950, Mauna Loa lava flows in South Kona rushed downslope about 20 km (12 mi) 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 0.8 to 5 km (0.5–3.5 mi) 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 1,000 m (3,300 ft), suggesting that the flow may have advanced 2–4 km (1.2–2.4 mi) 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 Hawai‘i, 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.


Volcano Activity Update

Kīlauea's summit lava lake level, which fluctuates in response to summit inflation and deflation, varied this past week between about 45 and 58 m (148–190 ft) below the vent rim within Halema‘uma‘u Crater.

Kīlauea's East Rift Zone lava continues to feed widespread breakouts northeast and east of Pu‘u ‘Ō‘ō. All active lava remains within about 8 km (5 mi) of Pu‘u ‘Ō‘ō. 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 the Island of Hawai‘i in the past week. On Thursday, August 27, 2015, at 4:39 a.m., HST, a magnitude-3.9 earthquake occurred 14.3 km (8.9 mi) south of Volcano at depth of 9.4 km (5.8 mi).