# Volcano Watch — Submarine eruptions - volcanoes on the rise

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Each of us has our own mental image of a volcano. For some, it may be a lofty, snow-capped peak like Mt. Rainier. For others, the horizon-dominating backbone of a volcano like Mauna Loa may come to mind. But not many of us imagine the watery edifice of one of the countless volcanoes that hide beneath the surface of the world's oceans.

Vapor billows from rocks appearing above the sea near the area where a one-kilometer high column of water vapor shot up in the Pacific Ocean at Fukutokuoka-no-ba. (Public domain.)

Although little is known about submarine volcanoes, or seamounts, they comprise the majority of the planet's volcanoes. Several thousand active submarine volcanoes have been discovered, and some scientists estimate that there may be more than a million active volcanoes hidden beneath the waves. If so, then for every active volcano on dry land, there are hundreds of active volcanoes located under water.

Here in Hawaii, we have a bit of both worlds - the volcanoes that make up the islands themselves, and Loihi, the active submarine volcano southeast of the Big Island. In 100,000 years or so, Loihi will poke its top above water, and a new Hawaiʻian iIsland will be born. When it does, it could be quite a sight to behold.

When volcanoes erupt in deep water, the weight of the overlying water prevents the explosive escape of volcanic gases. But as a volcano grows, and eruptions take place in shallower water, the water pressure can no longer confine the volcanic gases, thereby allowing them to escape. The presence of an unlimited supply of water also adds great quantities of steam to the eruption. If the conditions are right, the result can be explosive, with rock debris and steam blasting out of the ocean.

Earlier this month, Fukutokuoka-no-ba, a submarine volcano located about 1,125 kilometers (700 miles) southeast of Tokyo, Japan, began to erupt. Though apparently not explosive, the eruption created a steam plume that reached a height of about 1,000 meters (3,280 feet). Observation flights over the eruption site found the water strongly discolored by mud and other debris, with blocks of pumice floating on the surface.

Fukutokuoka-no-ba, the latest example of the rarely observed transition of a submarine volcano into a subaerial volcano, poses little threat to populated areas. A slightly more dangerous example of an active, near-surface submarine volcano is Kick em Jenny, about 8 kilometers (5 miles) off the north shore of Grenada, in the Caribbean. Though its historical eruptions have been rather small, the eruptions can be expected to be more violent as the volcano grows toward the ocean's surface. The modern cone of Kick em Jenny is also built within a horseshoe-shaped depression left by a giant underwater landslide. Therefore, both tsunamis and explosive eruptions threaten neighboring islands.

Although Fukutokuoka-no-ba and Kick em Jenny are good examples of active, shallow-water volcanoes, the volcanic island of Surtsey, which appeared off the southern coast of Iceland in 1963, provides a better example of what might happen when Loihi emerges from the waves. The quiet growth of Surtsey from the sea floor to within a few meters of the ocean surface went largely undetected. When shallow depths were reached, however, the eruption became explosive, hurling cinders and lava bombs to heights of at least 250 meters (820 feet).

The billowing ash and steam cloud over the vent reached as high as 10 kilometers (6 miles). After several months of explosive eruptions, the growing cinder cone finally blocked seawater from reaching the vent, and the steam explosions stopped. The eruption, however, continued to produce lava fountains and flows, much like those seen here at Kīlauea, until the eruption ended three years later.

Fukutokuoka-no-ba, Kick em Jenny, and Surtsey all illustrate an important step in an interesting and unique process. Other volcanoes will undergo the same basic process, and, eventually, Loihi will have its turn. If Loihi is anything like its neighbor, Kīlauea, erupting about 300,000 cubic meters (400,000 cubic yards) of lava each day, then it will certainly put on a good show when it emerges from the depths. Too bad none of us will be around to see it.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone.

The PKK lava tube continues to efficiently carry lava from its source near Puu Oo to the ocean. Thus, the amount of surface lava visible is very low, with only a handful of surface flows scattered from above the top of Pulama pali to the ocean. Two ocean entries, at East Laeapuki and East Kamoamoa, were active as of July 14. The East Laeapuki bench has suffered partial collapses three times in the last three weeks. One of these collapses took a good-sized chunk of an older bench. Access to the ocean entries and the surrounding area has been closed due to significant hazards. There are currently no easily accessible surface flows at the eruption site. If you do visit the eruption, check with the rangers for updates to this situation, and remember to carry lots of water when venturing out onto the flow field.

During the week ending July 13, four earthquakes were felt on Hawaii Island. At 11:29 p.m. on July 10, a magnitude-2.2 micro earthquake located 8 km (5 miles) north of Mauna Kea summit occurred at a depth of 18 km (11 miles); this earthquake was felt at Volcano (was Waimea sleeping?). A magnitude-3.4 minor earthquake occurred at 11:47 p.m. that same night 5 km south of Puu Oo crater at a depth of 10 km (6 miles); it was felt locally at Royal Hawaiʻian Estates and Volcano. At 10:12 p.m. on July 11, a magnitude-2.8 micro earthquake located 9 km (6 miles) east of Kaena Point occurred at a depth of 41 km (26 miles); it was felt at Volcano. At 5:01 a.m. on July 13, a magnitude-3.4 minor earthquake located 11 km (7 miles) south of Paauilo occurred at a depth of 23 km (14 miles); it was felt locally at LauPāhoehoe and Papa`aloa.

Mauna Loa is not erupting. During the week ending July 13, eleven earthquakes were recorded beneath the summit area. Four were deep and long-period in nature. Inflation continues at a slightly increased rate over the last few weeks.