Volcano Watch — Volcanic Spreading Induces Flank Collapse at Mauna Loa

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We already know that shield volcanoes like Mauna Loa grow gradually over time by erupting large volumes of lava onto their own flanks. Another force is at work, however, that spreads the volcano sideways, sometimes with catastrophic consequences. Mauna Loa's flanks are sliding seaward and, like unstoppable snow plows, are bulldozing sea-floor sediments in their paths. This is called "volcanic spreading." But how do we know this is happening?

Scientists have been able to survey the submarine flanks of Mauna Loa through the use of both manned and robotic submersibles. This is where the plowing action happens, where the base of the volcano meets the original, older oceanic crust. At the foot of Mauna Loa's western flank, geologists have found something very interesting—layers of volcanic debris that are tilted in toward the volcano, rather than parallel to the volcano's slopes like the lava flows on land. These sediments were once draped on the sea floor, but as Mauna Loa spread laterally, it scraped, thrusted, and tilted them upward as great slabs.

We can also theorize that outward-sliding of the volcano's flanks must happen. Consider how a shield volcano first forms. Lavas are erupted from a submarine vent onto an already-existing ocean floor, and flows are added over time to create a huge volcanic pile. They merely sit on top of the oceanic crust, with a weak contact between the two. When you take a load as heavy as Mauna Loa and inject magma into it from below, the volcano must inflate and, thereby, spreads along this weak zone to accommodate the ascending magma.

Gravity also plays a part. As you stack lava flows to an elevation of 12 km (7.5 miles) above the seabed, the tremendous downward-pushing force causes spreading. Submersible dives have shown that part of the volcano's submarine bulk is composed of broken fragments of rock and debris—much like the lava deltas at Kīlauea. Such material does not form a solid, stable mass. It is, rather, more like a gravel pile, which is a lot more prone to sliding as you load it than a single block would be. It is still not fully understood, however, if the driving force is gravity or magma forcing its way into the volcano—it is probably a combination of both.

As if Mauna Loa's massive lava flows and explosive potential are not hazardous enough, this volcanic spreading ups the ante. While the plowing action occurs very slowly as a barely noticeable creep, the force that drives the slow spreading occasionally spurs on a more serious event called "sector collapse"—a sudden detachment of the volcano's flank.

Such a catastrophic landslide was responsible for the formation of Kealakekua Bay, perhaps 100,000 years ago. The sheer cliffs that form its back wall are the surface from which the sliding flank detached. If you go snorkelling along the base of these cliffs, you can see the seabed plunging steeply into the abyss. What used to be a more gently sloping shoreline has literally broken off and slid into the sea, leaving a deep scar in the volcano's flank.

By mapping the seabed, we can see that the resulting debris travelled up to 100 km (62 miles) offshore. It may have generated a megatsunami hundreds of meters high. Such a wave would have been a colossal hazard, not only for the Big Island, but for the rest of the Pacific, as well, along with the tremendous power of the accompanying earthquake.

Should we worry about such an event happening again? Probably not. Though mapping of the sea floor around the Hawaiian Islands has revealed many ancient landslide deposits like this one, they occur, on average, only once every 32 km (20 miles) along the chain. Since this chain is moving northwestward with the Pacific Plate, measuring distance is like measuring time. Therefore, scientists have calculated that giant landslides occur, on average, every 350,000 years—not a timescale we should lose sleep over.

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Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is erupting elevated amounts of sulfur dioxide gas and very small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the south part of Kīlauea caldera and produced occasional air quality alerts in more distant areas, such as Pahala and communities adjacent to Hawai`i Volcanoes National Park, during kona wind periods. There have been two small explosive eruptions from the Halema`uma`u vent in the last week (on October12 and 14), both ejecting minor amounts of fresh lava onto the crater rim. There have also been several small ash-emission events from the vent, lasting only minutes, in the last week.

Pu`u `Ō`ō continues to produce sulfur dioxide at even higher rates than the vent in Halema`uma`u Crater. Trade winds tend to pool these emissions along the West Hawai`i coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

Lava continues to erupt from the Thanksgiving Eve Breakout (TEB) vent and flows toward the ocean through a well-established lava tube. Lava breakouts in the Royal Gardens subdivision have been active throughout the past week, expanding the TEB flow field several hundred yards to the west. One of these flows briefly followed Ali`i Avenue and destroyed one structure. The ocean entry at Waikupanaha continues to produce a moderate plume.

Be aware that active lava deltas can collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Check Civil Defense Web site or call 961-8093 for viewing hours.

Mauna Loa is not erupting. Two earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano.

One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-2.9 earthquake occurred at 7:08 p.m., H.s.t., on Thursday, October 9, 2008, and was located 13 km (8 miles) east of Mauna Loa summit at a depth of 12 km (8 miles).

Visit our Web site for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov. skip past bottom navigational bar