Volcano Watch — What if it's a summit eruption?

Much has been written, and rightly so, about the worst-case scenario for future Mauna Loa activity, namely an eruption on the lower half of the southwest rift zone. Such an event could unleash multiple flows that would cut Highway 11 and enter residential areas in a few hours' time. The next-worse case scenario would be an eruption on the northeast rift zone that persisted for many months-long enough for flows to traverse the great distance to Hilo.

But let's set aside the doom and gloom for a moment and explore the best-case scenario- a summit eruption of Mauna Loa. Such eruptions are limited to the summit caldera, Moku`aweoweo, and the uppermost regions of the two rift zones. For those of you who've never made it to the top of Mauna Loa, Moku`aweoweo looks similar to Kilauea's summit caldera except that it is more elongate and as much as 60 m (200 ft) deeper.

Of the 15 Mauna Loa eruptions in the last century, seven were summit events. The most recent of these, in 1975, lasted only a single exciting day. Fountains erupted from fissures that spanned the length of Moku`aweoweo and extended to the upper ends of both rift zones. An `a`a flow came within 100 m (350 ft) of cutting the road to NOAA's climate-monitoring observatory before its source vent died.

Skipping back a few decades, the two next-most recent summit eruptions were also the two longest of the last century. The eruptions of 1940 and 1949 lasted 134 and 145 days, respectively. The first few days of each eruption were the most dramatic, with a series of fissures 5-6.5 km (3-4 miles) long propagating from near the center of Moku`aweoweo to the upper end of the southwest rift zone. Lava fountains initially played along almost the entire length of the fissure system, but within a few days the fountaining was restricted to a short segment of fissure near the southwest end of the caldera. In 1949, the activity diminished so quickly that geologists estimated that two-thirds of the total volume of lava produced during the eruption-37 million cubic meters (48 million cubic yards)-erupted in the first 24 hrs.

In both eruptions, fountain heights grew as the active part of the fissure constricted. In 1949, fountains reached 300 m (1,000 feet) in the third week of the eruption, forming a cinder cone 450 m (1,500 feet) in diameter against the southwest caldera rim. Lava overflowed the southwest end of the caldera and fed a flow that advanced 8.8 km (5.5miles) to the southeast. Activity was intermittent during the last half of both these eruptions.

How does this compare with summit eruptions of Kilauea? Astute readers may have noted that there was no mention of a lava lake, such as was active in Kilauea's summit crater, Halemaumau, from the early 1800s through 1924. It's true that lava from fissures ponded to a considerable depth inside Moku`aweoweo. A bona fide lava lake, however, forms directly over the vent that feeds it. The influx of new lava from below creates convection currents that keep the surface of the lake in nearly constant motion. Other examples of lava lakes (or ponds) on Kilauea besides Halemaumau are those that formed at both the Pu`u `O`o and Kupaianaha vents during Kilauea's ongoing eruption. As far as we know, Moku`aweoweo has not hosted such a lava lake since the summit eruption of 1872.

The summit eruptions of 1940 and 1949 were followed in short order by rift zone eruptions in 1942 and 1950. This summit-flank eruption pattern has occurred several times in recent history, but the pattern was disrupted after 1975, when the next rift eruption didn't occur for 9 years. It's impossible to say with any certainty where the next eruption will be located.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Surface breakouts occur in the coastal flats within 525 m (575 yds) of the Chain of Craters road, and the National Park Service is allowing visitors to get up close to the action. Two lava deltas, Wilipe`a and West Highcastle, are growing with multiple ocean entries from each. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

Two earthquakes were reported felt during the week ending on October 10. Residents of Hamakua and Kohala were shaken by an earthquake at 10:41 p.m. on Saturday, October 5. The magnitude-2.8 earthquake was located 18 km (10.8 mi) southeast of Waimea at a depth of 35.7 km (21.4 mi). Residents of Pahoa, Pohoiki, and Leilani Estates subdivision felt an earthquake at 8:58 a.m. on October 10. The magnitude-2.4 earthquake was located 5 km (3 mi) northeast of Pu`ulena Crater at a depth of 4.5 km (2.7 mi).

Mauna Loa is not erupting, and the summit region continues to inflate. During the past week, seismicity was low with only four earthquakes catalogued.