Volcano Watch — A petrologist explains Kīlauea's eruptive condition

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Well, we've certainly learned a lot about how Kīlauea works, but there's always a caveat when we suggest what it's likely to do next. Pele could surprise us!

Since January 1983, Kīlauea has been erupting from vents on the east rift zone. This eruptive era really started in 1982, when new magma began steadily rising into the shallow volcanic edifice. The volcano responded by swelling at the summit as magma accumulated beneath the caldera. Several months after the April and September 1982 eruptions in the summit, a subterranean magma pathway was forged once again into the east rift zone, and the eruption commenced in Nāpau Crater, eventually concentrating the vent now known as Pu‘u ‘Ō‘ō.

Throughout the last 28 years, surges and lulls in the magmatic forces that drive eruptive activity have been transmitted along a 10-mile-long magma pathway that extends from the reservoir beneath the summit caldera to that beneath Pu‘u ‘Ō‘ō. This long-lived "eruptive plumbing system" remains pressurized as long as it is fed by new magma from depth.

For the last decade, most erupted lavas have contained a blend of hotter and cooler components. The higher-temperature component is fresh magma comparable to that erupted from the summit vent; the cooler component is fresh magma that has cooled and partly crystallized within the rift zone. This perpetual mixing condition implies that magma from the summit region circulated through a shallow magma reservoir on its way to eruption at Pu‘u ‘Ō‘ō.

In March 2008, when the summit eruption began, Kīlauea unveiled a new portal into its volcanic plumbing. This is an unprecedented opportunity for a petrologist (think "rock"ologist) to evaluate properties of lava at both ends of Kīlauea's active magma plumbing system. By tracking changes in the chemical composition, temperature, and crystallinity of lava, we are able to assess changing magma conditions between Halema‘uma‘u and Pu‘u ‘Ō‘ō.

Subtle, long-term changes in characteristics of rift lava indicate that remnants of older magma have been gradually purged from the eruptive plumbing system. Matching geochemical signatures of lava erupted from the summit and from the east rift zone have confirmed our suspicion that the volcanic edifice is fully inundated with a burgeoning supply of new magma.

More dramatic and sudden changes in magma composition occur when the well-established magma pathways are physically, and often surprisingly, disrupted.

The spectacular five-day Kamoamoa fissure eruption in March 2011 was one of Pele's surprises—very similar to one that occurred nearby in Nāpau Crater on January 29-30, 1997. Both fissure eruptions resulted from the abrupt opening of the rift zone upstream from the active Pu‘u ‘Ō‘ō vent. In both cases, magma throughout the pressurized plumbing system was drawn toward the zone of rift rupture. Lava drained from the Pu‘u ‘Ō‘ō crater and from the summit, and magma rose to the surface near Nāpau Crater.

Like the 1997 Nāpau Crater lava, the Kamoamoa fissure lava does not match lava from the Pu‘u ‘Ō‘ō vent. Once again, chemical and mineralogic analysis has confirmed that the lava erupted in this zone is a mixture of recent magma with cooler remnants of magma stored underground, and isolated from the active plumbing system, since the 1963 and 1968 fissure eruptions in the same area.

The upper-middle east rift—where the initial fissure fountains of the current eruption were first propagated in January 1983—is apparently a "soft-zone" prone to rupturing. This is, in part, because this segment of the rift still bears the magmatic remains of eruptions in past decades.

Our petrologic studies also indicate that a shallow, active magma chamber in this area of the rift may have grown like an "aneurysm" in Kīlauea's conduit, an artery over-pressured by a surge in magma supply deep within the heart of the volcano. This artery eventually burst and bled through fissures containing the molten remnants of previous eruptions.

Kīlauea is still recovering from the Kamoamoa eruption. Magma continues to rise into Kīlauea from deep below the volcano; lava has reappeared in the Halema‘uma‘u and Pu‘u ‘Ō‘ō vents. Soon the volcano will regain its previous eruptive vigor—that is, assuming Pele isn't holding back any other surprises!

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Volcano Activity Update

Lava erupted continuously within Pu‘u ‘Ō‘ō over the past week, feeding a small lava lake in the center of the crater. Changes in eruptive output commonly resulted in overflows from the lake that slowly built up the crater floor, which is about 53 m (175 ft) below the east rim of Pu‘u ‘Ō‘ō. No lava is erupting outside the crater.

A small lava lake was also present deep within the Halema‘uma‘u Overlook vent during the past week, with minor fluctuations in lava level. Volcanic gas emissions remain elevated, resulting in relatively high concentrations of sulfur dioxide downwind.

One earthquake beneath Hawai‘i Island was reported felt this past week. A magnitude-2.1 earthquake occurred at 3:48 p.m., HST, on Friday, May 6, 2011, and was located 5 km (3 mi) southwest of Honomū at a depth of 45 km (28 mi).