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Volcano Watch: Petrologist explains Kilauea’s eruptive condition

The western Kamoamoa fountain-fed lava flows advanced over 1997 lavas (dark flow in foreground) with 1965 and 1968 lavas buried by Puu Oo cinder to the far left. Puu Oo is in the distant background. (Photo courtesy of HVO)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Kilauea is the most studied volcano in the world. You’d think that, after a century of scientific scrutiny, we’d have it completely figured out by now.

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

Since January 1983, Kilauea 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 Napau Crater, eventually concentrating the vent now known as Puu Oo.

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 Puu Oo.

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 Puu Oo.

In March 2008, when the summit eruption began, Kilauea 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 Kilauea’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 Halemaumau and Puu Oo.

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 Napau Crater on Jan. 29-30, 1997. Both fissure eruptions resulted from the abrupt opening of the rift zone upstream from the active Puu Oo vent.

In both cases, magma throughout the pressurized plumbing system was drawn toward the zone of rift rupture. Lava drained from the Puu Oo crater and from the summit, and magma rose to the surface near Napau Crater.

Like the 1997 Napau Crater lava, the Kamoamoa fissure lava does not match lava from the Puu Oo 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 Kilauea’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.

Kilauea is still recovering from the Kamoamoa eruption. Magma continues to rise into Kilauea from deep below the volcano; lava has reappeared in the Halemaumau and Puu Oo 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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