Volcano Watch — Crater-floor eruptions require suitably pressurized magma

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The question might be answered in part by explaining why lava erupts at all. Magma ascends through the earth's crust because it is hotter and therefore more buoyant than its surroundings. But once magma nears the surface, its heat alone isn't enough to drive an eruption.

Magma erupts because of gas dissolved in it at depth. The gas forms bubbles or vesicles and tries to escape at the low pressures near the earth's surface. The technical term for this bubble formation is "vesiculation." Trapped in the magma, the bubbles expand and force the magma toward the surface. Magma with insufficient gas bubbles won't erupt.

Approaching Pu`u `O`o from depth, the magma rises by vesiculation and bubble expansion. If the cone and its surroundings are highly fractured, the magma may rise only partway in the vent's throat. The gas escapes through the fractures before the magma expands enough to erupt in the crater. But with time, the magma may seal the system. Blockages may occur intermittently at the point where the lava tube normally drains magma away from Pu`u `O`o. These blockages cause changes in pressurization, allowing the magma to reach the surface and spill onto the crater floor as lava.

The ponded lava rarely overtops the crater rim. Instead it drains back into its conduit. This drainback occurs because the lava cools and loses its gas to the atmosphere. This colder, denser lava forms a cap that stalls further vesiculation in the underlying magma column. Meanwhile, bubbles coalesce within the maturing magma column. As they grow in size they rise more rapidly, hollowing out the core of the column. Upon intercepting the pond, this gas-rich core first enhances surface bubbling and spatter. But as gas pressure diminishes, the pond lava begins to drain into the void. The gas-driven rise and fall of the magma is termed "gas pistoning."

Each drainback event in the autumn of 1999 triggered an inflation in the underlying rift-zone conduit, as measured by a tiltmeter near Pu`u `O`o. The degassed lava flushing down from the pond disrupted the equilibrium of the vesiculating magma column. Gas, trapped and unable to rise and escape, caused momentary swelling underground.

Throughout the pond filling and drainback, magma was still feeding into the lava tube and onto the flow field. Flow-field discharge might have varied subtly in response to the pond drainbacks, but if it did, we did not observe it. From the flow field's point of view, Pu`u `O`o is merely a glory-hogging open snout on a pipe that feeds the lava tube.

The autumn 1999 pond existed because of a delicately balanced pressurization that ended when the supply of magma from the summit was interrupted briefly in November 1999. The pond probably disappeared because enough cracks opened in the cone so that a vesiculating magma column lacked the pressure to rise beyond its subterranean realm, instead swelling and shrinking underground beyond our sight. Perhaps the recently renewed activity at Pu`u `O`o indicates another round of pressurization favorable for several months of small-volume lava flows in the crater.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated and effusively at the Pu`u `O`o vent during the past week. Lava erupted intermittently from several vents in the crater of Pu`u `O`o as well as from Puka Nui, a small crater at the southwest base of the cone. The bright glow over the "rootless" shield area diminished slightly during the past week. No surface flows were observed on Pulama pali or on the fan at the base of the pali. There are no ocean entries.

One earthquake was reported felt during the week ending on April 25. Residents of Ka`u and Puna felt a magnitude-3.4 earthquake at 2:25 p.m. on April 21. The temblor was located 4 km (2.4 miles) northeast of Pahala at a depth of 12.5 km (7.5 mi).