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Volcanowatch

August 3, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Kilauea's magma budget

How much magma enters Kilauea every day? How much is erupted to the surface? How much stays underground?

These questions strike to the heart of what makes Kilauea work. But all important questions about nature are tough to answer, and those three have eluded definitive answers. Here's where we stand today.

The amount of lava erupted each day should be easy to determine, but it's not. Let's start with the simple case of an eruption in a pit crater, such as at Halemaumau in 1952 and 1967-68. Here the dimensions of the crater were well known, so it was an easy matter to calculate the volume of lava added to the crater. The volume, however, is made of liquid and gas. The bubbles puff up the lava and make it look more voluminous than it really is, just as the popped corn you eat won't fit in the bag the corn came in. So, the calculated volume of lava in the crater must be adjusted for the amount of gas bubbles it contains-an inexact estimate, at best.

Another way to estimate the amount erupted is to calculate the amount of lava flowing through tubes. That is the way we make the estimate during the ongoing eruption. The cross-sectional area of flowing lava is estimated using a geophysical method based on very low frequency radio waves. Then the velocity of the lava is measured with a radar gun. Multiplying the cross-sectional area by the velocity gives the flux of lava through the tube. If there is only one active tube, then this flux is the daily amount erupted by Kilauea. There is generally more than one tube, however, and the measurements are not exact, and the problem of bubbles in the lava is still there. So, the final answer, though sophisticated and the best we can do, is an estimate only.

Recently another method has been used-measuring the amount of sulfur dioxide gas given off in the Pu`u `O`o area. This amount is directly proportional to the amount of lava that loses the gas. Since the gas is lost near and on the surface, this volume is a credible estimate of the lava supplied to the surface. The flux determined in this way generally agrees to within 25 percent of that calculated from the tubes.

All the methods suggest a long-term (months) average daily supply of lava to the surface of about 300,000 cubic meters (400,000 cubic yards). That's the equivalent of 40,000 moderate-size (10-yard) dump trucks! This number comes up repeatedly and suggests a throttle within the volcano that governs the amount of lava that can reach the surface. Short-term bursts of activity from pressurized reservoirs can supply magma at much higher rates, but over the long haul the throttle controls the output to about 300,000 cubic meters (400,000 cubic yards) per day.

But is that all of the magma that enters Kilauea? Is some permanently stored within the volcano? What about the times between eruptions? Does the supply to the volcano shut off, or is it held within the volcano, awaiting eruption?

Recent modeling of ground deformation, focused on the eight years before the start of the current eruption but generalized to the present, suggests that magma does enter the volcano during quiet periods. The flux to the volcano is estimated to be about twice the amount supplied during long eruptions--about 600,000 cubic meters (800,000 cubic yards) per day. Before the eruption, most of this was stored within the volcano (some escaped during small eruptions); now about half is erupting and half remains stored. The modeling, done by HVO colleagues Valerie Cayol (France) and Jim Dieterich (USGS, Menlo Park, CA) aided by HVO staff scientists Asta Miklius and Arnold Okamura, provides the best estimate yet of the total magma flux into Kilauea. But it is only an estimate that future researchers will surely improve.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast near the eastern boundary of Hawai`i Volcanoes National Park. Breakouts from the tube system feed flows on top of Pulama pali and in the coastal flats. Lava is entering the ocean mainly at two locations: the coast near the site of the buried Waha`ula heiau and 1.2 km (0.7 mi) to the west of Waha`ula at Kamokuna. Several small entries, located to the east of Waha`ula, are weak and ephemeral. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

A resident of Leilani Estates subdivision felt an earthquake at 9:05 p.m. on July 31. The magnitude-2.0 earthquake was located 9 km (5.4 mi) southeast of Pu`ulena Crater at a depth of 7.0 km (4.2 mi).


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Updated: August 8, 2000