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August 13, 1998

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


Bubble, bubble? Toil and trouble!

Geology is an inexact science, and sometimes studying volcanoes seems like a game of chance. This is particularly true for those who study the way lava flows.

The study of how fluids change shape (deform), whether honey on a spoon, brie on a hot day, or lava in a tube, is known as rheology. Its applications range from making smoother ice cream to predicting the course of the next lava flow.

Knowing how magma and lava flow is fundamental to understanding how, why, and when volcanoes will erupt. Normally, studying the rheological properties of a fluid is easy. You place a sample of your fluid in a high-precision laboratory instrument, apply a carefully controlled force to it, and measure the way it deforms. Now try doing that with lava at 2,200°F (1200°C). Not so easy!

It is not practical to melt large quantities of rock in the laboratory. Fortunately, Pele has provided us with a magnificent outdoor laboratory - Kilauea's east rift zone - where lava has been erupting almost continuously for the last 14 years. Unfortunately, most of the lava travels all the way to the ocean in lava tubes, and the only chance to observe the subterranean flow is through small, naturally occurring holes, called skylights, in the roof of the tube.

Estimates of the size of the tube and the flow rate through it can teach us something about how the lava flows. Accurate measurements, however, are hard to obtain, and it is impossible to look beneath the surface of the flow this way. To get a good look at the internal deformation of the lava requires a new approach.

One technique currently being developed by scientists at HVO relies on the fact that most lava contains many small, round gas bubbles. As the lava flows, the bubbles are deformed and become football-shaped. The degree of deformation depends on the speed of the flow and the rheological properties of the lava. So, if you measure the speed of a flow and cool the rock quickly enough to preserve the shape of the bubbles, then you can measure the deformation of the bubbles in the laboratory and work out the rheological properties of the lava. It's simple in theory, but not in practice.

Suitable lava flows, such as slowly creeping pahoehoe flows, are hard to find. By the time you have heard about an active flow, gathered your equipment together and hiked out to it, the lava has invariably solidified, leaving a red-faced, muttering volcanologist with nothing better to do than carry everything home and wait for the next breakout.

Even if the flow is still moving, the procedure is not straightforward. Collecting a sample of lava from a moving flow without deforming it in the process proves easier to say than to do. In fact, just getting close enough to sample from a flow without losing your eyebrows is a struggle! Our first attempt to collect undeformed samples, using a scoop and a bucket of water, was unsuccessful, but then the first attempt usually is.

Plan B awaits the next breakout. It will involve one person dashing up to a likely looking pahoehoe lobe and lopping it off with a shovel. A helper will then douse the severed lobe with water to harden the skin and prevent it from sagging. Meanwhile, the shoveler will try desperately to prevent the rest of the flow from overwhelming the sample before it has cooled enough to be moved. Simple? Maybe we'd better start working on Plan C. . . .

Eruption and Earthquake Update

A pause in eruptive activity at the Pu`u `O`o vent started on Wednesday afternoon and continued through Thursday afternoon (time of this update). Only a dribble of lava was observed in the tube system and at the coastal entry at Kamokuna on Thursday. Pauses in activity generally last no longer than a few days, and lava may be flowing again by the time this is printed on Sunday. The public is reminded that the ocean entry area is extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

Three earthquakes were reported felt during the past week. Residents of Hamakua, Hilo, and Puna reported feeling an earthquake at 3:27 p.m. on Saturday, August 8. The magnitude 3.8 earthquake was located 13.5 km (8 mi) southeast of Kilauea summit at a depth of 7.5 km (4.5 mi). Early Wednesday morning at 4:59 a.m., a magnitude 3.7 earthquake was felt in Waikoloa, Waimea, and Ahualoa. The earthquake originated 5 km (3 mi) northeast of Pu`u Wa`awa`a from a depth of 15 km (9 mi). Later in the morning of August 12 at 6:34 a.m., a Ka`oiki fault earthquake with a magnitude of 3.6 was felt in the Volcano area. The earthquake was located 17 km (10.5 mi) northwest of Kilauea summit at a depth of 10 km (6 mi).

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Updated: 23 August 1998