February 24, 2005

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

A closer look at lava fountains

Volcanology is an inexact science. At least, that is what we are always led to believe. Increased sophistication in measurement techniques however, has enabled volcanologists to become more precise with their data and interpretations. The increased precision has either improved or overturned many long-held beliefs and models of volcanic processes.

Recently, Patrick Allard, Mike Burton, and Filippo Muré managed to achieve a "first" with infrared spectroscopy measurements on Mount Etna, a basaltic volcano in Italy. They imaged the gases roaring out within a basaltic lava fountain.

Lava fountains are sometimes called Hawaiian explosive activity, named after the type locality for this sort of eruption-that is, Kilauea Volcano, Hawai`i. This phenomenon often occurs at other basaltic centers in different tectonic settings, like at Mount Etna, Sicily. Fountains form when the gas contained within the magma cannot escape quickly enough as the magma is rising towards the surface, resulting in a jet-propelled mixture of lava and gas streaming into the air.

Magma erupts because of the gases contained in it. Whether they erupt quietly as lava flows or explosively as a lava fountain depends on the way in which gases escape from the magma. An analogy is shaking a bottle of soda and then opening it. If you unscrew the cap very quickly, a foam is produced, which will cover you in spray. If you open it slowly, the gases will escape bit by bit, and when you eventually get the cap off, you won't get wet. Lava fountains are produced when you take the cap off a column of magma quickly.

In January to June 2000, the southeast crater of Mount Etna produced 64 spectacular episodes of lava fountaining, with some up to 3,000 feet in height, or twice the highest fountains from Pu`u `O`o.

The key result of their findings is the ability to discriminate between two models of fountain generation-one in which the lava degasses during fountaining, and one in which a substantial amount of the gas is released at depth (greater than 1 km (3,000 feet)) beneath the volcanic edifice, prior to fountaining.

In the first scenario, gases bubble out as magma rises very rapidly towards the surface. The fountain forms within a few tens of meters (feet) of the surface.

In scenario two, the gas separates from the magma at greater depths, forming a pocket of gas in the form of a foam. When the amount of foam gets large enough, the foam collapses. All the little bubbles in the foam become one super-bubble, which rushes to the surface with magma surrounding it, generating the fountain. This is the model used to explain fountaining at Pu`u `O`o, Hawai`i.

The results of the work on Mount Etna appear to indicate that scenario two is the most accurate of the two models.

How do the results indicate this? It has to do with the composition of the gas. As magma decompresses (rises to the surface), different gases bubble out in different amounts. For example, at Kilauea, carbon dioxide begins to bubble out at very great depths-50-60 km-beneath the surface while sulphur dioxide will only bubble out substantially once the magma reaches a few hundred metres to a kilometer beneath the surface. Other gases, such as hydrogen chloride, bubble out at still shallower depths.

The gases emitted from the lava fountains on Mount Etna clearly showed higher proportions of the deep-derived magmatic gases, carbon dioxide, and sulphur dioxide. The gas composition indicates that the gas phase formed, and reached equilibrium, at depths of around 1.5 km (1 mile) beneath the surface, before rapidly ascending to the surface in the fountain without time to adjust to the lower pressures.

Advances of this nature are becoming possible with the advent of more sophisticated instrumentation with which to monitor volcanoes. Our rate of understanding these complex systems is increasing all the time. As the quality of our observations improves, volcanology is rapidly becoming an increasingly precise science.

Activity update

Eruptive activity at Pu`u `O`o continues. At least five of the vents inside Pu`u `O`o crater and on the west and southwest flanks of the cone were spattering vigorously this past week, producing bright glow on clear nights.

The PKK flow continues to host substantial breakouts from the 2,300-ft elevation to the coastal plain. A new ocean entry began on February 20, at East Lae`apuki, about halfway between the existing ocean entries at West Highcastle and Ka`ili`ili. The closest activity to the end of Chain of Craters Road in Hawai`i Volcanoes National Park is at West Highcastle, 2.6 km (1.6 mi) from the ranger shed. Expect a 1-to-1.5-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs.

During the week ending February 24, only one earthquake was felt on Hawai`i Island. The magnitude-3.4 quake occurred 4 km (3 miles) north of Kilauea summit at a depth of 22 km (14 miles) at 9 minutes after midnight on Saturday, February 19.

These felt Kilauea earthquakes are part of a swarm beneath Kilauea summit that started in mid-January. Kilauea summit continues to inflate, with the rate increasing from 8 cm/yr (3 inches/yr) to over 40 cm/yr (15 inches/yr) in January 2005.

Mauna Loa is not erupting. The summit region abruptly stopped inflating at the end of January 2005. Since July 2004, the rate of inflation and number of deep earthquakes has increased. Weekly earthquake counts have varied from 5 to over 150 in the last half of 2004 but have been less than 10 since the beginning of 2005. During the week ending February 23, nine earthquakes were recorded beneath the summit area. Unlike the past seven months, when nearly all the quakes were 30 km (18 mi) or more deep, and of the long-period type, most of these are shallower and the more typical short-period type.

Visit our web site (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time Hawai`i earthquake information.