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March 9, 2000

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


Flames from vents are hydrogen gas burning

Large and colorful flames sometimes play from vents on Kilauea volcano. These flames, which range from yellowish orange to greenish blue and reach as high as 1 m (3 ft), result from hydrogen gas burning in air. Hydrogen is normally a minor constituent of volcanic gases. It is generated deep inside a volcano by the reaction of iron compounds with water. The temperature at which hydrogen ignites in air depends on its concentration; generally the more hydrogen present, the lower the temperature required for ignition. For example, the great Hindenburg disaster of 1937 occurred when a dirigible containing pure hydrogen at typical outdoor temperature caught fire and expoded as a result of an errant spark. Tremendous temperatures are required, however, to ignite the low concentration present in volcanic gas.

The hot walls of Kilauea's vents have temperatures in excess of 1,000?C (1,830?F). Additional heat may be provided by solids in the fume. Flames occur most commonly after episodes of lava fountaining, when vents are especially hot. The large flames that follow immediately after fountaining from Kilauea's vents sometimes persist for several hours and then slowly diminish and disappear. Their disappearance probably results from cooling of the vents' throats below the ignition temperature of the dilute gas, rather than from a diminishing gas concentration.

We have seen, on rare occasions, subtle blue flames flickering in the throat of littoral cones. These cones form where lava interacts explosively with seawater at the ocean's edge. Most of Kilauea's littoral cones lie at great distances from the vents that erupted the lava, 10 km (6.25 mi) or more in the case of the present eruption. The lava has degassed greatly in its journey to the ocean. At littoral cones, the presence of water from the ocean provides a source that increases the concentration of hydrogen gas to the point where it can combust. Flames sometimes occur above lava lakes, too. For example, an intense yellow flame danced above the `Alae lava lake in December 1972. `Alae was a near-vent lava lake, and the hydrogen gas was escaping from the impounded lava.

Typically, however, lava flows themselves don't emit flames. Their broad surfaces allow gas to disperse quickly, and their temperatures, although hot enough to roast the skin of onlookers standing nearby, are too low to ignite the dilute levels of hydrogen gas found at the flow surfaces. Instead, the flames that visitors often see alongside lava flows result from burning vegetation. Trees may be rafted along the surface of active lava flows, burning their way to oblivion as they are sluiced down steep slopes onto the flat land of the volcano--a spectacle of flames unrelated to the hydrogen gas that characterizes magma. Vegetation trapped beneath hot lava flows literally cooks, releasing methane gas that combines with atmospheric oxygen, sometimes bursting explosively through the flows. We at the Hawaiian Volcano Observatory have used Volcano Watch to dispel myths about volcanoes. One of them is the association of fire with eruptions. The poetic but inaccurate phrases "curtain of fire" or "fire-fountaining" come to mind. In reality, fire and flame are a part of vent-related phenomena only during the brief episodes when hydrogen gas ignites.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week, and a glow from Pu`u `O`o crater was visible at various times. Lava erupts from Pu`u `O`o and flows through a network of tubes toward the coast. One lava entry at the ocean remains active at Lae`apuki. The other entry, at Waha`ula, has been inactive for most of the past two weeks. Lava escapes from the tube intermittently from sites above and on the face of Pulama pali, producing short-lived `a`a and pahoehoe lava flows. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying unpredictable 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.

There were no earthquakes reported felt during the week ending on March 9.

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Updated: 13 Mar 2000