Volcano Watch — "Methane" explosions-a volcanic hazard worth understanding

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Recent visitors to the coastal eruption site, especially those unwise enough to approach the flow margins where lava is encroaching on vegetation, are being greeted by a sometimes underrated volcanic hazard-the "methane" explosion.

Depending upon how close you are to the explosion site, your experience can range from hearing a far-off, deep-sounding boom, to being thrown several meters (yards) across the hard and abrasive lava as the ground beneath you explodes. Regardless of where you're standing, the sound of these explosions is a call for your respect -- and besides, they're cool to know about!

As a lava flow enters a vegetated area, grassland or forest, all the biomass in the flow's path becomes available for one or both of two processes: combustion and/or pyrolysis. Molten lava at 1,130 degrees C (2,066 degrees F) is four times hotter than your cooking oven's maximum temperature. Most natural materials on the surface of the flow field, such as grasses, trees, and shrubs are immediately burned up (combusted) as lava covers the area. The bases of very large trees often become encased in lava, charring the outside of the trunk but not completely burning up on the inside. This is the process that forms "lava trees" or lava-tree molds-a topic for another Volcano Watch.

The process of lava tree formation is instructive, however, for understanding what happens to root masses and tree trunks beneath the ground as the surface vegetation burns. Root material protruding downward through the thin soil and into the cracked pahoehoe surface beneath flows does not burn completely. Instead, intense heat from surface flows radiates slowly downward (remember, rock can act as an effective insulator) and "cooks" the subsurface vegetation rather than burning it.

The lava cooking temperature is high enough to accelerate chemical breakdown of biomass as it chars the buried roots and stumps. A similar commercial process, pyrolysis, cooks wood in large, very hot ovens to make charcoal and another fuel byproduct called "producer" gas. Both producer gas and the gas generated by the lava flow consist of a mixture including methane, hydrogen, and carbon monoxide. Commercially, the flammable producer gas is extracted and burned to generate heat or electricity.

Beneath the lava flow, our fuel-gas mixture from the root mass penetrates subsurface passages, such as old empty lava tubes, tumuli, and cracks. The fuel-gas combines with air in these empty spaces to form combustible gas pockets. Recall that if you have the right proportions of a fuel (such as methane), a source of oxygen (such as air), and finally a source of heat (such as a match), you can make a fire.

When the underground air/fuel mixture is between 5 and 15 volume-percent fuel, a spark or the heat from a lava flow can ignite it. If ignition occurs in a constricted space, such as a lava tube, we observe Kilauea's answer to another technological innovation-the internal combustion engine. In your car's engine, the energy released as the fuel/air mixture is ignited in the confined space of the engine's cylinders is ultimately transmitted to the drive wheels and propels you down the road. Likewise, if you're standing over a tube or tumulus when it explodes, which can happen if you ignore the National Park's warnings, well, go figure.

There have been hundreds of these "methane" explosions while the Mother's Day flow has invaded over four square kilometers (1000 acres) of vegetation. We encourage visitors to stay safe by heeding the Park's posted warnings--and stay well away from areas where lava is encroaching on even sparse vegetation. Respect those booms. They're trying to tell you something!

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. A large surface flow was visible on Pulama pali all week. Numerous breakouts occur in the coastal flats, and the National Park Service has marked trails out to the closest activity. Lava is entering the ocean at Wilipe`a and West Highcastle. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

There were no earthquakes reported felt throughout Hawai`i in the week ending on October 17.

Mauna Loa is not erupting. The summit region continues to inflate, but no earthquakes were located in the area for the last seven days.