Categorized | Featured, Sci-Tech, Volcano

Volcano Watch: What goes up must come down, at least at volcanoes

Time-lapse movie of Kilauea Caldera as seen from Hawawaii Volcano Observatory

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Hawaii Volcanoes National Park. 1960 eruption of Kilauea Volcano. Papaya trees stripped, fountain in the background at Kapoho. January 1960. Photo courtesy of USGS

Hawaii Volcanoes National Park. 1960 eruption of Kilauea Volcano. Papaya trees stripped, a fountain in the background at Kapoho. January 1960. Photo courtesy of USGS

In past Volcano Watches, we’ve emphasized a radically new way to view Kīlauea—as an explosive volcano. Though not explosive now, it was dominantly explosive during 60 percent (1,500 years) of the past 2,500 years. Its latest major period of explosiveness lasted 300 years between the dawn of the New World and the Napoleonic era.

Explosive eruptions eject particles weighing more than 10 tons to as little as a piece of Pele’s hair. We are concerned with how high such particles, mostly ash (less than 2 mm or 0.08 inches in diameter) and lapilli (2–64 mm or 0.08-2.5 in), can rise in an “eruption column” above the top of Kīlauea. Why? Because the height tells us 1) whether aircraft need to be concerned and 2) how far downwind ash will travel, since the higher ash goes, the farther it will be blown.

Sophisticated computer models calculate column height from the size and fallout distance of ash and fine lapilli. They give good estimates of column height in large, sustained eruptions, such as typify most explosive volcanoes. However, past explosions at Kīilauea were far smaller than those in the models, in terms of both eruption volume and duration—several hours (at most) rather than one or more days. Large volumes and long durations are assumed for most models to work; the more material is erupted and the longer the eruption continues, the more the air is heated and the longer it stays heated, creating buoyancy that may carry ash to great heights, sometimes more than 35 km (21 mi). Kīlauea is not like that, and so we can’t reliably use the models.

We have known for 10-12 years that some past eruptions at Kīlauea propelled ash and small lapilli into the jet stream. John Young’s observations in 1790 imply that; and ash is found far east and southeast of the summit, where only the westerlies of the jet stream could have blown it. That means that some eruption columns rose into the air at least 5 km (3 mi)—about the base of the jet stream—and likely much higher.

Another way to estimate the height of past eruption columns at Kīlauea is to calculate how far a falling particle could be blown by the wind. This distance depends on the time it takes the particle to fall to the ground. A particle can’t be blown when it’s no longer in the air!

Within a few seconds, falling particles reach a constant terminal fall velocity (TFV) controlled by air resistance. Particle size, shape, surface roughness, and density are crucial factors governing TFV. Knowing the TFV of a particle, we can determine how long it would take that particle to fall from a given height. Knowing wind velocity, we can calculate how long it would take that particle to be blown from the eruption column to its site of deposition. Balancing these two times—fall time and dispersal time—allows us to estimate the column height. If the column were not high enough, the fall time would be too short for the wind to blow a particle to its actual landing site. If the column were too high, wind dispersal would last too long and the particle would overshoot that site.

The TFV of a particle is the big unknown, but fortunately TFVs were recently calculated for ash and fine lapilli from one of Kīlauea’s 1790 eruptions by colleagues at the University of Geneva, Switzerland. The particles have physical characteristics typical of those in other Kīlauea explosive eruptions. A range of reasonable wind speeds is available, thanks to measurements made with twice-daily weather balloon releases by NOAA in Hilo.

Using these TFVs and wind speeds, we calculate column heights of 10-22 km (6-13 mi) for several past explosive eruptions. We estimate a height of 12-15 km (7-9 mi) in 1790 and 18-22 km (11- 13 mi) in an eruption 1,200 years ago. Pretty substantial, and finer ash would go still higher! These eruptions were powerful but of small volume and short duration, a seeming enigma currently under scrutiny as we try to figure out what causes such eruptions at our drive-in volcano.

Next week we track the pathways of two small lapilli in the same eruption, one that entered the jet stream and one that didn’t.

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