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Fallen ash has a story to tell

December 6, 2013 - 6:28pm

Last week we showed that Kilauea has explosive eruptions that can carry volcanic ash, less than 0.08 inch across, and small lapilli, 0.08 to 2.5 inches across, high into the sky. Today we track the flight of two small lapilli, one confined to the trade winds and one rising higher.

Sometime between 1790 and 1823, an explosive eruption deposited ash and lapilli in the area shown on the map. The dots are places where the average particle size was measured. Black lines, contours, connect equal sizes. For the lapilli described below, the red lines show rising pathways, the green lines show falling pathways.

All contours are initially aimed southwest, but those of smaller particles bend southeast. Why? We think the larger lapilli were too heavy to rise above the north-northeast trade wind. The smaller particles, however, went higher into the jet stream, where they encountered its strong northwest wind before later falling into the trades. Here’s how it works.

A particle of 0.2 inch diameter and terminal fall velocity of 30.5 feet per second landed at A on the map. If it rose 1.8 miles in the eruption column, it would be blown slightly southwest by the trades during its rise and then take about 5.5 minutes to fall. If the trades were blowing 16 feet per second, the particle would drift about 1 mile south-southeast during its fall; if the wind speed were 26 feet per second, the particle would blow about 1.5 miles. Given all plausible possibilities, the particle could have fallen 1.8 to 2.4 miles and reached its landing site in about 5.5 to 7 minutes.

A smaller particle of 0.1 inch diameter and terminal fall velocity of 28 feet per second landed at B after a more adventurous trip. It rose into the jet stream and was blown a little southeastward. It could have risen about 1.2 miles into the jet stream; any higher, and it would have overshot its landing site. It then took almost 4 minutes to fall back into the trades, as it drifted southeast 2.1 to 3 miles at wind speeds of 50 to 65 feet per second. Once in the trades, it fell 2.4 miles to the ground in nearly 8 minutes at a wind speed of 16.5 feet per second, being blown about 1.5 miles southwestward. From the top of the 3.6-mile eruption column, it took almost 12 minutes for the 0.1 inch particle to land.

These calculations are oversimplified but show how terminal fall velocity and wind speed control the distribution of small particles. We can estimate column heights and particle pathways for other explosive eruptions at Kilauea and find that small particles from many entered the jet stream, rarely even the stratosphere. Is it any wonder that we consider Kilauea an explosive volcano?

Kilauea activity update

A lava lake within the Halemaumau Overlook vent produced nighttime glow visible via HVO’s webcam during the past week. A deflation-inflation cycle occurred early in the week, and the lava lake level fluctuated correspondingly.

On Kilauea’s east rift zone, the Kahaualea 2 flow continues to advance slowly into the forest northeast of Puu Oo. The active front of the flow is about 4 miles northeast of Puu Oo.

There were no earthquakes in the past week reported felt on Hawaii Island.

Visit for current Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to

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

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