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Volcano Watch — Fallen ash has a story to tell

December 5, 2013

Last week we showed that Kīlauea has explosive eruptions that can carry volcanic ash (less than 2 mm or 0.08 in across) and small lapilli (2–64 mm or 0.08–2.5 in) high into the sky. Today we track the flight of two small lapilli, one confined to the trade winds and one rising higher.

Area covered by tephra deposit south of Kīlauea Caldera described in text. Dots show places where the average particle size in the deposit was examined. Black lines connect equal average particle diameters; heavy black lines indicate diameters of 6 mm (0.2 in.) and 3 mm (0.1 in.) discussed in text. A and B indicate landing location of the two tracked particles. Red lines show hypothetical pathways of the particles as they were rising into the air, and green lines, hypothetical pathways as they were falling.

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 (in millimeters) 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 6 mm (0.2 in) diameter and terminal fall velocity (TFV; see last week's Volcano Watch) of 9.3 m/s (30.5 ft/s) landed at A on the map. If it rose 3 km (1.8 mi) 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 5 m/s (16 ft/s), the particle would drift about 1.6 km (1 mi) south-southeast during its fall; if the wind speed were 8 m/s (26 ft/s), the particle would blow about 2.5 km (1.5 mi). Given all plausible possibilities, the particle could have fallen 3–4 km (1.8–2.4 mi) and reached its landing site in about 5.5–7 minutes.

A smaller particle of 3 mm (0.1 in) diameter and TFV of 8.5 m/s (28 ft/s) 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 2 km (1.2 mi) 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 3.5–5 km (2.1-3 mi) at wind speeds of 15–20 m/s (50–65 ft/s). Once in the trades, it fell 4 km (2.4 mi) to the ground in nearly 8 minutes at a wind speed of 5 m/s (16.5 ft/sec), being blown about 2.5 km (1.5 mi) southwestward. From the top of the 6-km (3.6 mi) eruption column, it took almost 12 minutes for the 3-mm (0.1 in) particle to land. What a ride!

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


Volcano Activity Update

A lava lake within the Halema‘uma‘u Overlook vent produced nighttime glow that was visible via HVO's Webcam during the past week. A deflation-inflation cycle (DI event) occurred early in the week, and the lava-lake level fluctuated correspondingly.

On Kīlauea's East Rift Zone, the Kahauale‘a 2 flow continues to advance slowly into the forest northeast of Pu‘u ‘Ō‘ō. The active front of the flow is about 6 km (4 miles) northeast of Pu‘u ‘Ō‘ō.

There were no earthquakes in the past week that were reported felt on the Island of Hawai‘i.