Volcano Watch — Volcanic ash lands at Jaggar Museum during south wind

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Visitors watching the lava from outside Jaggar Museum during a south (kona) wind may feel their skin prickle with excitement…or is it volcanic ash? Most often the former, but sometimes there really is ash in the air. What is this stuff?

Volcanic ash lands at Jaggar Museum during south wind...

A faint dusting of volcanic ash, produced by a small rockfall-triggered explosion in the Kīlauea summit lava lake on April 25, 2015, fell on this car parked near Jaggar Museum. Light rainfall afterward caused the ash to clump. USGS photo.

(Public domain.)

First, what it is not. Ash is a light, fluffy residue of fire, as anyone knows who has sat around a campfire. Ancient peoples believed that volcanoes produced fire, so any fine-grained material ejected by a volcano was called ash. The inaccurate association of volcanoes with fire continues to the present, with discredited terms such as fire fountain and curtain of fire still used today by some in Hawai‘i. Terms such as ash and cinder have worked their way into scientific usage, however, and we just have to live with them.

So what really is volcanic ash? It consists of fine rock particles, formally defined as less than 2 mm (0.07 in) in diameter, but often used for somewhat larger particles that are ejected from a volcano. In general, there are two types of ash particles at Kīlauea.

The most common type is made from the skin of bursting gas bubbles in lava. Tiny fragments of the skin fly into the air, where they rapidly cool to volcanic glass. These glass fragments can be Pele's hair, Pele's tears, tiny hollow spherules, and finer material that is simply called ash. Wind then blows them away from the vent, and a south wind carries them toward Jaggar, where Pele's hair can often be found.

When rocks fall from the walls of Halema‘uma‘u Crater, dust rises and is blown away by the wind. This dust comes from older rocks, not lava, and often is dull gray, not shiny. It is not volcanic ash, because it is not formed by a volcanic process. It is just dust such as one sees along dirt roads, farmer's fields, and construction areas.

In the current situation at Kīlauea's summit, however, rock-fall dust often mingles with fragments formed by bubble bursts to create a mixture containing both glass and dust. In this case we call the dust volcanic ash, because it is propelled from the crater by volcanic gas. Such mixtures often fall outside Jaggar Museum.

In fact, all recent explosive events, which result from rock falls and avalanches into the lava lake in the Overlook crater, produce such mixtures. Falling rocks break through crust on the lake and trigger, by a poorly understood process, vigorous bubbling in the lava that forms glassy volcanic ash. This glassy ash mixes with rock-fall dust to form the mixed type of volcanic ash.

All ash fragments, whether glassy or from older rocks, are abrasive. That is because the material itself is hard, and also because it has sharp edges very different from, say, beach or river sand grains, which are rounded by water action. Pele's hair can easily puncture skin. Be careful when removing ash from clothing or skin. Rinse, don't rub. Try to avoid inhaling ash or getting it in your eyes.

Heavy falls of ash from large eruptions, such as one sees on TV news, are not expected at Kīlauea under current conditions. In the past, however, Kīlauea has been very explosive and produced thick deposits of ash that encircle the caldera. Now, about all we can expect north of Halema‘uma‘u is a paper-thin dusting. This can be a nuisance, because ash doesn't melt like snow, but it is not a major hazard.

So, enjoy the light sprinkles of ash from time to time at Jaggar Museum, or anywhere north of there, if a south wind is blowing. You just stand there and the eruption sends a greeting to you!

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Volcano Activity Update

Kīlauea's summit lava lake was brimful throughout the week and occasionally overflowed onto the floor of Halema‘uma‘u Crater. A small explosive event on Sunday, May 3, threw spatter from the lava lake onto the rim of Halema‘uma‘u near the overlook and parking areas, which have been closed since 2008 due to ongoing and significant volcanic hazards there. Seismicity remains elevated beneath Kīlauea's summit and the upper East and Southwest Rift Zones.

Kīlauea's East Rift Zone lava flow continues to feed widespread breakouts northeast of Pu‘u ‘Ō‘ō. The most distant active lava was about 8 km (5 mi) from Pu‘u ‘Ō‘ō when mapped on Tuesday, May 5.

Four earthquakes were reported felt on the Island of Hawai‘i in the past week. On Friday, May 1, 2015, at 1:05 a.m., HST, a magnitude-2.6 earthquake occurred 3 km (1.9 mi) southeast of Pohoiki at a depth of 1.2 km (0.7 mi), and at 9:30 a.m., HST, a magnitude-3.2 earthquake occurred 4.9 km (3.1 mi) southwest of Volcano at a depth of 2.9 km (1.8 mi). On Monday, May 4, 2015, at 4:42 a.m., HST, a magnitude-3.6 earthquake occurred 5.1 km (3.2 mi) southwest of Volcano at a depth of 3.0 km (1.9 mi), and at 9:28 p.m., HST, a magnitude-2.5 earthquake occurred 4.8 km (3.0 mi) northwest of Captain Cook at a depth of 10.4 km (6.5 mi).