Tuesday, December 22, 2015

Explosive dangers at Kilauea volcano

- Janine

My first AGU Fall Meeting was so full of wonderful science - emergency management exchanges with Colombia to address hazards of lahars (Nevado del Ruiz), volcanic lightning, active monitoring of volcanoes, community preparedness, and all aspects of volcanic activity above, on, and below the surface. I had great conversations with people excited by their work and eager to communicate their work with the AGU masses.

One of the many posters that caught my attention was "Don't forget Kilauea: Explosive Hazards at an Ocean Island Basaltic Volcano" by USGS volcanologist Don Swanson. When I talk to people about volcanology the first thing to come up is usually how cool it must be to study lava flows at Hawaii. Well, I don't study effusive lava flows, I am on the explosive end of the spectrum with dome collapse block and ash flows, and I have not yet visited Hawaii. One thing is obvious, many people I talk to think of the relatively safe (with exceptions) pahoehoe lava flows when they think of volcanic activity, especially at Hawaii.

Kilauea Overlook, photo by NPS.
What many people are not aware of is the very dangerous explosive personality of Kilauea. Around 2.6 million people visit Kilauea volcano ever year according to the National Park Service, you may be one of them (I hope to be, one day). You may have even been lucky enough to see the beautiful lava flows like this one:

But do you know about Pele's more dangerous and explosive side?

According to Don Swanson (and references listed below) Kilauea's explosive nature includes: subplinian ash plumes up to heights of 10 km; phreatomagmatic (magma + water) and phreatic (steam) explosions; pyroclastic density currents (especially pyroclastic surge); volcanic ashfall; and ballistic ejecta (flying rocks) around the entire summit area with total eruption volumes up to 0.02 km3 (VEI 3) - based on studies of explosive events that occurred during the past 2,500 years.

These are relatively small events, but for any of those 2.6 million people visiting, or 2575 people living (2010 population) near the summit they can be deadly. Once these explosive episodes get going they can even last for centuries (Swanson and Houghton, 2015).

One of these phreatic (steam) eruptions occurred on November of 1790 produced a pyroclastic surge that killed a few hundred warriors and their families (Swanson et al., 2015). This same area is currently visited by around 5000 people per day.

Extent of the November 1790 pyroclastic surge with numbers indicating possible locations of groups impacted, Swanson and Houghton, 2015.
The below video is of a pyroclastic density current at Soufriere Hills Volcano on Montserrat. You can see the hot expansion of the ash and gas as it flows down the hill, similar processes to a pyroclastic surge that may be seen at Kilauea (this video is of a denser flow from a dome eruption). The 1790 event was likely composed mostly of fast moving superheated steam that took the victims by surprise (Swanson and Christiansen, 1973).

Eruptions that produce ballistic blocks are not uncommon at Kilauea. These are blocks that are thrown out of the crater during an explosion and deposit around the crater, as seen in the video below:
This small eruption at Tavurvur volcano shows ballistic blocks/bombs flying out of the eruption plume that then deposit around the crater.

The map below shows the locations of ballistic blocks from eruptions in 1790, 1924, and 2008 (Courtesy of Don Swanson):
Ballistic block locations from Swanson and Houghton (2015).
These blocks can get quite large - with measured blocks up to 195 cm in diameter, if you have visit the summit you would have walked right past them. See this HVO article for more information on ballistic blocks at Kilauea.

Block at the Kilauea Overlook, courtesy of Don Swanson, Swanson and Houghton, 2015.
The ash plume is estimated to have reached 30,000 feet high based on visibility reports. Depending on the wind direction at the time of eruption, ashfall could impact Ka‘ū, Puna, and Hilo, and very fine ash could reach Waikiki (Swanson and Houghton, 2015). An ash plume also produces hazards for the many trans-Pacific flights that travel near the island.

Why is this explosive potential not more widely known? There is little written history about the earlier eruptions for us to learn from. The work done by Swanson and others in uncovering the narrative and geologic evidence from the 1790 event give us a much different view of the seemingly effusive and "safe" Kilauea volcano. If you get the chance to visit Pele, keep her more explosive side in mind while you enjoy this beautiful effusive phase.

All images and information from Swanson and Houghton (2015) with the permission of Don Swanson, USGS Hawaii Volcano Observatory. Link to poster below.

References for more information:
Helz, R.T., et al., 2014. Microprobe analyses of glass from Kīlauea tephra: USGS Open-file Report OF2014-1090.

Mastin, L.G., 1997. Evidence for water influx in 1790: JGR, v. 102, p. 20,093–20,109.
Schiffman, P., Zierenberg, R., et al., 2006, Acid-fog deposition at Kilauea: Geology, v. 34, p. 921–924.

Swanson, D.A., Christiansen, R.L., 1793. Tragic base surge in 1790 at Kilauea Volcano. Geology, 1: 83-86.

Swanson, D.A., Rose, T.R., et al., 2014. Cycles of explosive and effusive eruptions at Kīlauea: Geology, v. 42, p. 631–634

Swanson, D.A., Weaver, S.J., Houghton, B.F., 2015. Reconstructing 1790 lethal eruption: GSA Bull., v.127, p. 503-515.

Swanson, D.A., Houghton, 2015. Don't Forget Kīlauea: Explosive Hazards at an Ocean Island Basaltic Volcano. AGU Fall Meeting, San Francisco, PA43C-2202.

Wolfe, E.W., Morris, J., 1996. Geologic map of the Island of Hawaii: USGS MI Map I-2524.

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