Kilauea eruption is classic example of ‘gentle’ volcano

Even with as much publicity as the current Puna eruption has garnered, there is still some misunderstanding about Kilauea Volcano’s eruptive behavior.

Fundamentally, there are two kinds of volcanoes, explosive and gentle, with a spectrum of intensities in both groups.

Hawaii volcanoes characterize the gentle type, known as shield volcanoes.

Shield volcanoes occur far from the crushing compressive edges of tectonic plates. They are common in Hawaii and other Pacific Ocean island chains, and in Iceland, the Canary Islands and the East African rift, to name the most prevalent.

Explosive volcanoes are associated with convergent tectonic plate boundaries around the rim of the Pacific Ocean, in the Italian/Aegean region and in the Caribbean. These stratovolcanoes are more violent because their magma has a much higher silica and gas content. More silica makes the magma more viscous and therefore more difficult for the greater gas content to escape. They run the gamut in violence from gentle to highly explosive, although even the gentlest are more violently explosive than even the most explosive Hawaii type.

Eruptions of stratovolcanoes such as Volcan de Fuego in Guatemala usually begin with an explosive phase that releases large amounts of gas (mostly water vapor) that comes undissolved in the magma as it rises from a higher pressure deep inside the volcano to a lower pressure near the surface.

The eruption spews pulverized lava as fine ash particles high into the atmosphere along with glowing hot ash flows that race down the volcano’s slopes at 100 miles per hour, engulfing everything in their path indiscriminately and burying it in hot ash that solidifies as it cools into hard tuff.

Thick, relatively cool and viscous lava often follows, covering the ash, which over time builds a steep-sided symmetrical conical composite volcano consisting of alternating layers of ash/tuff and lava. Mount Fuji in Japan is the poster child for this type of volcanic edifice.

The low silica and higher temperatures of shield volcanoes produce eruptions of hot, fluid lava that oozes or fountains out of linear fissures along rift zones that radiate outward from a central vent. Some eruptions occur from the central vent, which often sits inside a caldera a mile or more in diameter.

Unlike violent stratovolcanoes, shield eruptions do not most often occur at the summit. When they do, the lava typically fills the summit crater and overflows. On rare occasions a single explosion occurs when groundwater contacts the magma underground and flashes to steam.

Hawaii eruptions typically begin as gentle fountaining along rift fissures, often forming a “curtain of fire” as fountains have reached 1,000 feet or more in height. Lava from the fountains collects in flows that follow the downhill path of least resistance. The curtain of fire might consist of small eruptive centers along the rift.

Typically, the eruptions along the rift become confined to a single vent, which could continue to erupt and cover the surrounding surface with lava.

The current eruption of Kilauea has followed this pattern as if scripted, with most of the activity now confined to fissure 8.

The actual eruption details are much more complex than summarized here, and a single eruption phase can provide enough data for years of analysis by volcanologists.

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Richard Brill is a professor of science at Honolulu Com- munity College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.