To the casual observer, lava that recently flowed into the outskirts of Pahoa might look a lot like the lava that grazed the village of Kalapana in November 1986 and completely overran it in 1990. In fact, these Kilauea flows were significantly different in most aspects that affect how far and how fast lava moves downslope.
Theoretically speaking, the surface extent (how far) and advance rate (how fast) of lava is regulated primarily by three factors: eruption rate; lava temperature and, therefore, “crystallinity,” at the time of eruption; and topography (slope of the land). Crystallinity refers to the abundance and types of phenocrysts (crystals that grow in magma before it erupts to the surface) and micro-phenocrysts (minute crystals that grow in a lava flow as it advances and cools) in lava.
Olivine, the first mineral to grow in magma as it rises into Kilauea’s summit reservoir system, crystallizes at a temperature below 2,219 degrees Fahrenheit. At lower temperatures, below 2,102 degrees, the minerals pyroxene and plagioclase also crystallize along with olivine.
When lava is erupted, micro-phenocrysts of plagioclase and pyroxene form within the molten rock as it flows over the ground and cools. As crystallinity increases, the flow becomes more viscous (pasty) and eventually stalls and solidifies.
From 1986 to 1992, lava flows were erupted from Kupaianaha, an active vent less than 7 miles from Kalapana. During that time, eruption temperatures ranged from 2,111 to 2,138 degrees and the lava contained few olivine crystals, about 0.04 inch in size.
Lava tubes from Kupaianaha carried lava down the steep southern flank of Kilauea. These tubes enabled efficient delivery of 390,000 to 520,000 cubic yards of lava per day to the coastal plain at temperatures of 2,093 to 2,120 degrees.
At those temperatures, the tube-fed flows that reached Kalapana were still relatively fluid and crystal-poor. This resulted in the fast-moving pahoehoe sheet-flows that quickly spread through Kalapana and covered the black sand beach at Kaimu Bay in 1990.
In contrast, the June 27 lava flows erupted from the northeast flank of Puu Oo are moving down the crest of Kilauea’s East Rift Zone toward Pahoa, more than 12 miles away. Since 2011, eruption rates have been estimated at about 230,000 cubic yards per day, the lowest sustained rate in more than 30 years of eruption, and the eruption temperatures have been 2,085 to 2,095 degrees.
The June 27 lava flows contain a mix of olivine, plagioclase and pyroxene phenocrysts, often as crystal clusters 0.04 to 0.2 inch in size. In contrast, the higher temperature Kalapana flows contained only olivine phenocrysts.
Despite its cooler temperatures and lower eruption rates, the June 27 lava flow traveled nearly twice the distance of the 1986 and 1990 Kalapana flows. But, the Kalapana flows were cut short when they flowed into the ocean, so how much farther they could have traveled is not known.
As with the abrupt termination of the 1986 Kalapana-bound flows, the leading edge of the June 27 flow stagnated when lava was tapped to supply pahoehoe breakouts at higher elevations near Puu Oo. The relative contribution of lava temperature, crystallinity, eruption rate and topography to this stagnation is now the subject of ongoing research. Whether additional lava will advance farther than the distance the June 27 flow has already reached remains to be seen.
Based on our recent analyses of the June 27 lava flow, current eruption conditions do not favor a Kalapana-like scenario in which broad sheet-flows inundate large swaths of land. However, we must keep in mind that eruption conditions — for instance, eruption rate, lava temperature and vent location — can change unexpectedly. Because of this, the USGS Hawaiian Volcano Observatory continues to closely monitor Kilauea and will notify the public of any significant changes in the eruption.
Kilauea activity update
Kilauea’s East Rift Zone lava flow continues to feed three areas of breakouts near Puu Oo. The breakout farthest downslope had reached to about 4 miles northeast of Puu Oo by Wednesday based on satellite imagery.
There have been no major changes at Kilauea’s summit vent, which continues to host an active lava lake. The lava lake level continues to fluctuate and was 140 feet below the rim of the Overlook crater on Thursday.
Visit the HVO website at hvo.wr.usgs.gov for Kilauea daily eruption updates and other volcano status reports, current volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary update; email questions to askHVO@usgs.gov.