The pāhoehoe flows that moved through Kalapana in 1990 (left) were hotter, more fluid, and faster-moving than the lava flow that reached to within 155 m (170 yds) of Pāhoa Village Road in October 2014 (right). USGS photos.

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

To the casual observer, lava that recently flowed into the outskirts of Pāhoa 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 Kīlauea 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 Kīlauea’s summit reservoir system, crystallizes at a temperature below 1215 degrees Celsius (2219 degrees Fahrenheit). At lower temperatures, below 1150 degrees Celsius (2102 degrees Fahrenheit), 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 12 km (7 mi) from Kalapana. During that time, eruption temperatures ranged from 1155 to 1170 degrees Celsius (2111– 2138 degrees Fahrenheit) and the lava contained few olivine crystals, about 1 mm (0.04 in) in size.

Lava tubes from Kupaianaha carried lava down the steep southern flank of Kīlauea. These tubes enabled efficient delivery of 300,000–400,000 cubic meters (390,000–520,000 cu yds) of lava per day to the coastal plain at temperatures of 1145–1160 degrees Celsius (2093–2120 degrees Fahrenheit).

At those temperatures, the tube-fed flows that reached Kalapana were still relatively fluid and crystal-poor. This resulted in the fast-moving pāhoehoe sheet-flows that quickly spread through Kalapana and covered the black sand beach at Kaimū Bay in 1990.

In contrast, the June 27th lava flows erupted from the northeast flank of Pu‘u ‘Ō‘ō are moving down the crest of Kīlauea’s East Rift Zone toward Pāhoa, more than 20 km (12 mi) away. Since 2011, eruption rates have been estimated at about 175,000 cubic meters (230,000 cu yds) per day, the lowest sustained rate in over 30 years of eruption, and the eruption temperatures have been 1140–1145 degrees Celsius (2,085–2,095 degrees Fahrenheit).

The June 27th lava flows contain a mix of olivine, plagioclase and pyroxene phenocrysts, often as crystal clusters 1–5mm (0.04–0.2 in) in size. In contrast, the higher temperature Kalapana flows contained only olivine phenocrysts.

Despite its cooler temperatures and lower eruption rates, the June 27th 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 27th flow stagnated when lava was tapped to supply pāhoehoe breakouts at higher elevations near Pu‘u ‘Ō‘ō. 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 27th flow has already reached remains to be seen.

Based on our recent analyses of the June 27th 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 Kīlauea and will notify the public of any significant changes in the eruption.