Volcano Watch: How do lava flows cool and how long does it take?

  • This aa flow erupted from fissure 8 on Kilauea Volcano’s lower East Rift Zone on June 1, shows how the interior of a lava flow remains incandescently hot even though surface cooling forms a crust of solid rubble. Based on studies of lava flow cooling rates, it will take more than 130 days for a flow this thick (about 15 feet) to cool to a temperature of about 200 degrees Celsius (290 degrees Fahrenheit). (USGS photo courtesy / A. Lerner)

Since the end of the 2018 lower East Rift Zone (LERZ) eruption on Kilauea Volcano, questions have surfaced concerning how long it will take for the new lava flows to solidify. This is a difficult question to answer, because the initial eruptive temperatures along with many different factors can influence the rate of cooling.

Eruptive lava temperatures of the 2018 LERZ eruption reached a maximum of approximately 1,140 degrees Celsius (2080 degrees Fahrenheit). When the entire flow cools below about 1,000 degrees Celsius (1,800 degrees Fahrenheit), it has solidified, but the interior is still very hot.

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Arguably the most influential factor determining how fast lava cools is the thickness of the flow. Other factors include heat loss from both the top (to the atmosphere) and bottom of a flow (into the ground). Contributing to heat loss at the flow’s surface are air temperature, rainfall and wind.

The initial contact between a lava flow, the air above it, and ground surface below it, quickly hardens the outer crust (top and bottom) of the flow. This is apparent in the silvery crust that forms on active pahoehoe flows and the rubbly clinker that surrounds active aa flows. As the crust cools and thickens, it retains heat within the flow’s interior. This is because the crust is a good insulator, meaning it poorly conducts heat — similar to how an insulated thermos keeps liquid inside it hot.

After the initial formation of crust, the flow continues to lose heat through radiation and conduction, facilitated by wind and rain. As rain water percolates into cracks in the flow’s surface and encounters the hot interior, it produces steam, forming the billowy white plumes often seen over active (or recently active) flows. This steaming can persist for decades, long after the lava has solidified, depending on the thickness of the flow and the temperature of its interior.

Based on a study of crustal cooling of pahoehoe lava flows in Kalapana erupted from the East Rift Zone Kupaianaha vent in 1990, we can estimate the solidification time for the 2018 LERZ flows. Because the equation only looks at cooling of the lava flow’s upper crust, the basal crust thickness is assumed to equal 70 percent of the upper crust according to this study.

The Kalapana measurements were made on thin pahoehoe flows, but most of the 2018 LERZ lava is aa. But, because the core of each flow type should cool at similar rates, we are basing 2018 cooling rates on the 1990 study. Also, of note, the flows studied in 1990 were much thinner with shorter cooling rates and may not account for long-term changes in wind and rain patterns.

Preliminary analyses of the 2018 LERZ eruption flow thicknesses, suggest that the average flow thickness is around 33 to 50 feet. Based on the cooling rate calculation, it could take roughly eight months to one and a half years for flows of these thicknesses to solidify.

Solidification of flows ranging 65-100 feet thick could take about two and a half to six years. The thickest LERZ flows on land, which are approximately 180 feet thick, may take roughly 20 years to reach a completely solid state.

Because flow thickness, wind speeds, rainfall amounts, air and ground temperatures, and other factors all affect lava cooling rates, there is a range of uncertainty on how long the interior of a flow remains liquid. For example, after the 1959 Kilauea Iki eruption, the approximately 440 feet deep lava lake took about 35 years to completely solidify, and the interior of the lake could still be hot enough today that the rock is incandescent. This is why, on rainy days, you can see steam rising from the Kilauea Iki crater floor, as well as the Kilauea caldera floor.

With flow crust being such an efficient insulator, it can take years to decades for lava within thick flows to solidify. It takes much longer for the flow to cool to ambient temperatures.

Next week’s Volcano Watch will address in more detail the thicknesses of lava flows from the 2018 LERZ eruption.

Volcano Activity Updates

Kilauea is not erupting. Rates of seismicity, deformation, and gas release have not changed significantly over the past week.

Two earthquakes with three or more felt reports occurred in Hawaii during the past week: a magnitude-3.0 quake 4 miles south of Leilani Estates at 5 miles depth on Feb. 22 at 2:31 a.m. HST, and a magnitude-3.2 quake at 11 miles southeast of Volcano at 4 miles depth on Feb. 21 at 11:32 a.m. HST.

Deformation signals are consistent with refilling of Kilauea Volcano’s deep East Rift Zone (ERZ) magma reservoir. Sulfur dioxide emission rates on the ERZ and at Kilauea’s summit remain low.

Hazardous conditions still exist at both the lower ERZ and summit of Kilauea. Residents and visitors in the lower Puna District and Kilauea summit areas on the Island of Hawaii should stay informed and heed Hawaii County Civil Defense closures, warnings, and messages (http://www.hawaiicounty.gov/active-alerts). HVO continues to closely monitor Kilauea for any sign of increased activity.

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The USGS Volcano Alert level for Mauna Loa remains at normal.

Please visit HVO’s website (https://volcanoes.usgs.gov/hvo) for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Call 967-8862 for weekly Kilauea updates. Email questions to askHVO@usgs.gov.

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