HILO — As roads are recut into Kilauea’s 2018 lava flow field, many have been surprised at how hot the lava remains under the surface, even though it is solidified. Why is it still so hot? The short and simple answer is that lava insulates itself very well.
Since last writing about this topic in a Feb. 28, “Volcano Watch” article, we now have more accurate calculations of lava thickness, cooling times, and the relative proportions of the internal molten core to the exterior solid crust for Kilauea’s 2018 lava flow field.
Previous work by Hawaiian Volcano Observatory (HVO) scientists published in 1994 measured the cooling rate of pahoehoe lava at Kalapana. They found that the upper crust of a basalt lava flow grows thicker as a function of the square root of time. In other words, the lava flow crust grows more slowly with time. Therefore, thicker lava flows will take longer to become completely solid.
Lava erupts from Kilauea at a temperature of 1,150° C (2,100° F). In 1917, Thomas Jaggar published results from the then-active Halemaumau lava lake that indicated basalt can remain molten at temperatures as low as 750–850° C (1,380-1,560° F). For the calculations here, crust is considered solid when it is below 850° C (1,560° F) and crust is viscoelastic (semi-solid or malleable) at 850–1,070° C (1,560–1,950° F).
Additional insight comes from previous HVO studies of active lava lakes in Kilauea Iki, Makaopuhi, and Alae craters. By drilling into the cooled upper crusts of lava lakes within these craters, scientists documented that solidification takes decades — specifically the 140-foot thick 1959 Kilauea Iki lava lake took about 35 years to fully solidify. Today, its core is still hotter than 540° C (1,000° F).
Therefore, although the top surface of Kilauea’s 2018 lava flows are already solid and cool to the touch, just beneath the surface is still very hot.
Unmanned Aircraft System (UAS) mapping in 2018 helped HVO create a lava flow thickness map. This map indicates that at the intersection known as “Four Corners” there is approximately 50 feet of lava. Using this value and the equations from the 1994 study of the Kalapana lava flows, we can calculate how much of the 2018 flows have solidified.
Over the 14 months since the end of the eruption last year, the upper 24 feet and lower 8 feet at “Four Corners” should already be solidified crust, and the middle 5.5 feet should still be malleable.
It will take about three more years for the remaining 5.5 feet of malleable lava over the “Four Corners” intersection to reach 850° C (1,560° F) and be completely solid.
When construction of new roads in the area expose hot temperatures just below the surface, the lava cooling profile is effectively reset. The newly exposed surface will quickly lose heat, as if it were the original cooling surface.
This matches recent observations by road construction crews, who noticed hot rocks being exposed at a road cut along Highway 132. HVO geologists confirmed this in August, when temperatures of 425° C (800° F) were measured at the newly cut road site. Hot temperatures will remain several feet below the surface for now and will likely generate steam when there is rain.
Lava retains heat so well that thick flows take years to decades to completely solidify, and even longer to cool off. The UAS lava-flow thickness map indicates that over Kapoho Bay, the flow is over 160-feet thick. The maximum thickness of 820 feet is located east of the bay in an area that was formerly offshore.
At both locations, the solid crusts will be the same thickness as at “Four Corners,” but the viscoelastic layer will be 15 feet. A thick molten inner core should exist within the aa flow delta and have a thickness of 105 feet in the bay and an astonishing 761 feet at the eastern area of maximum thickness.
Although Kilauea’s 2018 eruption ended 14 months ago, it will be years before the lava flows emplaced on land are entirely solidified below 850° C (1,560° F), and over a century before the 820-foot thick area offshore fully solidifies.
Please visit HVO’s website (https://volcanoes.usgs.gov/hvo) for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Call 808-967-8862 for weekly KILAUEA updates. Email questions to askHVO@usgs.gov.
Volcano Watch (https://volcanoes.usgs.gov/hvo/hvo_volcano_watch.html) is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.