Investigating physical and thermal interactions between lava and trees: the case of Kīlauea’s July 1974 flow

Abstract

To examine whether there was any physical or thermal interaction between trees and lava when a lava flow inundates a forest we studied the Kīlauea's July 1974 lava flow. We mapped the location of ∼600 lava-trees and the lava type (pāhoehoe versus 'a'ā), and sampled an additional ten lava-trees for chemical and textural analysis to infer flow viscosity and dynamics. The emplacement event lasted 3.5 hours and markers on the outer surface of the lava-trees allowed us to define initial high effusion rate and velocity (∼400 m 3 /s and 5-10 m/s) that then declined to 9 m 3 /s and 4 m/s during a waning phase. We find that lava passing through the forest underwent an initial cooling rate of 4 °C/km which increased to 10 °C/km late in the eruption. This is no different to cooling rates recorded at Kīlauea for tree-free cases. There thus appears to be no effect on cooling for this case. The lava-trees did, though, form a network of vertical cylinders obstacles and evidence for local diversion of flow lines are noticed. However, this varies with lava type, as almost no lava-trees form in ʻaʻā. We find a relation between the percentage of ʻaʻā and the number of lava-trees per hectare. The pāhoehoe-ʻaʻā transition for this flow occurs at a viscosity of 10 3 Pa s and this appears to be a threshold below which lava-trees can form so as to behave as a network of obstacles, and above which they cannot.

Chemical, Textural and Thermal Analyses of Local Interactions Between Lava Flow and a Tree – Case Study From Pāhoa, Hawai’i

“…One of the most visited sites to observe lava-trees and tree molds is the July 1974 lava flow of Kîlauea, which flowed through two kilometers of Ohia forest leaving behind more than a thousand lava-trees (Lockwood and Hazlett, 2010;Jones et al, 2017;Chevrel et al, 2019). However, Ohia trees have thin trunks and the wood is relatively dry, both of which are factors that likely restrict their influence on lava flow cooling (cf.…”

“…Also known as the "Monkeypod", Albizia typically grows up to a height of 15-24 m (Kepler, 1990). In addition to samples of the 2014 flow collected around the Albizia tree, we used some samples collected from lava-trees formed during the July 1974 eruption of Kilauea (here after called J74) that inundated a forest of Ohia trees (Chevrel et al, 2019). Ohia trees, or Mestrosideros polymorpha of the Myrtaceae (Myrtle) family, are different from Albizia trees in that they are endemic to Hawaii (Ziegler, 2002), and are smaller (being as short as 3 m; Greenwell et al, 2009) and denser (dry wood density 3 : 915 versus 600 kg m −3 ) (Wood Data Base, 2019).…”

“…Many effusive centers (e.g., Hawaii, Etna, the Virunga region, La Réunion) are densely vegetated and in climatic settings that favor rapid vegetation regrowth after inundation by lava (e.g., Ziegler, 2002;Smathers and Mueller-Dombois, 2007). Forest inundation by lava flows is thus a common phenomenon, which has been frequently described during eruptions at, for example, Etna (Andronico et al, 2005;Carveni et al, 2011), Piton de la Fournaise (Bertile, 1987) and Kilauea (Moore and Richter, 1962;Lockwood and Williams, 1978;Bell and Williamson, 2017;Chevrel et al, 2019). As a result, it is well known that when lava invades a forest, it causes widespread combustion of the trees, shrubs and undergrowth comprising the biotic system as, for example, during the eruption of Kilauea Iki (Kilauea, Hawaii) in 1959 (Richter et al, 1970).…”

“…When relatively low viscosity lava (< 10 3 Pa s, Chevrel et al, 2019) contacts a cold, damp tree, it has been long established that the engulfing lava cools rapidly to form a cast around the tree trunk (Finch, 1931;Jaggar, 1945;Moore and Richter, 1962;Lockwood and Williams, 1978;Macdonald et al, 1983). Isolated within its cast, the tree heats to combustion and burns slowly to leave a mold with the same size and form as the trunk within the solidified lava flow (Finch, 1931;Lockwood and Williams, 1978).…”

“…However, Searle (1958), Honda (1998Honda ( , 1999 and Wasilewski et al (2000) have hinted at potential chemical and thermal interactions between a combusting tree and the engulfing lava. Chevrel et al (2019) constrained the possible influence of a forest on the emplacement of an entire lava flow system, showing that lava-trees and tree molds can act as obstacles capable of locally diverting the flow path. Although no impact on the overall cooling of the lava flow was found, a distinction was noted 1 https://www.youtube.com/watch?v=WpQyjVEfx5c between the down-flow cooling rate of lava that was deposited against tree trunks and lava that cooled in the far field (Chevrel et al, 2019).…”