Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā

TLDR: In the early stages of Kilauea Volcano, lava cooling and crystallization was observed to occur in the upper parts of the lava channel as mentioned in this paper, and the transition from pahoehoe to pyroxene microlites occurred at a distance of 1.9 km from the vent.

Abstract: Samples collected from a lava channel active at Kilauea Volcano during May 1997 are used to con- strain rates of lava cooling and crystallization during early stages of flow. Lava erupted at near-liquidus tem- peratures (F1150 7C) cooled and crystallized rapidly in upper parts of the channel. Glass geothermometry indi- cates cooling by 12-14 7C over the first 2 km of trans- port. At flow velocities of 1-2 m/s, this translates to cooling rates of 22-50 7C/h. Cooling rates this high can be explained by radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel. Crystallization of plag- ioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20-50% per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of F10 4 /cm 3 s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due to stirring of the flow. The transition of the flow surface morphology from pahoehoe to 'a'aoccurred at a distance of 1.9 km from the vent. At this point, the flow was thermally stratified, with an interior temperature of F1137 7C and crystallinity of F15%, and a flow surface tempera- ture of F1100 7C and crystallinity of F45%. 'A'afor- mation initiated along channel margins, where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that the transition involved crossing of a rheological threshold. We suggest this threshold to be the develop- ment of a lava yield strength sufficient to prevent vis- cous flow of lava at the channel margin. We use this concept to propose that 'a'aformation in open chan- nels requires both sufficiently high strain rates for con- tinued disruption of surface crusts and sufficient groundmass crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite poor on eruption, these combined requirements help to explain two common observations on 'a'aformation: (a) 'a'aflow fields are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava issues from the vent as pahoehoe and changes to 'a'aonly after flowing some distance, thus permitting sufficient crystallization.