Kilauea Lava Lakes: Natural Laboratories for Study of Cooling, Crystallization, and Differentiation of Basaltic Magma

Abstract

Three Kilauea eruptions have produced accessible lava ponds in the pit craters Kilauea Iki (1959). Alae (1963). and Makaopuhi west pit (1965). These have provided a unique laboratory in which to study the cooling and crystallization of basaltic magma. The results of field and laboratory studies conducted at Kilauea place constraints on processes that take place during cooling of shallow magma chambers and of thick basaltic lava flows. Field methods of study include repeated core drilling to determine the thickness of upper crust. collection of gas and measurement of temperature and oxygen fugacity in uncased drill holes. sampling and measurement of viscosity of molten basalt, and periodic measurement of changes in surface altitude and crack configuration. Laboratory studies include measurement of density, chemical composition, and conductivity of drill core, megascopic and microscopic study of crystal and glass content as a function of depth, time, and temperature, and finite-element modeling of the thermal history. Liquidus temperatures of each magma are at or above 1200°C depending on MgO content. Solidus temperatures are all near 980°C. The interface between rigid ‘crust' and fluid ‘melt' occurs at 1070 ± 50°C. The thickness of upper crust in each lake during the first several months increased at a nearly linear rate of 40 cm per unit change in measured in days. Finite-element modeling of the cooling of Alae lava lake has established that cooling is controlled principally by conductive heat transfer. Most measured temperatures there can be computed within 50°C with a model assuming a latent heat of 80 ± 5 cal/gm and a constant diffusivity of .006 cm2/sec and taking into account the heating and vaporization of the measured rainfall of approximately 250 cm/yr. The rainfall considerably hastened post-solidification cooling of the lake. Measured temperatures can be more closely duplicated using a diffusivity based on the measured density and calculated heat content of the lava and on a conductivity that decreases with porosity and increases with temperature. The cooling rate of Makaopuhi and Kilauea Iki may have been increased by convective heat transfer in the melt, as indicated by measured temperature fluctuations and melt differentiation in Makaopuhi. Three kinds of crystal-liquid differentiation are observed in the lava lakes as inferred from chemical analysis of drill core: (1) gravitative settling of olivine phenocrysts, (2) filter-pressing of low-temperature (l030–1070°C) liquids into open fractures, and (3) downward concentration of augite and plagioclase during inferred convective flow. Similar chemical variation observed in Kilauea rift eruptions suggests that these differentiation processes occur on a larger scale within the Kilauea conduits. Some features are unique to each lake. One example is the pattern of vesiculation represented by changes in crust and melt density with depth and by surface altitude changes. Vesiculation is important in determining the density stratification in each lake, and hence the convective history. The distribution of vesicles depends in part on the initial volatile content and on differences in the mode of filling of each lake. These observations made for any single lava lake cannot, therefore, be directly applied to other basaltic lava flows without consideration of evidence for initial gas content and vesiculation history.