Abstract
The Halema'uma'u lava lake in Hawai'i, USA provides a natural laboratory for testing imaging of molten and recently molten basalt. The thermal emission spectrum from basalt at high temperatures has been measured by collecting simultaneous thermal Infra-Red and multispectral visible to near-infrared (VNIR) images of an active lava lake at night. In this study, we attempt to constrain the VNIR ( nearly equal 475-842 nm) spectrum due to thermal emission to evaluate the usefulness of multispectral data for detecting changes in crystallinity. Reflectance in the VNIR has proven to increase with increasing crystal abundance in solidified basalt. However, the incandescent molten basalt adds significant signal in the VNIR range which influences attempts to measure the absolute reflectance. In this study, we present side-by-side thermal IR and VNIR images of the Halema'uma'u lava lake from the evening of February 22nd, 2021 to evaluate the effect of temperatures ranging from nearly equal 400-1100 degrees C on the brightness in the five bands, blue, green, red, red edge, and near-IR. VNIR images were collected with a Micasense RedEdge sensor that captured 5 bands simultaneously. We calibrated this sensor on a blackbody furnace at the USGS Astrogeology Science Center in Flagstaff. Radiance values of the lava lake ranged from 0.001-0.008 W/m2/sr/nm. We present emissivity values of five bands over a range of temperatures for effectively molten and recently solidified basalt. These values will provide greater accuracy when interpreting VNIR satellite images of hot volcanic deposits, a necessary step towards potentially providing constraints on crystal content, glass content, proportion of altered material, and chemical compositional data for dangerous or difficult to access eruptions.