Heat Loss Measured at a Lava Channel and its Implications for Down-Channel Cooling and Rheology

Channelized lava flow on Mt. Etna (Sicily) was observed during May 30-31, 2001. Data collected using a Forward Looking Infrared (FLIR) thermal camera and a Minolta-Land Cyclops 300 thermal infrared thermometer showed that the bulk volume flux of lava flowing in the channel varied greatly over time. Cyclic changes in the channel's volumetric flow rate occurred over several hours, with cycle durations of 113-190 minutes. Profiles of the surface temperature and heat-loss of the lava flow were extracted from 2016 thermal images acquired by the FLIR over a period of ~8 hours. Flow surface temperatures declined from ~1070 K at the vent to ~930 K at 70 m. Heat losses were dominated by radiation (5 × 104 W m2) and convection (~104 W/m2). These compare with a heat gain from crystallization of 6 × 103 W/m2. The imbalance between sinks and sources gives core cooling (δT/δx) of ~110 K/km. However, cooling rate per unit distance also depends on the varying flow conditions, where we distinguished: (1) unimpeded, high-velocity (~0.2 m/s) flow with low δT/δx (0.3 K/m); (2) unimpeded, low-velocity (~0.1 m/s) flow with higher δT/δx (0.5 K/m); (3) waning, insulated flow at low velocity (~0.1 m/s) with low δT/δx (0.3 K/m); and (4) impeded flow at low velocity (<0.1 m/s) with higher δT/δx (0.4 K/m). Our data allow us to define three thermal states of flow emplacement: insulated, rapid, and protected. Insulated is promoted by the formation of hanging blockages and coherent roofs. During rapid emplacement, higher velocities suppress cooling rates, and δT/δx can be tied to mean velocity (Vmean) by δT/δx = aVmean-b. In the protected case, deeper, narrow channels present a thermally efficient channel, where δT/δx can be assessed using the ratio of channel width (w) to depth (d) in w/d = aδT/δx-b.