Heat loss measured at a lava channel and its implications for down-channel cooling and rheology
Andrew Harris, John Bailey, Sonia Calvari, Jon Dehn, 2005. "Heat loss measured at a lava channel and its implications for down-channel cooling and rheology", Kinematics and dynamics of lava flows, Michael Manga, Guido Ventura
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During May 2001 we acquired 2016 thermal images over an ∼8-h-long period for a section of active lava channel on Mount Etna (Italy). We used these to extract surface temperature and heat-loss profiles and thereby calculate core cooling rates. 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 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 (V mean) by δT/δx = a V mean −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.