Non equilibrium structures in lava flows
| 18 Gennaio 2016 | ore 11 | Simone Tarquini – INGV PI | Sala seminari – INGV Sezione di Pisa
Lava flows are open systems which, under favorable conditions, can attain a non-equilibrium stationary state. Although this evidence is not new, the open system nature has never been fully considered in a model which accounts for both the observed dynamics and the thermal regimes of lava flow.
In an open, non-equilibrium system, the regression or amplification of fluctuations discriminates between a linear and a nonlinear phase in the dynamical regime. A similar dualism is
also found in lava flows, in which fluctuations in lava flux are typically observed and can have a relevant impact. Within the general framework of the thermodynamics of irreversible processes, a conceptual model is presented, in which the dynamic of lava flows can evolve in a linear or in a nonlinear regime on the basis of the constraint active on the system: a low constraint promotes a linear dynamic, a high constraint a nonlinear one. Two cases are considered as end-members for a linear and a nonlinear dynamic in lava flows: the typical “Hawaiian” sheet flow and the classic “Etnean” channelized flow (respectively). In lava flows, the active constraint is directly proportional to the slope of the topography and to the thermal conductivity and thermal capacity of the surrounding environment, and is inversely proportional to the lava viscosity and to the supply rate. The constraint indicates the distance from the equilibrium conditions of the system (Δeq ), and determines the rate of heat dissipation per unit volume. In subaerial flows, the heat dissipated during the emplacement is well approximated by the heat lost through radiation, which can be retrieved through remote-sensing techniques and can be used to correlate dynamic and dissipation. The dynamical structures of non linear lava flows are intrinsically characterized by a structural relaxation time τs which can be viewed as an analog of the relaxation time τα of the micro structure in silicate melts. In Maxwell liquids, τα is directly proportional to the melt viscosity η (η ∝ τα) and inversely proportional to the melt temperature Τ (τα ∝ 1/Τ). As the distance from the equilibrium conditions Δeq is the non-equilibrium analog of the temperature in equilibrium systems, it is not surprising to found that in real lava flows (typical non equilibrium systems) the relaxation time of the macro structures τs tends to be inversely proportional to Δeq. The existence of a relaxation time in the macro-scale structures of lava flows suggests the existence of a macrostructure viscosity ηs (ηs ∝ τs), and it is found that this property of macro-structures can explain the emplacement dynamic of a very large spectrum of terrestrial lava flows, ranging from pillow lavas to flood basalts. At constant micro-scale viscosity, different macro-structure viscosities produce contrasting emplacement dynamics which lead to contrasting lava deposits. The model presented recombines previously unrelated concepts regarding the dynamics and the thermal regimes observed in different lava flows, providing a global consistent picture.