Effusion rate controls on lava flow length and the role of heat loss: a review

Walker (1973; Phil. Trans. R. Soc. Lond., 274, 107) argued that, for a limited set of compositions and flow types, effusion rate (E) was the principal influence on flow length, sparking a series of studies into the volume and cooling limits on flow extension. We here review these works, as well as the role of heat loss in controlling flow length. We also explore the applicability of Walker’s idea to a larger compositional and morphological range. Heat loss plays a fundamental role in determining flow core cooling rates, thereby influencing cooling-limited flow length. Field measurements allow classification of four flow types with respect to heat loss. In this classification as we move from poorly insulated to well insulated regimes, decreased heat losses increase the length that a flow can extend for a given E, composition, morphology, or amount of cooling: (1) immature tube-contained, basalt – thin tube roofs provide minimal insulation, allowing cooling rates of c. 10 8C s so that at low E, these flows extend only a few hundred metres; (2) poorly crusted, basalt – open channels with hot surface crusts also exhibit cooling rates of c. 10 8C s so such flows extend a few kilometres at E , 1 m s; (3) heavily crusted, dacite – heat losses are reduced when thick crusts form, reducing core cooling rates to c. 10 8C s so these flows can potentially extend several kilometres even at low E and despite very high viscosities (10–10 Pa s); (4) master tube-contained, basalt – thick tube roofs insulate flow, reducing heat losses and cooling rates to c. 10 8C s. These cooling rates mean that at low E, tube-contained flows can extend tens to hundreds of kilometres. Basically, if composition, insulation, and morphology are held constant flow length will increase with effusion rate. Our aim is to review G. P. L. Walker’s pioneering work into the emplacement of lava flows, focusing on his studies of the relationship between effusion rate and eventual flow length, and the subsequent work that this inspired. Walker’s ideas on this topic followed observations of active lava flow emplacement during a terminal (summit) effusive eruption at Mt Etna (Italy) during 1966, and the work was completed during 1967–1973. The work comprised a series of three papers. The first, ‘Thickness and viscosity of etnean lavas’ (Walker 1967), was published in Nature and examined the influence of viscosity and slope on flow emplacement, dimensions and morphology. The second, ‘Compound and simple lava flows and flood basalts’ (Walker 1972), published in Bulletin of Volcanology, set up standard definitions and nomenclature for lava flow units and compound lava flows, and examined the conditions under which different flow architectures are built. Finally, ‘Lengths of lava flows’ (Walker 1973; in the Philosophical Transactions of the Royal Society of London) rounded the series off by demonstrating that effusion rate is the principal factor that influences not only flow length, but also flow field type, with viscosity being a secondary factor. With higher effusion rates, long simple flows tend to be emplaced, whereas at lower effusion rates, multiple short flows pile up around a vent to build compound flow fields. We focus on the main theme of this third paper and the influence that this has had on subsequent study and debate, before examining the role of thermal insulation and heat loss on determining and modifying effusion rate–flow length relationships. A relationship that governs the interplay between flow length, heat loss and effusion rate was recognized by Walker (1973). In the conclusion of the ‘Lengths of lava flows’ paper he stated: There seems to be a tendency for the relatively highfluidity pahoehoe flows to be shorter than the somewhat more viscous aa flows, and it was this tendency which first suggested to the author that some factor other than viscosity controls the lengths of lava flows . . . This tendency, if real, could be related to a relatively high rate of heat loss per unit volume from thin flows of pahoehoe as compared with From: THORDARSON, T., SELF, S., LARSEN, G., ROWLAND, S. K. & HOSKULDSSON, A. (eds) Studies in Volcanology: The Legacy of George Walker. Special Publications of IAVCEI, 2, 33–51. Geological Society, London. 1750-8207/09/$15.00 # IAVCEI 2009. thicker flows of aa. The strong tendency for the former to build up compound lavas could be due to the same cause. This elegant statement sums up the complexities involved in determining how far a flow can extend, with effusion rate, rheology, heat loss, flow morphology and eruption duration all playing roles, and all being subject to complex feedbacks with each other. Heat loss, for example, will determine cooling rates and hence temperature-dependent viscosity and velocity, eventually limiting a flow’s ability to move. Velocity in turn will influence the coherence of the surface crust, thereby affecting heat loss. Because of this complex interplay it is unlikely that flow length can be attributed to individual flow parameters in a universal way. Instead, different cases defined by specific rheological, heat loss and/or emplacement conditions need to be considered separately.