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Abstract

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. 10−2 °C s−1 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. 10−2 °C s−1so such flows extend a few kilometres at E < 1m3 s−1; (3) heavily crusted, da-cite - heat losses are reduced when thick crusts form, reducing core cooling rates to c. 10−4 °C s−1 so these flows can potentially extend several kilometres even at low E and despite very high viscosities (109−1010 Pa I); (4) master tube-contained, basalt - thick tube roofs insulate flow, reducing heat losses and cooling rates to c. 10−3 °C s−1. These cooling rates mean that at low BE, 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.

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