Lava tubes: Their morphogenesis and role in flow formation.

An attempt is made to understand the morphology, formation and operation of lava tube systems in pahoehoe lava flows from evidence contained in lava tube caves (landforms resulting from the segmental drainage of lava tube systems). Twelve caves from five contrasting lava flows in Tenerife, Iceland and Sicily (Mt. Etna) are described and the morphogenesis of each is worked out from relationships between flow structure and cave form. The structural evidence adds support to the tube-forming processes previously observed by vulcanologists during periods of effusive volcanic activity. No evidence is found to support the currently popular 'layered lava' theory of cave genesis; instead, each cave is seen to be derived from a lava tube system which is a network of varying tube types. Knowledge of cave forms enables a visualization of the morphology of lava tube systems and the dynamics of the lava rivers they transport. Ideally, each system is sinuous and partly braided along the flow axis and terminates at a delta-like front, though complexities frequently arise as a result of such processes as stream piracy, the development of overflow tubes and the extension of the axial tube across former deltaic regions of the flow. Discussion of channel forms, comparison with other fluvial systems and knowledge of the efficiency of lava tubes in maintaining flow temperature and mobility suggests that lava tube systems are 'adjusted' forms: it is only through their construction that temperature and mobility are maintained sufficiently to enable the continued advance of the flow front downslope. At the front lava emerges from the tube system as a jet flow. As a result, it is argued that the development of pahoehoe lava flows is predictable and amenable to future quantification through the application of jet theory.