Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms

Lava tubes play a pivotal role in the formation of many lava flow fields. A detailed examination of several compound 'a'a lava flow fields on Etna con firmed that a complex network of tubes forms at successively higher levels within the flow field, and that tubes generally advance by processes that i nclude flow inflation and tube coalescence. Flow inflation is commonly foll owed by the formation of major, first-order ephemeral vents which, in turn, form an arterial tube network. Tube coalescence occurs when lava breaks th rough the roof or wall of an older lava tube; this can result in the unexpe cted appearance of vents several kilometers downstream. A close examination of underground features allowed us to distinguish between ephemeral vent f ormation and tube coalescence, both of which are responsible for abrupt cha nges in level or flow direction of lava within tubes on Etna. Ephemeral ven t formation on the surface is frequently recorded underground by a marked i ncrease in size of the tube immediately upstream of these vents. When the l ining of an inflated tube has collapsed, 'a'a clinker is commonly seen in t he roof and walls of the tube, and this is used to infer that inflation has taken place in the distal part of an 'a'a lava flow. Tube coalescence is r ecognised either from the compound shape of tube sections, or from breached levees, lava falls, inclined grooves or other structures on the walls and roof. Our observations confirm the importance of lava tubes in the evolutio n of extensive pahoehoe and 'a'a flow fields on Etna. (C) 1999 Elsevier Sci ence B.V. All rights reserved.