Morpho-Structure of the 1982 Lava Flow Field at Mount Cameroon Volcano, West-Central Africa

Abstract

Basaltic eruptions have been observed to produce structurally complex, compound 'a'ā lava flow fields but their mor phometry has only rarely been systematically documented. We document the morphology and structures that developed during the emplacement of the 1982 basaltic lava flow field at Mount Cameroon (MC) volcano over a period of one month. Topographic cross-sections (13 in total) were made from the main vent (~2700 m above sea level (a.s.l)) down to a distance of 5.5 km on the cooled lava surface. Details obtained from these cross-sections include: channel width and depth, levee slope, lava surface morphology and structures. These details enabled us to describe the physical char acteristics of the 1982 lava flow field. The inclined (12 ° - 19°) underlying slopes on which this flow field was emplaced resulted in a characteristic channelized basaltic 'a'ā flow field morphology. This includes a proximal zone characterised by reduced flow width and depth with no subsidiary channels. Slab-crusted lava dominates the proximal channel dis tinctively bent into convex upward shapes. 7 secondary vents were observed for the first time ~2.5 km from the main vent, with heights of 3 - 15 m. This is a very significant observation since it points to the fact that the flow field em placement may have been a product of 2 eruption sites as observed at other historical MC lava flow fields. This suppo sition was ruled out by further evidence obtained from other surface features within the flow field. The presence of these secondary vents still has an important bearing in lava flow hazard assessment. Field observations also revealed the presence of tumulus. This is a novel feature for MC lava flow fields. It displayed a close similarity to those observed at other basaltic volcanoes occurring in association with clinker 'a'ā lava, lava tubes, squeeze-ups and pressure ridges. Channels are well-defined, bounded by levees. Accretional and overflow levees dominate in this flow field. This lava flow-field attained a final length of 7.5 km, an area of 2.6 × 10 6 m 2 and volume of 1.3 × 10 7 m 3 . The presence of tumulus indicates internal inflation together with structures such as pressure ridges and squeeze-ups which are also attributed to compressive forces. Our observations suggest that real-time monitoring of compound lava flow fields evolution at MC may reveal the emplacement mechanisms of complex structures such as the secondary vents (~2180 - 2011 m a.s.l . ) observed within the flow field. In addition, documenting the occurrence, morphology and link between lava tubes, tu mulus and squeeze-ups may allow us to determine the risk of reactivation of a stalled flow front. This will thereby en hance the ability to track and assess hazards posed by lava flow emplacement from MC-like volcanoes.