Abstract
Basaltic eruptions have been observed to produce structurally complex, compound 'a'ā lava flow fields but their morphometry 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 characteristics 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 distinctively 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 emplacement may have been a product of 2 eruption sites as observed at other historical MC lava flow fields. This supposition 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 × 106 m2 and volume of 1.3 × 107 m3. 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, tumulus and squeeze-ups may allow us to determine the risk of reactivation of a stalled flow front. This will thereby enhance the ability to track and assess hazards posed by lava flow emplacement from MC-like volcanoes.
Highlights
The new found interest in the morphology and physical characteristics of historical Mount Cameroon (MC) lavas [2,8,11] has revealed a variety of lava structures which has resulted in an increased understanding of the nature and dynamics of these lava flow fields relevant in lava flow hazard assessment
This eruption produced a dominantly channelfed compound 'a'ā lava flow field supplemented by a network of 7 secondary vents and lava tubes similar to other basaltic 'a'ā lava flow fields emplaced at Mount Etna [5,6,12,13,26]
Lava morphologies at the 1982 'a'ā flow field exhibited a broad spectrum of surface types ranging from slab-crusted lava, squeeze-up, tumulus, to clinker and blocky 'a'ā emplaced on inclined slopes which are challenging to interpret when already emplaced
Summary
The advance of lava flows produced by volcanic eruptions has been studied through field observations [1,2,3,4,5,6,7,8,9,10,11,12]; remote sensing techniques [13,14,15,16,17], as well as through analytical or numerical modelling [18,19,20,21,22,23]. The above cited methods have greatly improved understanding of the emplacement dynamics of lava flows as they provide clues about key processes occurring during eruptions even for flows not witnessed. The physical volcanology of most lava flow fields in developing countries has received limited systematic attention. Lava morphological data are significant to understand lava emplacement mechanism and anticipate impacts from effusive eruptions.
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