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.
The advance of lava flows produced by volcanic eruptions has been studied through field observations [1-12]; remote sensing techniques [13-17], as well as through analytical or numerical modelling [18-23]. 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. However, 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. Lava flows can cover long distances in 3 - 6 days, damaging properties and threatening lives [2,24,25]. The complex morphologies characteristic of long duration basaltic 'a'ā lava flow fields reflect the importance of processes such as inflation, formation of lava tubes and secondary vents [
Advances in lava flow modelling in the past 25 years show that it is increasingly possible to anticipate the final lengths and spatial spread of single flow units [18,28]. However, during basaltic eruptions, complexities like new flow units are emplaced alongside and on top of earlier units resulting in compound flow fields [5,6]; channels crust over and lava tubes, secondary vents, squeeze-ups and tumulus develop. The development of these features hinders accurate modelling of long-lived (>1 week) basaltic 'a'ā lava flow fields [
Studies of terrestrial lavas suggest that the overall development of flow fields is systematic and that a general, normalized relationship can be established linking the final dimensions of a flow field to underlying slope, eruption duration, discharge rate, gravitational acceleration, lava density and rheology [27,30,31]. Linking qualitative and quantitative measurements of lava flow surface morphology with historical observations of eruptions is an important, but yet underexploited route, to constrain emplacement mechanism of basaltic lavas.
Basaltic lava flow fields often demonstrate compound morphology. That is they are comprised of several flow units and lobes with some superposed on each other [5, 17,32]). A flow lobe here refers to an individual package of lava surrounded by a chilled crust [
Eruptions that produced basaltic lava flow fields from MC have been associated with significant impacts over the past years. The 1922, 1959 and 1999 eruptions posed major threats to agro-allied complexes and road infrastructure around the SW [24,25] and NE [
In October 1982, an eruption that produced seismic swarms widely felt around Buea (SE flank;
The goal of this paper is to describe the morphology and structures observed within the stable and transitional channel zones of the 1982 lava flow field as defined by [
The 1982-lava flow field offers an excellent opportunity to examine the large-scale structural evolution of a compound flow field as well as the complex surface features that are ubiquitous in most 'a'ā flows. Such intermediate-long lived eruptions emplaced on inclined slopes offer good opportunities to investigate the effect of slope on 'a'ā lava flow fields in terms of surface morphology and inferred emplacement. So far few studies have described long-lived basaltic lava flow field morphologies and emplacement processes on inclined [11,31] and extremely steep slopes [
MC is the largest and most active of the continental volcanoes of the Cameroon Volcanic Line (CVL;
MC is a steep volcanic shield covered by successions of lava flows [11,53] and subsidiary scoriae deposits (
been confined to the summit, SW and NE flanks of MC (
Historical lava flows have flow lengths from ~850 m (upper 2000) to 11.5 km (1922, lower 1999;
The next most abundant products observed at MC are pyroclastic cones (~340 cones have been mapped) aligned along a NE-SW trend (
On 16 October 1982, new effusive activity started at MC and continued until 23 November 1982. This eruption occurred after 23 years of quiescence and emplaced a ~7.5 km-long basaltic 'a'ā lava flow field (
flank of MC. The observed fissure extended over a distance of ~1 km down slope.
Moderately explosive activity at the upper end of the fissure led to the formation of a 25 m high scoria cone by November 7 (Figures 3(a) and (b)). A lava channel with an initial width of 3 m was observed in the first few days of eruption (
Between 30 October and 4 November, a landslide occurred just below the main cone (
The emplacement of this lava flow field on moderately inclined slopes (12˚ - 19˚) resulted in distinctive flow field morphologies. The development of a compound lava flow field characterised by several lobes (
Field observations and measurements (channel width, depth and levee angle) were made at the proximal, medial and dispersed flow portions of the flow field. Details of the surface morphology for the different lava types and structures observed in this flow field were obtained across transversal profiles made within the stable and transitional flow zones. 13 profiles (
Morphology of the flow field at a distance (length) above 5.5 km (that comprised the dispersed and flow front zones) could not be observed because this portion of the flow is buried by the lower 1999 vents and lava (Figures 1 and 2). Several bifurcations, islands and flow lobes are observed. Our focus is on the WSW branch of the flow field (
Lava thickness was derived from trigonometry. These values were substituted in equations from [55,56] and into Jeffrey’s equation to estimate yield strength (levees)
and mean channel velocity (Equations (1) and (2)).
where t is yield strength, r is the dense rock equivalent density, g is acceleration due to gravity which is equal to 9.8 m∙s−2, t is levee thickness and a is the gradient of slope (slope here represents pre-eruptive down flow slope).
where V is mean velocity, a is the pre-eruptive down flow slope (obtained from the field and the 30 m Digital Elevation Model (DEM) for MC) and B is the shape constant considered to be equal to 3 (value is representative of wide channels).
From the estimated velocity (V), channel depth (d) and width (W), effusion rate for lava flowing in these channels was derived (see [
For the purpose of reconstructing the stable channel geometry, profiles were made at intervals of 100/200 m (for the first one kilometre) and at 1 km interval from then onwards (
The 1982 compound 'a'ā lava flow field (Figures 4(a)-(d)) has 6 main and uncountable subsidiary lava flow lobes which branch out in several directions, with some of them meeting up again towards the flow front (
1) Scoria Cone The development of the main cone thought to have fed the greatest volume of the lava flow field was partially documented by [
The blockage (
2) Secondary Vents Field investigations revealed a series of 7 late-forming secondary and/or ephemeral (short-lived) vents (Figures 5(b) and (c)). These vents are observed ~2.5 km away from the main vent (Figures 5(b) and (c)) at ~2180 m a.s.l, emplaced on slopes of 15˚ - 19˚, all aligned in the E-W direction within the flow field. These small vent constructs are 3 to 15 m high (
3) Levees Few initial levees were preserved because of variations in flow level and blockages within channels that fa-
voured over flow events and formation of overflow levees. These events modify levees (
(Figures 4(a)-(d), Profiles 1 - 4). The inner walls of these levees show horizontal layering and shearing features (
Overflow levees could be distinguished from initial levees because they showed layering of different lava levels believed to represent different flow episodes. Overflow material is basically clinker and blocky 'a'ā lava (
Accretionary levees (Figures 6(b)-(e)) are the dominant levee type. Accreted/agglutinated lava surfaces were observed both on the outer and inner walls of channels. They are characterised by brecciated surfaces (
4) Lava Channels In this flow field, as for other 'a'ā dominated lava flow fields at MC (e.g. 1959, 1999) channels were the dominant preserved transport pathway. Lava in proximal channels (first 100 m) flowed with initial estimated velocity and effusion rate of 4 m∙s−1 and 3.3 m3∙s−1 respectively. Channels are 0.85 - 5 m wide, dominated by slab-crusted lava with clinker 'a'ā margins (
Within a few metres (~20 m) from vent, secondary channels were observed at the sides of the primary or main channel (
hosting sealed lava tube surfaces, characterised by reduced velocity (4.5 m∙s−1) and high flow rates (68 m3∙s−1). Channel material is clinker 'a'ā emplaced on a slope of 15˚.
Lava channels were still considerable wide (10 - 22 m) from this distance but dropped to 6 m, 5.5 km from vent. At this distance, velocity and flow rate had dropped to 0.3 m∙s−1 and 13 m3∙s−1 respectively. The lava type here is clinker and blocky 'a'ā lava emplaced on slopes of 12˚ - 14˚. From the estimates made, an average velocity and flow rate of 4.5 m∙s−1 and 26 m3∙s−1 were produced for this flow. Taking an average lava thickness of 5 m (±2 m; field estimates), an area of 2.6 × 106 m2 (GIS estimate), this eruption produced lava with a volume of ~1.3 × 107 m3.
5) Lava Tubes Lava tubes were first observed in channels at a distance 600 m from vent (
secondary vents were observed.
Here, on the basis of channel geometry fluctuation, we split the stable channel zone of the flow into three sections: 1) an upper section characterised by wider than deep channels; 2) an intermediate section governed by deeper than wide channels and 3) a lower section where channels resume their normal trend as in the upper section being wider than deep. This zone is followed by the transitional channel zone observed ~4 km down-flow.