Construction dynamics of a lava channel

Abstract

We use a kinematic GPS and laser range finder survey of a 200 m-long section of the Muliwai a Pele lava channel (Mauna Ulu, Kilauea) to examine the construction processes and flow dynamics responsible for the channellevee structure. The levees comprise three packages. The basal package comprises an 80-150 m wide ′a′a flow in which a ∼2 m deep and ∼11 m wide channel became centred. This is capped by a second package of thin (<45 cm thick) sheets of pahoehoe extending no more than 50 m from the channel. The upper-most package comprises localised ′a′a overflows. The channel itself contains two blockages located 130 m apart and composed of levee chunks veneered with overflow lava. The channel was emplaced over 50 h, spanning 30 May-2 June, 1974, with the flow front arriving at our section (4.4 km from the vent) 8 h after the eruption began. The basal ′a′a flow thickness yields effusion rates of 35 m 3 s −1 for the opening phase, with the initial flow advancing across the mapped section at ∼10 m/min. Short-lived overflows of fluid pahoehoe then built the levee cap, increasing the apparent channel depth to 4.8 m. There were at least six pulses at 90-420 m 3 s −1 , causing overflow of limited extent lasting no more than 5 min. Brim-full flow conditions were thus extremely shortlived. During a dominant period of below-bank flow, flow depth was ∼2 m with an effusion rate of ∼35 m 3 s −1 , consistent with the mean output rate (obtained from the total flow bulk volume) of 23-54 m 3 s −1 . During pulses, levee chunks were plucked and floated down channel to form blockages. In a final low effusion rate phase, lava ponded behind the lower blockage to form a syn-channel pond that fed ′a′a overflow. After the end of the eruption the roofed-over pond continued to drain through the lower blockage, causing the roof to founder. Drainage emplaced inflated flows on the channel floor below the lower blockage for a further ∼10 h. The complex processes involved in levee-channel construction of this short-lived case show that care must be taken when using channel dimensions to infer flow dynamics. In our case, the full channel depth is not exposed. Instead the channel floor morphology reflects late stage pond filling and drainage rather than true channel-contained flow. Components of the compound levee relate to different flow regimes operating at different times during the eruption and associated with different effusion rates, flow dynamics and time scales. For example, although high effusion rate, brim-full flow was maintained for a small fraction of the channel lifetime, it emplaced a pile of pahoehoe overflow units that account for 60% of the total levee height. We show how time-varying volume flux is an important parameter in controlling channel construction dynamics. Because the complex history of lava delivery to a channel system is recorded by the final channel morphology, time-varying flow dynamics can be determined from the channel morphology. Developing methods for quantifying detailed flux histories for effusive events from t...

Morphology and structure of the 1999 lava flows at Mount Cameroon Volcano (West Africa) and their bearing on the emplacement dynamics of volume-limited flows

“…Although blocky flows are commonly associated with the more viscous lavas of andesite and rhyolite flows (Macdonald, 1953), primarily as a function of their slower emplacement times (and hence opportunity to thermally mature), they are also witnessed at long-lived basaltic flows (e.g. Booth & Self, 1973;Harris et al 2009 and references therein), as documented in this study. The upper surface of the flow as well as an increasing fraction of the marginal flow interior deform more as a granular flow, with blocks milling, fracturing and rearranging to accommodate channel movement.…”

“…An active channel is a stream of lava contained within marginal zones of static lava termed levees (cf. Harris et al 2009). Channel width varies greatly; proximal pahoehoe channels (Fig.…”

“…Internal levees can be formed through a variety of processes. Typically they represent subsequent flow pulses, or further inflation in the channel through rheological stiffening down-slope exerting a local back-pressure, which in turn leads to lava flow thickening (Harris et al 2009). The draining of this channel would expose another set of younger levees, and so the process can continue.…”

“…The two factors promoting accretion are both greatest at the upper portions of the channel sides: shear, aiding the smearing and consolidation of clinkers, and cooling, which promotes the solidification and stabilization of the levee. Successive rises in flow level will thus accrete material from the channel, and build outwardly sloping inner walls unless a flow surge or blockage downstream causes a more rapid rise, in which case overflows will likely occur (Harris et al 2009). There is often considerable interplay between initial, accretionary and overflow levee-building processes.…”

“…Detailed studies of basaltic or basaltic andesite lava flows have been dominated by investigations at only a few volcanoes, for example, Hawaiian volcanoes (Soule, Cashman & Kauahikaua, 2004;Harris et al 2009;Ball, Pinkerton & Harris, 2008), Longquimay, Chile (Naranjo et al 1992) and Etna, Italy (Polacci & Papale, 1997Calavari & Pinkerton, 1998Guest et al 1987;Guest & Stofan, 2005;Bailey et al 2006). There is need to examine case studies elsewhere.…”