Partial Collapse of Plinian Volcanic Jets and the Production of Multiply Layered Ash Clouds
Thursday, 18 December 2014
Powerful explosive volcanic eruptions inject ash high into the atmosphere, which spreads as an intrusion to form characteristic umbrella-shaped clouds. An enigmatic feature of a number of recent eruption clouds (e.g. Popocatepetl, 2012; Soufriere Hills, 2010; Mt. St. Helens, 1980 and Puyehue, 2011) is that they are constructed of multiple layers (Figure 1, left). How such layering emerges within an advancing gravity current of initially well-mixed ash is unclear. Potential major controls include the strength and structure of the atmospheric density stratification, the particle size distribution within the ash cloud and the entrainment of ambient atmosphere into the rising plume. Accordingly, we conduct analog experiments in which saltwater jets with mono- and bi-disperse suspensions of fine and coarse silica particles are injected into a saltwater tank with a linear density stratification. Whereas classical umbrella clouds are produced for strong jets (low source Richardson number, -Ri0) under all particle-loading conditions, multiply layered clouds emerge for weak jets (high -Ri0) and relatively concentrated bi-disperse and coarse mono-disperse suspensions. In particular, at high -Ri0 coarse particles inhibit entrainment and enhance the partial collapse of rising jets to form gravity currents that intermittently descend along the jet margin and spread at varying neutral buoyancy heights to form layers. For high concentrations of coarse sand gravity currents can reach the tank floor. Collapse and compaction of this material to form a deposit expels buoyant interstitial fluid that rises to form additional layers below and within the overlying multiply layered cloud. One layer and multiply layered clouds have distinct depositional patterns and present unique risks to air traffic.