CLASTOGENIC FLOW AS A RESULT OF REACTIVATION OF AGGLUTINATED SPATTER

Clastogenic lava flows form by remobilization of accumulated spatter. The main factors controlling clastogenesis are yield strength and basal shear stress, which depend on accumulation rate, cooling rate, cone height, and topographic slope. When basal shear stress exceeds yield strength, clastogenesis may occur. Four spatter cones were investigated at Krafla, Iceland. Height and topographic slope measured from these cones were used to calculate basal shear stress. Yield strength was calculated as a function of the spatter cones’ internal temperature. Results from Rader and Geist (2015) were used to derive a linear relationship between accumulation rate and cooling rate; higher accumulation rates cause greater insulation of the deposited clasts and slower cooling. A strength parameter accounting for the strength of the vitreous interclastic framework formed by the quenched rinds on the spatter clasts was added to the yield strength. Whole rock major oxide data were used to calculate and plot the melt viscosity of Krafla spatter as a function of temperature following Giordano et al. (2008). Those viscosities and a vesicle strain analysis conducted with Geological Image Analysis Software (GIAS; Beggan and Hamilton, 2010) were used to determine the amount of strain preserved in vesicles and to estimate the necessary strain rates for melt mobilization. Only one of the cones exhibited convincing evidence of clastogenesis (slope: 40°, spatter thickness: 3.5 - 4 m). Compaction of vesicles is evident in thin section but there is no quantitatively consistent increase in strain towards the base of the cone, suggesting strain may be accommodated by viscous flow within the cone. Based on these strain rates (0.0001 – 0.0015 1/s) and those predicted using the Bingham flow law (1.39 – 36.16 1/s), the vesicles record a maximum of only 0.017% of the total strain, since many bubbles break into smaller bubbles and reset the strain signature. Accumulation rates greater than or equal to 11 m/hr on 40° slopes, 14 m/hr on 30° slopes, and 18 m/hr on 20° slopes can produce clastogenic flows. On 40° slopes, agglutinate failure thicknesses range from 4.5 to 4.8 m depending on accumulation rate. These modeled heights are comparable to the height of the clastogenic cone at Krafla.