FROM SCORIA CONE SHAPE AND VOLUME TO MAGMA CHAMBER VOLUME AND SUPPLY

Many volcanic arcs contain volcanic fields dominated by smaller volcanic types, particularly small shields and scoria cones. Whether truly monogenetic or merely short-lived relative to the larger stratovolcanoes, these smaller cones account for much of the magma volume and offer insight into the tectonic factors driving volcanism. The growth of scoria cones during historic eruptions was modeled using exponential curves aka Wadge 1981: V=Q*(a*(1-exp(-t/a))-b*(1-exp(-t/b))) where V is cone volume, Q controls the maximum magma supply, t time since start of eruption, and a, b are coefficients controlling how fast the magma supply increases and decreases over the eruption. Constant values of a (2x10⁷) and b (1x10⁶) fit this equation to several historically erupting cones, including Paricutin, Mexico, Laghetto, Mount Etna, Sicily, and Tolbachik, Kamchatka, with magma supply rates varying from 1 to 100 m³/s. That the estimated magma supply rate generally increases with cone volume suggests that scoria cone volume could be indicative of magma supply rates despite being a variable (and sometimes) small percentage of the magma erupted. Although scoria cones are neither piles of lava flows and nor usually associated with magma chambers, their heights scale with their basal radii much as the Castruccio, Diez, and Rho 2017 model predicts for shield volcanos which can have a similar magma plumbing. This study explores the factors controlling scoria cone growth to determine whether magma chamber volume, geometric constraints, or scoria avalanching controls scoria cone size and shape. For scoria cone populations in several volcanic fields, including the Guatemalan-El Salvadoran Volcanic Field, there is some shift in the height-to-basal radius ratios with overall cone size for both scoria cones alone and scoria cones compared to other volcanic types. Placing individual scoria cone growth histories in the context of volcanic cone populations (of all types) suggests that magma chamber depths, magma supply rates, and tectonic stress states may exert a dominate control on the size distribution of volcanic populations.