The depths and thicknesses of the magma bodies that fed some of the largest Quaternary eruptions on Earth

Guilherme Augusto Rosa Gualda , Bradley W Pitcher, Sarah Lynette Smithies , Lydia Harmon , Ayla S Pamukçu

Volcanic systems are critical for the evolution and dynamics of the Earth; yet, our understanding of the plumbing systems that feed eruptions is hampered by our inability to directly observe magmas at depth. We present a compilation of matrix-glass data and rhyolite-MELTS pre-eruptive storage pressures for eleven Quaternary VEI 6+ caldera-forming eruptions worldwide with volumes ranging from 35 to 1,500 km3, and we assess the depths and thicknesses of the magma bodies that fed these eruptions. We show that magmas that feed VEI 6+ eruptions are typically stored at median depths of 3.4-5.1 km, with magma body thicknesses typically in the range of 1.5 to 4 km. Two smaller eruptions (i.e., < 50 km3) in the TVZ were fed from magmas stored at deeper levels (~10-12 km) and are characterized by much smaller calculated thicknesses (≤ 1.5 km), which may be facilitated by the extensional tectonic environment and their eruption during the early stages of magmatic cycles. In combination, results suggest that the interval between 3 and 5 km depth in the shallow crust is the most favorable for the accumulation of large volumes of magma. Comparison of thicknesses calculated using geobarometry with volume:area values shows that most magmas that feed VEI 6+ eruptions are found in melt-dominated magma bodies, rather than fed by mobilization of melt dispersed in magma mush on eruption timescales. Finally, magma thickness is constant as a function of volume for volumes > 200 km3, which implies growth of magma bodies that feed supereruptions via lateral spreading. The characteristics outlined above suggest that the behavior of magmatic systems shifts when magma volume reaches ~200 km3.