Control of Volcanic Activity by Crustal Structure: Inference from the Izu‐Bonin‐Mariana and Northeast Japan Arcs.

Volcanic activities in the Izu‐Bonin‐Mariana (IBM) oceanic arc and the Northeast Japan (NEJ) continental arc are different in spite of rather constant subduction rates of the same Pacific plate. IBM volcanoes are much more voluminous than NEJ volcanoes; IBM magmas are predominantly basaltic whereas NEJ magmas are andesitic. We propose that the crustal density structures play a key role in those differences. The crustal density structures were estimated based on seismic P‐wave velocity structure and thermodynamic calculation, suggesting that the neutral buoyancy level of a primitive, mantle‐derived basalt, where a magma chamber forms, is shallower in the IBM arc than the NEJ arc. If a magma that differentiates and attains water oversaturation could erupt at the surface, a larger amount of crystallization should occur for the magma in a deeper chamber. This idea explains that erupting magma in the IBM arc is more voluminous and more mafic than those in the NEJ magma. Plain Language Summary: This paper is concerned with mechanisms governing volcano size and lava composition. Features of volcanoes on the two arcs in Japan, the Izu‐Bonin‐Mariana (IBM) and the Northeast Japan (NEJ), are clearly different although the volcanic activities are caused by subduction of the same Pacific plate. Volcanoes on IBM are much more voluminous and mostly consist of less silica lava than those on NEJ. We propose that the density of Earth's crust plays a key role in those differences. Generally, a magma forms a reservoir at a level where the magma is equal to the crust in density and then partially solidifies there by cooling. As solidification proceeds, the rest of the magma decreases in volume and increases in silica component. Considering gas features dissolved in magma, the magma in a shallower reservoir can bubble by less solidification and erupt to the surface. Our estimation of crust densities on the two arcs indicates that the reservoir level in IBM is shallower than that in NEJ. Namely, erupting magmas in IBM are affected by less solidification and hence are more voluminous and have less silica component than those in NEJ. Key Points: Two arcs in Japan have different volcanic activities: one has voluminous, basaltic volcanoes and the other small, andesiticWe propose that the crustal density structures and hence magma chamber levels cause the differences on the volcanoesIn a shallower chamber, a magma can be saturated in water with less differentiation and erupt as voluminous, basaltic magma [ABSTRACT FROM AUTHOR]