High‐resolution studies of the summit of Axial Volcano

Abstract

Sea Beam bathymetry, Sea MARC I side scan (30 kHz), towed camera, and submersible data from Axial Volcano, Juan de Fuca Ridge (JdFR) have been integrated to produce detailed maps of the volcano's summit region in order to elucidate its volcanic and tectonic evolution. The proximity of Axial Volcano, the youngest edifice of the Cobb‐Eickelberg Seamount Chain, to the JdFR superposes a hotspot‐derived radial stress pattern onto the linear stress field of a mid‐ocean ridge spreading center. The morphology of the summit of Axial Volcano reflects the differing eruptive styles produced from this superposition. Long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south. Axial Volcano has a distinct summit plateau similar to those found on many other submarine volcanos. The process by which summit plateaus are constructed is illustrated by the lava flows originating from the South Rift Zone, which have partially filled the southern half of the caldera and have smoothed the flanks. The summit plateau contains penetrative NE‐SW fracturing that is approximately parallel to the JdFR. The unusual rectangular caldera is 3×8 km in size, and its trend is about 160°. Three possible mechanisms are discussed for the caldera's formation: (1) Canary Island‐type “trapdoor” formation, (2) overlapping spreading center interaction, and (3) magma source migration. Although none of these models is fully compatible with the present data base, the overlapping spreading center model is preferred. Postcaldera volcanism probably began with the formation of the Central Caldera Eruptive Complex, which was characterized by relatively low‐volume, high‐viscosity flows. The most recent volcanism has apparently taken place along the northern and southern rift zones, which have erupted more fluid lavas resulting in areally extensive flows covering up to 60% of the caldera floor. Extensive low‐temperature venting and areas of high‐temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of Axial Volcano.