A SeaBeam and SeaMARC I survey of a seamount group located several kilometres west of the axis of the East Pacific Rise (EPR) at lat 9°53′N defines the existence of a large submarine lava field at the base of one of the volcanoes (MIB). The field must be relatively young with respect to the surrounding terrain because lavas lap up against the base of MIB seamount, obscuring older erosional terrain, and extend out over the faulted, sedimented terrain of the flank of the EPR. Volcanic cones within the lava field are aligned with the strike of fault scarps that are buried by the lavas. MIB seamount is characterized by a large, well-developed caldera, and we suggest that the lava field was created during a recent volcanic episode when magmas migrated laterally away from the seamount's reservoir along preexisting faults parallel to the ridge axis. The withdrawal of magmas from the MIB reservoir into the flanking parasitic eruptive plumbing system created the lava field and deflated the seamount summit. This event contributed to the development of the MIB caldera. Draining of shallow-level magma chambers under seamounts by pressure-controlled basal-flank eruptions that form lava fields is an important mechanism for the development of seamount morphology.

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