Isla San Martín, 6 km off the west coast of Baja California, is the westernmost volcano of the San Quintín volcanic field and the only one that is located offshore. The 2-km-wide island is a shield volcano capped by scoria cones that reach 230 m above sea level. Wave erosion has truncated part of the volcano, forming the sea cliffs seen at the left on the S side of the island.