A youthful shield volcano forms Isla Tortuga, which lies in the Gulf of California, 40 km off the Baja coast. There is a circular, 1-km-wide caldera, west of the center of the oval 4-km-long island. Circumferential faults cut the rim of the more than 100-m-deep caldera, which is floored by a solidified lava lake and contains youthful-looking spatter cones. The volcano was constructed during two north-migrating periods of activity that began with submarine eruptions and included formation of a circular tuff-ring complex enclosing the caldera. The sides of a small volcanic horst in the center of the caldera display lava flows overlying a 2-m-thick halite bed, formed when sea water filled the caldera, creating an evaporite basin. Recent lava flows cover most of the flanks, and fumarolic activity has continued into historical time. A submarine cone with a 2-km-wide summit caldera lies about 5 km ESE of the island on the flanks of the edifice.