Alcedo is one of the lowest and smallest of six shield volcanoes on Isabela Island. Seen here from coast of Fernandina Island to its west, Alcedo has a broad 7-8 km wide summit caldera. Most of the flanks and summit caldera are vegetated, but young lava flows are prominent on the northern flank near the saddle with Darwin volcano. Alcedo is the only Galapagos volcano known to have erupted rhyolite as well as basalt. Only one eruption is known during historical time. An active hydrothermal system is located within the caldera.