The Galápagos Archipelago constitutes one of the world’s largest and most active groups of oceanic volcanoes. The oldest rocks are on Barrington, Hood, and Indefatigable islands; they are uplifted submarine lavas of Pleistocene age. Recent volcanoes fall into three general geologic and petrographic groups. The large western islands are built of fluid tholeiitic lavas erupted from the summit and flanks of shield volcanoes, and each volcano has a summit caldera. The smaller islands along the northeastern side of the archipelago have a more complex history; they show at least two periods of activity and the rocks include both tholeiitic and magnesium-poor alkali basalts that commonly contain extremely abundant plagioclase phenocrysts. The southernmost of the central islands, including the western part of Chatham Island, are built mainly of magnesium-rich alkali basalts that form large, mature volcanoes. None of the islands in this group shows clear evidence of having developed a caldera, although parasitic vents are common. The north-central islands — James, Duncan, and Jervis — have complex histories and contain the greatest variety of rock types, including the most strongly differentiated lavas and accidental plutonic blocks found in the archipelago. Chemically the rocks differ from those of Hawaii; they are probably typical of igneous rocks related to the East Pacific Rise. Plutonic and effusive tholeiitic rocks follow parallel courses of differentiation with enrichment in Fe/Mg, total alkalis, and excess silica. Alkali basalts are derived from a deeper source than are the tholeiitic rocks and show little evidence of differentiation. The islands were never connected to the mainland of South or Central America. They grew from a broad shallow platform near the crest of the East Pacific Rise, and the location of individual volcanoes appears to have been controlled by two major fracture systems, one trending north-northwest and the other nearly east-west.