Isabela, westernmost and largest island of the Galapagos, lies 650 miles west of Guayaquil, Ecuador; it is shaped like the letter “J”, and measures 40 miles at its widest, 70 miles at its longest. It is entirely volcanic; several of its 5 craters and shield volcanoes, about 2½–6½ miles in diameter, have erupted during this century. The lavas, scoria, and tuff are mainly basaltic, except at one coastal locality east of Volcan Alcedo where there are possible andesites. Lava chiefly constitutes the lower, relatively barren slopes of the island; the upper, grassy or lightly wooded parts are more scoriaceous with some pumice, but late flows abound, especially in and near the craters. Shell-rich sands appear on protected beaches but coral reefs are absent.
Outstanding volcanic features include spatter cones, slickensided flows, rifts, crevasses, solfataras, fumaroles, and strikingly linear eruptions. Breaching of cones follows local patterns—e.g., toward the south on the southeasterly side of Isabela. Sulphur deposits are associated with breaching and with vents emitting steam, hydrogen sulphide, and sulphur dioxide. Apparently oxidation of the hydrogen sulphide yields steam and sulphur.
The general relations of the volcanoes and the relative newness of the western ones suggest that the Archipelago is being extended westward from Cocos Island and the mainland of Costa Rica along a southwestward rift line, one of the two predominating crater alinements. Available soundings suggest linkage of the Galapagos group to Central America rather than to Ecuador.