The beautiful volcanic formations you’ll see on the islands are often quite different from those found on the mainland. This is due to basaltic lava which, rather than producing high cones such as Cotopaxi, makes shield-shaped volcanoes, such as the one at Fernandina. The broad tops of these can collapse into empty magma chambers below, leaving enormous calderas, huge depressions many times the size of the original vents, or craters, circular basins rimmed by lava walls at the volcano’s summit; Volcán Sierra Negra on Isabela has a crater 10km across, one of the largest in the world. Like steam vents on the volcanoes, sulphur-encrusted fumaroles send puffs of gas into the air, as at Volcán Chico on the north side of Sierra Negra.
Much of the Galápagos landmass consists of lava flows, and you’ll find two particularly interesting types on several islands. Pahoehoe lava, from the Hawaiian word meaning “ropey”, describes the rippled effect caused when molten lava in contact with the air begins to solidify, but is then ruffled up by molten lava passing beneath it into tongue or rope-like shapes; Sulivan Bay, at Santiago, has excellent examples of this. Aa lava, named after the Hawaiian for “hurt”, occurs when the surface of the lava flow buckles, breaks and then gets bulldozed by the continuing movement of the flow, resulting in layers of small, sharp rocks that can be very difficult to walk on. It can form a natural barrier to animals, as at the Perry Isthmus on Isabela, and castaways and buccaneers told of how this lava shredded their boots. If a lava flow hardens on the outside, and then the strength of the flow decreases, lava tubes are sometimes formed; there are several large enough to walk down on Santa Cruz.
Cones of various sizes and types frequently appear on the islands: hornitos (less than 1m high), resemble burst pimples solidified on a lava bed; the larger spatter cones give Bartolomé its spectacular lunar landscape; and the impressive tuff cones are often made up of stripy layers of rock-hard compacted ash. The uplift at Urbina Bay, on Isabela, is one of the more startling products of volcanic activity. In 1954, a five-kilometre stretch of reef was shunted 4m into the air by movements of magma beneath the crust, leaving its marine inhabitants drying in the sun. Islands including Plazas, Baltra and Seymour Norte are entirely the result of uplifts.