The Galápagos Island region is one of the most volcanically active areas in the world. The first islands were formed between three and five million years ago, when underwater mountains formed by successive volcanic eruptions began to emerge from the sea, and island formation has continued ever since.
Each major island is a large shield volcano, with the exception of Isabela, which was formed by the unification above sea level of six volcanoes. Since 1535, when the islands were discovered, over 50 eruptions have been recorded, the most recent of which was Isabela’s Sierra Negra in October of 2005.
Geologists rely on two widespread geological theories—the theory of plate tectonics and the hotspot theory—to explain island formation and evolution. According to the theory of plate tectonics, a series of mobile pieces of the earth’s outer crust, or plates, are continually spreading away from one another, sliding past one another or colliding with one another.
Near the Galápagos, tectonic activity follows an interesting yet complicated pattern influenced by the convergence of three plates—the Pacific, the Nazca, and the Cocos Plates. The island archipelago is located on the northern boundary of the Nazca Plate near its junction with the Cocos Plate, but it is not at rest there. Sea-floor spreading along the Galápagos Rift causes the islands to move south and east at the rate of 7 cm/year. This may not seem that fast, but in a million years (a geologic heartbeat), that amounts to 70 km (43.5 miles).
The hotspot theory says that, in certain areas around the earth, there are stationary areas in the mantle that occasionally get superheated. When the heat from these hot spots increases enough to melt the earth’s crust, it will produce a volcanic eruption with enough magnitude to project molten lava toward, and eventually above, the ocean’s surface.
As tectonic plates move across the fixed hot spot, a trail of volcanoes follows, each aging and eroding with time. Since the Galápagos are moving southeast over the hot spot, geologists speculate that the oldest islands must be San Cristóbal and Española, which are found in the southeast archipelago. Following the same logic, the newest islands must be the northwestern islands; according to form, the western islands of Fernandina and Isabela, which are situated directly over the hot spot, are still in the process of formation.
Most of the volcanic rocks and magma forming the eruptions in the Galapagos are basaltic, which have low levels of silicon and oxygen and flow more easily. Since eruptions thus take the form of lava flows instead of explosions, the volcanoes that develop tend to have smooth, shield-shaped outlines with rounded tops instead of the more well-recognized cone-shape.
As lava flows, its surface layer comes into contact with the air, cools, and slows down. The result is the formation of a crust with a distinctive surface type: pahoehoe (braided), aa (jagged, ow! ow!), or schrict (ropy). These lava formations are commonly seen at Sullivan Bay on the eastern side of Santiago.