Geology of the Galápagos
Movers and Shakers
The Galápagos Islands are on the move — and relocating quite rapidly, in geological terms. In fact, they risk sliding under the South American continent altogether.
It’s a startling fact that gets attention, but that scenario won’t happen anytime soon. It will take millions of years. When it does, however, the spot on the globe where the islands are now won’t be empty. New islands are constantly forming to the west. In other words, the Galápagos Islands are still being made.
The western side of the Galápagos is the younger side of the archipelago. Fernandina and Isabela are thought to be the newcomers, with Española and San Cristóbal the older “siblings.” San Cristóbal, the easternmost island, lies about six hundred miles west of Ecuador in South America. The area the Galápagos Islands occupy measures 3,093 square miles, spread out in about thirteen major islands and many smaller islands, islets, and rocks.
The Equator cuts across the craters of Wolf Volcano and the extinct, eroded Ecuador Volcano in the northern section of Isabela Island. The islands are purely “oceanic,” meaning they have never been connected to the mainland by a land bridge. That explains why there is a conspicuous lack of land mammals on the islands. But that absence and the newness of the Galápagos are what make visiting the islands so fascinating. Here you can see land that is fresh and brand-new; a world being made.
Volcanoes and Eruptions
The Galápagos Islands began to form ten to fifteen million years ago when volcanic eruptions started to break through the ocean floor. Continuing eruptions caused underwater mountains to build up.
The bases of these mountains eventually joined together to form the Galápagos Platform, a basaltic submarine plateau located at between two hundred and five hundred fathoms (or a depth of twelve hundred to three thousand feet). When the tips of these mountains began to emerge from the sea two to five million years ago, several of the Galápagos Islands came into being.
Basically, each major island consists of a single, large shield volcano, which culminates in a collapsed crater or caldera. Long stretches of shoreline may be only slightly eroded, but in many places faulting and marine erosion have produced steep cliffs and lava, coral, or shell sand beaches.
There are some exceptions, however. Most of the central islands, such as Baltra, North Plaza, the northeastern edge of Santa Cruz, Santa Fé, Seymour, and South Plaza are uplifted submarine lava. Isabela is formed from six volcanoes that have flowed into each other, filling in the Pacific gap between them.
There are two distinct types of lava on the islands. `A`a (pronounced “ah ah”), said to be a Hawaiian term for “hurt,” is very rough and hard to walk over. `A`a lava flows have a spiny, rubble-like surface and are made of broken lava blocks called “clinkers.” The clinkers cover a dense core, which is the most active part of a lava flow. As the gooey lava in the core travels down the volcano’s slope, the clinkers are carried along on the surface. At the leading edge of an `a`a flow, however, these cooled blocks tumble down the steep front and are subsequently buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an `a`a flow.
In contrast, the second type of lava, called by the Hawaiian word pahoehoe (pronounced “pahoyhoy” and which means “ropy”) is smooth and looks like rope. It’s the result of lava that contains a lot of gas. A pahoehoe flow typically advances in small lobes and “toes” that continually break out from a cooled crust. A pahoehoe flow may create bizarre shapes, often referred to as “lava sculptures.” You may see excellent examples of pahoehoe lava on Fernandina at Punta Espinosa.
Today the Galápagos are one of the largest and most active groups of oceanic volcanoes in the world, with more than fifty eruptions in the past two hundred years. Six volcanoes are still active (one on Fernandina and five on Isabela). In 1968, there was a major geologic event when the floor of the caldera on Fernandina fell more than 984 feet, accompanied by a large ash eruption, which covered most of the northwestern slope of La Cumbre Volcano. There was another large eruption of La Cumbre in 1994. Isabela experienced an eruption on the southern flank of Cerro Azul in 1998 and on the northern rim of Sierra Negra in 2005. In 2009, the southern flank of La Cumbre on Fernandina erupted.
A visit to the Galápagos means witnessing geology in action.
Some of the Volcanic Eruptions in the Galápagos Islands
|2009||Fernandina Island (April 11–30)|
|2008||Cerro Azul on Isabela Island|
|2005||Fernandina (May 11–13); Sierra Negra on Isabela (October 22)|
|1998||Cerro Azul on Isabela|
|1991||Alcedo on Isabela (March 21); Fernandina (April); Marchena Island (September)|
|1982||Wolf on Isabela (August–September)|
|1979||Cerro Azul and Sierra Negra on Isabela|
|1954||Alcedo on Isabela|
|1813||Isabela and Fernandina|
|1801||Alcedo on Isabela; or on Darwin Island|
The Earth’s crust (or outer shell) is composed of several rigid pieces of continental or ocean floor, called “tectonic plates.” As a result of geologic processes deep in the Earth, these plates move in relation to one another at rates of about a half-inch to three inches per year.
The Galápagos Islands are on the Nazca Plate, close to its junction with the Cocos Plate. At the plate edges, many important geologic events occur. Where two sea-floor plates are moving apart, long, underwater mountain ranges — such as the Mid-Atlantic Ridge or the East Pacific Rise — occur. At the center of these ridges, new molten rock rises from beneath the crust and solidifies at the surface to form new sea floor. This gradually causes points on the two plates to move farther apart, in a process called “sea-floor spreading.”
When a sea-floor plate moves toward a continental mass, the sea-floor plate is forced under the continent. Termed subduction, this process is occurring on the west coast of South America where the Nazca Plate is colliding with the American continent. It’s responsible for the many earthquakes along this edge of South America, the building of the Andes Mountains, and the presence of volcanoes in the Andes. It’s speculated that the plate movement is driven by convection currents caused by heat deep inside the Earth, which drag the plates along with them.
At times, two plates move past one another with little or no destruction or generation of plate material. Here, transform faults are made. Another type of boundary is called a collision zone, which happens in areas of former subduction zones where the continents, which are transported on the sea-floor plates, are colliding.
In the Galápagos, not only are three plates in close proximity (the Cocos, the Nazca, and the Pacific), but there is also sea-floor spreading and subduction zones — as well as transform faults. As a result of the sea-floor spreading along the Galápagos Rift (an uneven line of ridges and chasms that lies between the Cocos and Nazca Plates) and the East Pacific Rise, the islands are moving south and east at more than 2.5 inches per year. Over a million years, this amounts to more than thirty-nine miles of movement.
One million years is a short time in geologic terms, especially when compared with the estimated age of the Earth: 4.6 billion years. Relatively speaking, the oldest islands in the Galápagos are thought to be “youngsters,” no more than five million years old. And they may have originally appeared above water as much as 217 miles northwest of their present positions.
The Galápagos lie at the meeting point of two submarine ridges: the Carnegie Ridge, running west from South America; and the Cocos Ridge, extending south from Central America. The meeting point of these two ridges is known as the “Galápagos Hotspot.” It’s located to the east of Fernandina, the youngest of the islands.
Hotspots occur in certain places around the Earth. They are stationary areas of intense heat in the mantle (which lies beneath the crust and above the central core) that wax and wane in strength, related to a weakness of the oceanic crust. Hotspots cause the crust and mantle to melt, making fissures on the ocean floor and giving rise to volcanoes.
In the Galápagos, because of the hotspot, there are fractures on the ocean floor that resemble a checkerboard. And under the Nazca Plate, the hot magma is always in motion due to convection currents. As the magma under the hotspot rises upward into the ocean through a weak part — a fissure — in the tectonic plate, it cools off rapidly upon contact with the cold ocean water. It begins to build a platform. It keeps rising, eventually creating a volcanic cone. In a few years, the cone reaches the surface of the ocean.
While the hotspot stays stationary, the tectonic plate shifts. The Nazca Plate is migrating to the southeast at a rate of 2.5 to three inches per year. As the plate moves, the volcano breaks free from the stationary hotspot, forming an independent island. As the hotspot continues to emit material, a new island is born. Soon, as the plate continues to move (with the volcano on top of it) and new plumes of magma continue to pierce the ocean floor, a chain of islands (or archipelago) begins to appear. The consequence is that the oldest islands will be on the east and the youngest on the west. (The northern line of islands — Genovesa, Marchena, Pinta, Wolf, and Darwin — are not the products of the hotspot plume. A major transform fault occurs north of the Galápagos Hotspot, resulting in widespread volcanism there.)
Eventually, the Nazca Plate will be forced under the continental plate of South America, in subduction. The Cocos Plate will migrate northeast and dive in the subduction zone of Central America.
Geologists believe the Galápagos Hotspot has been active for at least twenty million years — and maybe as long as ninety million years.
Caldera — A large, bowl-like depression, a caldera is formed when magma beneath the summit of a volcano cools and contracts, causing the entire peak to lose its structural support. It collapses inward, back into the magma reservoir, thus forming the caldera.
Calderas differ from craters, which are smaller, circular depressions created by the explosive excavation of rock during eruptions. On Isabela Island, a three- to four-hour hike will take you up to one of the largest calderas in the world on Sierra Negra. It measures about six miles in diameter.
Cinder Cone — Sometimes called parasitic cones, cinder cones are nearly perfect cone shapes. They are the result of explosive eruptions from a new vent, where there is a lot of gas in the magma. Liquid lava is thrown into the air in fountains; and small gassy, glassy particles solidify, rain down, and form a hill around the vent. Lava flows may appear around the cone’s base. The cinder (or scoria) is used on inhabited islands to construct roads, which are very dusty when dry. Cinder cones are typically found farther inland that tuff cones and on the flanks of a main volcano.
Driblet Cone — A driblet cone is a small cone formed by gummy lava, which is thrown out in dollops. Such lava solidifies into smooth and often colorful cones that look like melted toffee.
Fumarole — Fumaroles are volcanic vents or “steam valves” which emit volatile gases — such as hydrochloric acid, sulfur dioxide, and ammonium chloride — at high temperatures in powerful jets. You can view fumaroles near the volcanoes of Alcedo and Sierra Negra on Isabela Island.
Hotspot — A hotspot is an upwelling center of molten basalt, a narrow plume or pipe of mantle material that rises and spreads out radially in the asthenosphere.
Lava Bomb — Lava bombs are the result of flight. They are generally round, spherical, disc, teardrop, or radial in shape. Often found some distance away from their points of origin, lava bombs are rocks that have been ejected with explosive force during an eruption.
Lava Tunnel (or Lava Tube) — Sometimes, as lava flows, the exterior portion will cool and harden into a crust, even though molten material continues to flow in the interior. When the lava flow abates, there is not enough liquid to fill the inner cavity, and a nearly circular tunnel is formed. In the Highlands of Santa Cruz Island, it’s possible to climb down and walk through a lava tunnel.
Magma — Magma is molten, volcanic material while it is still beneath the ground. The molten magma is stored in a magma chamber before an eruption.
Pit Crater (or Collapse Crater) — Once a lava flow subsides, surface features directly above pockets of magma become somewhat unstable, causing depressions called pit craters. Pit craters are formed in the same manner as calderas, but they are not located over a central vent. In the Santa Cruz Highlands, you may see the pair of giant pit craters called Los Gemelos (The Twins).
Pumice — A low-density, volcanic glass full of cavities, pumice is the result of a very violent eruption involving water. In the Galápagos, pumice is only found on Alcedo Volcano on Isabela Island. However, because pumice floats, it may be washed up on the beaches of many islands, especially after the rainy season.
Shield Volcano — Shield volcanoes are named for their broad, dome shapes. As a lava flow emits from a central vent, it pours out in all directions, creating a gently sloping, bulging profile reminiscent of an ancient Teutonic war shield. The volcanoes build up slowly as the result of thousands of fluid lava flows. The lava cools into thin layers, which continue to accumulate for years, eventually giving the volcano its height.
Most of the Galápagos Islands were made from a single shield volcano, creating a high point on each island, with gentle slopes and a flat coastline. Española and Fernandina are typical shield volcanoes. Isabela, the largest island, was created from the fusion of six shield volcanoes.
Spatter Cone — A larger version of a driblet cone, spatter cones form when a limited amount of basaltic lava erupts from an active volcanic vent. Molten rock falls back to the ground in liquid gobs that fuse together. The result is a steep-sided cone up to a few dozen feet high. They may be part cinder.
Tuff Cone — A tuff cone is a vertical rock formation of volcanic origin, which is a consolidation of hardened ash particles, called tuff. During an eruption, as hot lava pours into cold water, explosions occur. Fragments are sent flying in all directions and come spattering down to the ground, hardening into a cone shape. Off Bartolomé Island is Pinnacle Rock, a remnant of a tuff cone.
Uplift — When magma flows through a subsurface geological fissure (or fault), it exits the fissure as lava, hardening and gradually lifting the landmass up past the ocean’s surface. Uplifts are usually associated with a previous or impending eruption.
Volcanic Plateau — Volcanic plateaus are created when lava pours quickly from fissures rather than from central vents. The lava surrounds the area; and after a succession of lava flow upon lava flow, broad plateaus are formed.