PART 1: GEOLOGY

The Galapagos Archipelago comprises 13 large islands, 6 small islands, 42 islets and a number of small rocks and pinnacles, which make up a total land surface of 8,000km2.

The Galapagos Islands, located on the equator about 1000km (600m) west of Ecuador, were never part of mainland South America. They are a group of submarine volcanoes that grew progressively from the ocean floor, until they finally emerged above sea level about 4.5 million years ago and formed a group of islands. The islands have been added to and new islands have been forming ever since. Each island is formed from a single volcano, with the exception of Isabela, which comprises 6 volcanoes strung together.

Tectonic situation of the Galapagos Islands

The Galapagos Islands are not formed at the junction of two or more tectonic plates, as are many of the world’s volcanoes. They occur within the Nazca Plate, and are interpreted to be the result of a ‘hot spot’. A “hot spot” is region of high thermic flux due to the presence of a magmatic plume ascending from the earths’ mantle. The rising magma pierces the oceanic crust in a weak part of the plate (e.g. where the plate is fractures) and magma is extruded onto the sea floor. Another classic hot spot is responsible for the formation of the Hawaiian Islands.

The Galapagos Archipelago is a chain of islands. This is not the result of movement of the hot spot, rather, the hot spot remains stationary and the Nazca plate drifts over it to the southeast (at a rate of about 3 inches, or about 6.5cm, per year), taking the older islands with it, while new islands form the to the North west. Thus the oldest island is Isla Espanola in the South west, while Fernandina and Isabela in the northwest are the youngest and most volcanically active.

Pahoehoe lava – with a ropey surface

Like the Hawaiian Islands, the Galapagos are basaltic in composition. Basalt has a relatively low viscosity and typically forms volcanoes with gently sloping flanks (

The submarine, or seamount stage of growth is represented by basaltic pillow lavas, hyaloclastites (quenched fragmented lava), and, as the seamount approaches the surface, by coherent submarine lavas. Above sea level, shield volcanoes are composed of lava flows, with limited scoria fall and spatter deposits.

The Galapagos shields have gentle lower slopes that rise to steeper central slopes (34 degrees) and ultimately flatten off to form spectacular summit calderas between 3 and 9km in diameter, the largest being on Sierra Negra. Calderas are large, broadly circular volcanic depressions that are usually formed by the collapse of the roof of a subsurface magma chamber. Collapse often occurs during or after the evacuation of the magma chamber by an eruption. An event of this type occurred for example, on Volcan Fernandina in 1968, when the caldera floor subsided by 300m.

Galapagos shield volcano

The dome-like shape of the Galapagos shields has been likened to an overturned soup plate, in comparison to the gently sloping overturned saucer-shape of the Hawaiian shields. Scientists have suggested that the presence of intrusive rocks (e.g. basalt dykes and sills injected into the lava pile)at a high level may account for their characteristic shape.

The Galapagos Islands are among the world’s most active volcanic areas today. There have been over 50 eruptions in the last 200 years, and many are recent fro example; Fernandina has erupted on a regular basis, every 4-5 years since 1968, with the last eruption being in 1995 when lava flowed into the sea, also Volcan Cerro Azul on Isabella has erupted regularly over periods of approximately tens years since the 1950’s (intervals were closer together before that), with the last eruption being only last year, when lava flowed down the south flank of the volcano.

PART 2: CLIMATE

One of the things that makes the Galapagos Islands so special is its climate. Firstly the islands themselves are isolated and are surrounded by several hundred miles of open ocean. Thus their climate is determined almost exclusively by ocean currents, which are themselves influenced by the trade winds that push them. The marine biota is also affected by these currents.

The Galapagos Islands are situated at a major intersection of several ocean currents, the cold Humboldt current (which predominantly influences the climate), the cold Cromwell current (also known as the Equatorial Countercurrent, which is responsible for much of the unique marine life around the Galapagos) and the warm Panama current. The intensity of these currents varies during the year, as the respective trade winds that blow them weaken. Thus two distinct seasons occur depending on which current is dominant at the time.

For most of the year the Galapagos is cooled by the upwelling of the cold Peruvian oceanic and the Peruvian coastal currents (known collectively as the Humboldt current), which sweep northwards from the Antarctic, pushed by the Southeast trade winds. The Humboldt current has a mean temperature of 15 degrees centigrade. Upon reaching the Galapagos platform, the cold nutrient rich waters surface from a depth of 100m. The cold waters cool the air above them, producing a temperature inversion. That is, instead of the air gradually cooling with increased elevation, as is the norm, the air at, and above the ocean surface is cooler than that above, thus a temperature inversion occurs.

Ocean currents affecting the Galapagos Islands

The inversion layer has several effects. Firstly it contains a high concentration of moisture droplets that have evapourated from the ocean. Since some Islands have volcanoes high enough to intercept the inversion layer this results in condensation of the moisture which produces a continuous mist at high level – this is called the Garua and gives its name to the cool, dry season, which lasts from May to December. Although the Galapagos highlands are keep damp during this period, the lowlands and low-lying islands remain bone dry.

The cold Cromwell current, also known as the subequatorial Countercurrent, is also a very important influence on the Galapagos islands. It is the principal reason why the marine environment around the Galapagos islands is so unusual. The Cromwell current is a deep flow of oceanic waters originating in the western Pacific. It runs beneath the Equator in the opposite direction to bulk westward movement of the surface waters, which form the South Equatorial Current. The Cromwell current has a temperature of only 13 degrees centigrade at its core. It runs thousands of miles along the depths of the ocean gathering nutrients which rain down from the surface layers above. When the current encounters the submarine Galapagos platform it upwells to the west of Fernandina and Isabela Islands and dissipates towards the center of the archipelago.

When the Southeast trade winds slacken, usually around December the Humboldt current looses its driving power. The north east trade winds become dominant sweeping the warm, but nutrient poor, waters of the Panama current south. The waters around the Galapagos are warmed to about 27 degrees centigrade and as a result the inversion layer breaks-up, allowing the tropical weather pattern to reassert itself. Cumulus clouds build up during the morning and a downpour occurs most afternoons, this is known as the rainy or warm season and lasts from December to May.

El Niño Events

This is the normal state of affairs for the Galapagos Islands. Some years however (every four to seven years) the south east trades winds do not develop sufficiently to sweep the Humboldt current northwards and the Panama current drastically warms the waters around the islands. This phenomenon is known as El Niño, named by the Spanish, meaning ‘the child’ because it typically begins around Christmas. On the positive side it brings lots of rain and favours vegetation growth, however it also has a drastic negative effect. Under normal circumstances the upwelling of the cold Humboldt current brings nutrients to the surface ensuring a plentiful supply of plankton for animals low down in the food chain, e.g., fish and squid. Without this occurrence the food chain is broken at a critical point and many larger animals such as the Galapagos fur seals, sea lions and marine birds such as Bobbies, Flightless cormorants and penguins starve to death as their food departs for deeper cooler waters.

Cold, nutrient-rich waters are warmed during El Niño events

In 1982-83 the Galapagos suffered the effects of an exceptional El Niño, which brought nine months of continuous rain to the Islands, very high humidity and sea surface temperatures of 30 degrees centigrade – and resulted in very high animal mortality. In 1997-98 the Galapagos suffered another drastic El Niño event and many animals were again effected, for example the populations some sea lions and boobies fell by as much as 50%. The Marine Iguanas also suffered even though they are vegetarian. The abnormally warm waters prevented the growth of the algae they feed on and although they switched to alternative food in many cases they were unable to digest it and literally starved to death with full bellies. A classic example of nature ‘bloody in tooth and claw’.

References / Acknowledgements

Cas RAF, Wright JV (1987) Volcanic successions: modern and ancient. Allen and Unwin.

Simkin T, Howard KA (1970) Caldera Collapse in the Galapagos Islands, 1968.

Simkin T, L Siebert, L McCelland, D Bridge, C Newhall and JH Latter (1980) Volcanoes of the world. A regional directory gazetteer and chronology of volcanism during the last 10,000 years. Smithsonian Institution. Stroudsburg, Pennsylvania. Hutchinson and Ross.

Perry R (1984) Key Environments: Galapagos. Pergamon, UK

Thanks to Dr Jon Dehn (Alaska Volcano Observatory) and Dr Mike Branney (University of Leicester, UK) for searching the web for information on Galapagos Volcanoes.

Those who are interested can look at an up to date satellite image of Galapagos volcanoes at:

http://rsd.gsfc.nasa.gov/goese/autogvar/goes8/galapagos

Dr. Janet Sumner-Fromeyer