How to assess accurately the rate of current?
It is difficult to judge currents, but there are some indicators, which will tell a good observer more or less the rate of current. Fish react according to how strong the current is. Some seek shelter, others thrive in strong current. Observe small schooling fish like anthias or basslets and watch bigger fish like mackerels or trigger fish.
The small fish are swimming in every direction, in large schools distributed both vertically and horizontally over the reef.
Light current (to 1 knot):
The small fish are aligned, all facing up-current. If they are still in large, spread out schools, the current is around a half-knot. lf the schools are low and wide, swimming close to the coral, the current is closer to one knot. You are able to fin against this kind of current for a short time.
Medium current (1 to 2 knots):
The small fish are now hovering in a school spread out just barely above the coral and finning madly. A current of this strength begins to show in the behavior of the larger fish as well. They face the current and tend to concentrate behind coral heads or in other lee areas (lee =away from the wind / current). Most fish will now swim against the current, so if you do a drift dive, schools of fish will come towards you. In this kind of current it is already difficult to swim against.
Strong current (2 to 3 knots):
In this kind of strong current, the small fish are not seen, because they are all hiding among the branches of the coral. The big fish are gathered in lee areas, or very close to the bottom. On a drift dive in this kind of current you won’t be able to stop and fin against if you want to look at something close up – so just enjoy the ride!
Very strong current (3 knots):
Now you won’t need the fish as an indicator anymore. You are either swept along on a very fast drift dive or hiding behind a coral head. If you move your head around and face the current full on, your mask is fluttering and threatening to fly off, and your regulator begins to free-flow.
Anything over 3 -4 knots is to strong and this is very dangerous for a diver! In a matter of minutes you can be kilometers away from the boat and if the seas are big, then it will become increasingly difficult to sea a signal tube. This is when GPS devices are a good idea.
If you don’t hang on, use reef hooks or gecko dive then the currents in the galapagos will = very short dives. 3 knots = 5.4 km per hour, so you travel 2 km in 22 min.) or the current couldn’t have been much more than a knot (2 km per hour is 1.1 knot). An example of this is when we dove Darwins arch, we had to Gecko dive for the first 30 min. This starts with a negative (no air in your BCD) backwards roll off the zodiac a quick decent in the current to about 50′. Then you would do what I call upside down bouldering. As you pull yourself along the reef and over to underwater cliff edges. You would hang out in these spots at around 80′ while looking out to see whale sharks and schooling hammerheads.
Then you would release and quickly shoot along the plateau (sandy bottom and reef between the arch and darwin island.) I would often choose to descend into the sand and dig in for a while as Hammerheads would fly overhead, other would continue to ascend and do there safety stops. I would ascend over the plateau, but could see members who were next to me 5 – 8 minutes earlier would be at the far end of Darwin island. Almost 1km away. It is a lot of work for the surface crew, and requires a lot of concentration from the divers. While surfacing you want to ensure you keep clear of caves or eddies that might be present along Wolf and Darwin
When I was in the Galapagos diving off of Darwin’s Arch we figured we were in a 4-5 knot current. We had 16 members on our boat and after our first dive, we only had 8 members will to test the rest of the dives at Darwin that day.
This is a picture of the ocean currents that was created in 1943
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 IslandsThe 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.
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:
Dr. Janet Sumner-Fromeyer