GEOLOGY & MARINE CURRENTS

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.

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.

Like the Hawaiian Islands, the Galapagos are basaltic in composition. Basalt has a relatively low viscosity and typically forms volcanoes with gently sloping flanks (<10 degrees), known as shield volcanoes. In plan, shield volcanoes are  roughly circular or elliptical in shape. They are built up by frequent eruptions of fluidal basaltic lava issuing from a central vent or the flanks. Two main types of subaerial basaltic lava have been distinguished; Pahoehoe (which is the Hawaiian word meaning ropey) which is characterised by smooth, billowy, ropey and toe surfaces; and Aa (the Hawaiian name for hurt) which has a spinose and fragmented surface. 

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.

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 for 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.

MARINE CURRENTS

The Galapagos Islands are not only a marine biogeographic province on their own (due to the high proportion of unique marine species), but are also an area subdivided into different ecoregions. Warm an cool oceanic currents meet in the islands and produce this unique setting, where fur seals and penguins meet tropicbirds and tropical flamingos. The Galapagos Marine Reserve is one of the largest marine protected areas in the world and is becoming a top destination for international diving tourism. Of course, climate and weather throughout the Galapagos depend directly upon oceanic conditions.

Despite their tropical location, two moving currents affect the islands. The cold Humboldt Current (or Peru Current) produces the cool and dry garúa season from June to November and the warm marine Panama Current which produces the warm and wet season from December to May. During the garúa season, cooler waters from the Humboldt Current are driven to the Galapagos by the southeast trade winds, with an average sea temperature of 71°F. As a result, there is warm tropical air passing over cool water. The moisture evaporating from the sea is concentrated in an inversion layer (300 to 600 m above sea level) and the higher parts of the islands, which intercept this layer, receive precipitation in the form of garúa (mist rain). While lowland areas remain dry though cool. During the warm season the southeast trade winds diminish in strength and warmer waters from the Panama Basin flow through the islands. The average sea temperature rises to 77°F. Warmer waters cause the cool season inversion layer to break up, and Galapagos experience a more typical tropical climate with blue skies and occasionally heavy showers. In some years, the flow of warm water is much greater than normal, and an "El Niño" year results. Surface water temperatures are higher and rainfall can increase greatly. Life on land blossoms but seabirds and sea life, which depend on the more productive, cooler waters, may experience dramatic breeding failures.

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