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 (<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.
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.