Galapagos Islands volcanoes
Mid-ocean islands like the Galapagos are formed from basalt, the most basic of all types of lava. Basalt has a very different chemical composition from the lavas that erupt from continental volcanoes, and is much more fluid. Consequently, as the lava flows build up to produce a volcanic cone, the island cones have a much shallower slope than those on the mainland. These shallow-sloped volcanoes are called shield volcanoes and in the Galapagos, they are often compared to over-turned soup bowls. Such shield volcanoes can clearly be seen in the younger western islands of Isabela and Fernandina. To the east, the volcanoes are lower and more eroded.
Many volcanoes are topped by a caldera, a large circular depression derived from the original crater (sometimes this is subsequently filled in by new lava). During an eruption, the crater is fed from a magma chamber, but as activity dies down, the magma withdraws, leaving a large, open cavity. The ceiling periodically collapses, lowering the crater floor and widening the diameter. There was a major caldera collapse on Fernandina in June, 1968, when the floor dropped 300 meters! The largest caldera in the islands is that of Volcan Sierra Negra, Isabela, which is 7 by 10 km.
Isabela is not only the largest but also the Island with most volcanoes in the Galapagos archipelago with 6 volcanoes:
Two distinct types of volcanoes occur in the Galapagos. In the west, on the islands of Isabela and Fernandina, large volcanoes with an "inverted soup-bowl" morphology and deep calderas occur. In the east, smaller shield volcanos with gentler slopes occur. The difference between these two volcanic morphologies appears to be due to the difference in lithospheric thickness. The fracture zone at 91° W separates oceanic crust and lithosphere of distinctly different age. West of the fracture zone at 91°W, the lithosphere is older and thicker, and therefore able to support the load imposed on it by a large volcano. East of the fracture zone, the lithsosphere is too young and weak to support large volcanic edifaces.
The "inverted soup-bowl" morphology of the large western volcanoes is quite unusual (though not entirely unique) and its origin is not entirely certain. The Hawaiian volcanoes, which are the largest on Earth and much larger than the largest of the Galapagos volcanoes, are more similar to the shields of the eastern Galapagos volcanoes. According to one theory, this morphology results from way in which eruptive vents are distributed on the volcanoes. Most of the vents occur either on circumferential fissures near the flat summits, or on radial fissures on the lower flanks and aprons of the volcanos. Relatively few vents occur on the steep upper flanks of the volcanos. Thus the volcano grows outward at the bottom and upward at the top, resulting in this distinctive morphology. The location of vents and fissures primarily reflects the stresses within the volcano. Why stresses in Galapagos volcanoes should differ from those of other volcanos and lead to this distribution of vents remains unclear. An alternative hypothesis for the morphology of Galapagos volcanos is that it reflects the pattern of intrusion of magma within the volcano. In essence, magma intruded into the volcano inflates the central part, pushing the summit region upward and steepening the slopes on the upper flanks.
Another unusual characteristic of the western Galapagos volcanoes is the large size of their calderas, particularly in comparison to the size of the volcano. The presence of a caldera is responsible for the volcano's flat top; this flat top is well illustrated by Alcedo. Calderas form as a result of collapse of an underlying magma chamber. Magma within a magma chamber contributes to the support of the overlying edifice; when magma is withdrawn, the surrounding rock may not be able to bear the load and collapse results. Almost certainly, none of the calderas formed in a single event; instead they are the result of numerous episodes of collapse, as is evidenced by the uneven floors of some and benches and the walls of others. A partial collapse of the caldera on Fernandina occurred in 1968, when the northern part of the caldera floor dropped 200 meters. Collapse occurred several weeks after a brief eruption. It was observed from a distance and scientists arrived shortly after the event, so that this is one of the best documented examples of a caldera collapse. Once formed, caldera may broaden as parts of caldera wall collapse. This occurred on Fernandina in 1988. Calderas may also occasionally fill entirely with lava, then reform. Marchena, in the northeast, has a caldera that has been very nearly filled with lava. The floor of Genovesa's caldera is below sea level and breaked on the south side, forming Darwin Bay.
Historic eruptions have occurred on many of the Galapagos volcanoes, including Fernandina,Volcan Wolf, Alcedo, Sierra Negra, Cerro Azul, Santiago, Pinta, Floreana, and Marchena. Eruptions in the recent geologic past (the last ten thousand years or so) have also occurred on Volcan Darwin, Volcan Ecuador, Genovesa, San Cristobal, and Santa Cruz. A number of submarine volcanoes many have also been active in this time. It is quite unusual for a mantle plume to produce so many similtaneous active volcanoes. In Hawaii, for example, only 6 volcanos (including the seamount Loihi) have erupted in this same time, and most of the activity in Hawaiian is located on just 3 volcanos). In Reunion, only a single volcano has been active. It should be noted, however, that the magma output of Mauna Loa, the largest of the Hawaiian volcanos, has probably exceeded the output of all the Galapagos volcanos combined.
The islands of Espanola and Santa Fe are remenants of extinct volcanoes. In both cases, only part of the volcanic structure has been preserved, the remaining parts having been faulted away. Espanola and Santa Fe have been extinct for several million years . Pinzon and Rabida are both small extinct shield volcanos that have not been active for about 1 million years. Though Santa Cruz and San Cristobal remain active volcanoes, parts of their edifices are much older, more than a million years in the case of Santa Cruz (including its small neighbors of Baltra, Seymour and Las Plazas), and nearly two and a half million years in the case of San Cristobal.