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The Galapagos Islands

The Galapagos Mantle Plume

The Galapagos are a group of volcanic islands located on the equator roughly 1000 km (600 miles) west of the South American coast. Like many oceanic islands, such as Hawaii, the Azores, and Reunion, the Galapagos are thought to be the product of a mantle plume. Mantle plumes are columns of hot rock, roughly 100 km in diameter, that rise from deep within the Earth. These plumes rise because they are hotter (by perhaps as much as 200 degrees centigrade) and therefore less dense, than the surrounding rock. The rate of ascent is about 10 cm/year or so. The depth from which mantle plumes rise is, however, still a matter of scientific debate; some believe that plumes originate at a shallower depth, such as the boundary between the upper and lower mantle at 670 km, others believe they come from greater depth. One idea is that mantle plumes form at the base of the Earth's mantle, at a depth of 2900 km, where a layer of rock called D'' (D-double prime) is heated by the Earth's liquid iron core beneath it. One reason scientists believe that mantle plumes come from great depth is that they remain fixed relative to one and other over many tens of millions of years, even though the lithospheric plates above them move thousands of kilometers in this time. Thus the distance between the active Galapagos and Hawaiian volcanoes has remained fixed, even though the volcanos are carried off in opposite directions by lithospheric motion.

Magma Generation and Volcanism

As plumes near the surface, they begin to melt. The melting occurs as a result of decompression (the decrease in pressure experienced by the plume as it rises) rather than any heating. Melting probably begins at a depth of 150 km or so and continues until the plume is prevented from further rise by the overlying lithosphere. Lithosphere is the relatively cool and rigid outer layer of the Earth that extends to depths as great as 100 km beneath oceans and 200 km beneath continents. Lithosphere forms as the underlying asthenosphere, which, though solid, is hot enough to flow, cools. The lithosphere beneath the Galapagos is relatively young, and therefore thin, perhaps no more than 15 km or so thick. Thus the region of melting beneath the Galapagos probably extends from depths of 100 or 150 km to 15 km. The temperature at these depths is 1400 C and more. By the time the melts reach the surface, however, they have cooled to 1100-1200 C.

The plume does not melt completely. At a maximum, only about 20 percent of so of it melts. The melt, or magma, is initially present as microscopic channels wetting the surface between mineral grains. Because it is less dense than the surrounding rock, however, it quickly aggregates and and begins to rise to the surface. Rising into the lithosphere, it eventually becomes trapped in large pools, called magma chambers at depths between a few kilometers and ten kilometers beneath the surface. Occasionally, the magma in the chamber is able to force its way to the surface, producing a volcanic eruption. Successive eruptions over hundreds of thousands and years produce a volcano. Some of the magma crystallizes within the magma chamber, thickening the crust beneath the volcano.

The upward motion of the mantle plumes pushes the overlying lithosphere upward. This, together with magmatic thickening of the crust, is responsible for the Galapagos Platform, an anomalously shallow region of the ocean upon which the Galapagos Islands sit.

The Plate Tectonic Setting of the Galapagos

The lithosphere is broken up into about 2 dozen or so plates, which move with respect to one and other. This plate motion, together with the flow of the underlying asthenosphere, is part of a system of convection that is the principal way in which the Earth looses heat. (Part of this heat is produced by decay of radioactive elements within the Earth, the other part is left over from formation of the Earth, 4.5 billion years ago.) Mid-oceans ridges are located at the edges of plates moving away from one and other. One such mid-ocean ridge, the Galapagos Spreading Center, is located just north of the Galapagos archipelago. Mid-ocean ridges are often offset by fracture zones or transform faults. A major transform fault is located just north of the Galapagos at 91 W. Subduction zones occur where plates collide. A major subduction zone is located along the west coast of Central and South America, where the Nazca and Cocos Plates are subducting beneath the South American and Carribean plates. Subduction zones are marked by deep trenches and overlying chains of volcanoes (the Andes, for example).

As a lithospheric plate moves over a mantle plume, a chain of volcanoes is created. The volcanoes get older in the direction of plate motion. The Hawaiian mantle plume has created a chain of volcanic islands and seamounts (known as the Hawaiian-Emperor chain) thousands of kilometers long over the past 80 million years. The Hawaiian mantle plume is located beneath the Pacific plate, which is moving to the west-north-west, hence the Hawaiian Islands get older to the west-northwest. Other chains on the Pacific Plate, such as the Society Islands, are parallel to the Hawaiian-Emperor chain.

The Galapagos Islands are located beneath the Nazca Plate, which is moving east-southeast. The Galapagos plumes has not produced such as simple linear chain as the Hawaiian Islands or the Society Islands. Nevertheless, the islands do get older to the south-southeast (Espanola is the oldest Galapagos island), and it has produced a chain of seamounts known as the Carnegie Ridge. A second seamount chain, the Cocos Ridge, extends northeast from the Galapagos Spreading Center. This ridge was also produced by the Galapagos plume because up until about 5 million years ago, the Galapagos Spreading Center was located directly over the Galapagos mantle plume. This a chain of volcanos was produced on both the Cocos and Nazca plates. The Galapagos Spreading Center has since migrated to the north.

Motion of the lithosphere eventually carries a volcano away from the plume and its magma source, so the volcano then becomes extinct. The volcano and the lithosphere beneath it then begin to cool. As it cools it contracts. As a result of this contraction, the volcano slowly sinks beneath the sea. Thus the youngest Hawaiian volcanoes are islands, but the older ones are now seamounts, the tops of which become progressively deeper to the northwest. Many of the seamounts, however, were once islands. Because the Carnegie and Cocos Ridges disappear into subduction zones, it is uncertain how old the Galapagos mantle plume is. A 1990 oceanographic expedition, however, did locate an 8 million year old seamount on the Carnegie Ridge that was certainly once an island. This volcano, though now 1500 m beneath sealevel, has rounded cobbles on a flat top, which provide clear evidence of wave erosion. Thus there have been islands in the Galapagos for at least 8 million years. The plume, however, is certainly even older. Many scientists believe that the Galapagos mantle plume is responsible for the abundant volcanic rocks of Creteceous age that occur in the Carribean and on the northwest margin of South America. Thus the Galapagos mantle plume could be as old as 90 million years and there may have been islands in this locality this long. This is of great importance in understanding the origin and evolution of the unique animals that occur on the Galapagos.

Galapagos 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 91W, 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.