Galapagos Islands geology
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.