The Galapagos Islands, which are also familiar by the name of Islas Encantadas, are situated almost 1000 km (600 miles) west of Ecuador, located across the equator. The archipelago comprises 13 major islands, six smaller islands and numerous islets and reefs, with a total area of 7,882 km square. These are volcanic islands. In fact, the largest island in the west, Fernandina, and the largest island, Isabela, are often associated with volcanic activity.
Shield Volcanoes constitute the major islands; these are composed mainly of huge piles of basaltic magma built up, away from the sea floor. Most of the shield volcanoes have large, non-steep surface that curves outward visibility, this shape can be revealed when a Viking’s sheet is laid flat on the ground. The Fernandia Island shows varying degrees of difference in the shape compared with others, where the volcano, which has been deemed as the most active volcano historically, has built up a sharp slope which is almost vertical, contrary to the usually low- profile island. The cactus is one of the earliest colonizers of new lava terrains and belongs to genus Brachycereus. Occasionally, the cactus is swallowed by the new lava terrains, which is shown by the frame of Brachycereus preserved in some flows.
Most of the giant shield volcanoes feature an amazingly large crater at their tops. The largest caldera belongs to Volcán Sierra Negra. This immense oval area which is lowered then the surface surrounding it almost 10 kms long and covered with lava flows. Calderas are different from the normal volcanic craters because they are not just openings in earth’s crust through which the magma erupts, but rather they are formed when the vertical support underneath them is removed after the volcanic eruptions on a larger scale take place.
Flank Eruption in Volcanoes:
Not necessarily all the eruptions take place from the highest openings of the volcanoes but many Flank eruptions may also occur, and these flank eruptions occur specially in the larger volcanoes, where the pressure needed to raise the volcanic products to the highest point openings in volcanoes, may exceed the pressure needed to split it open from the lower sides of the volcanoes. The main stack of volcanoes Fermandia in the above diagram is dotted with little parasitic cones indicating the point where lower side eruptions on volcanoes occurred.
These shield volcanoes are produced at thermal geological features, known as Hot Spots. Hot spots on the earth’s surface are characterized by extremely high heat flows, which often occur in volcanic activity. The hotspots are believed to overlap mantle plumes. Mantle plumes are slowly rising round pillars, containing hot and viscous mantle material and have a diameter of about 150 kms. The rising mantle plumes rise from centre to outwards beneath the tectonic plates which cause the overlying lithospheric plates to heat up. Volcanoes may form on the lithospheric plates, which can be removed only by the heat source by plate movement.
The Hawaiian Islands are the authoritative example of shield volcanoes existing in a linear form, existing at a hot spot over a mantle plume, with younger and active volcanoes are at the southeast end of the Hawiian volcanic chain while the older and eroded volcanoes at the northwestern end away from the hot spot. In a similar fashion, the Galapagos Islands are also shield volcanoes formed over a hot spot, with young and busy volcanoes at the west end. Plate movement in the chain is in the east- southeast on a bearing on 102 degrees. Read the full story