Galapagos Islands landforms
The Galapagos Islands are a volcanic archipelago of long-isolated flora and fauna. (Photo: Images )
The Galapagos Islands off the western coast of Ecuador are justifiably renowned as a biological laboratory: Rich in species found nowhere else in the world, they have been celebrated since Charles Darwin famously sang their praises as showcases of natural selection. But that floral and faunal richness stems from the islands’ isolation, and that isolation is a function of a dramatic geological story – one as worthy of experiencing, through Galapagos landscapes, as marine iguanas and giant tortoises.
Like the Hawaiian archipelago, the Galapagos Islands – which include 16 main islands and many islets – derive from volcanic activity associated with a so-called hotspot in the Earth’s mantle, a source of molten rock that rises to the surface. Many geologists describe the mechanism as a mantle plume. The swelling of molten rock has produced a bulge called the Galapagos Platform peppered with the volcanoes that form both above-water islands and submerged seamounts.
Crystallizing magma surfaced in the mantle plume of the Galapagos hotspot builds shield volcanoes, which are massive, broadly sloping fire mountains named for their resemblance, in profile, to a battle shield. The volcanoes of the western Galapagos tend to be steeper-sided than their eastern counterparts, but because the basaltic lava ejected by shield volcanoes are typically very fluid and non-explosive, shield volcanoes are far gentler in form than their steeply conical and violent cousins, stratovolcanoes. The loftiest of the Galapagos volcanoes, and one of the most active, is Volcán Wolf, the 5, 600-foot crown of Isabela, biggest of the islands and a popular tourist destination.
A volcanic crater that collapses due to failures in the underlying magma chambers forms a broader summit depression called a caldera. Huge calderas top volcanoes of the western Galapagos: Volcán Wolf has a 2, 000-foot-deep caldera. These ruined mouths give the mountains their level-looking summits in profile. A caldera atop an active volcano may be flooded with lava in a subsequent eruption, as seen on Marchena Island. A number of Galapagos calderas contain lakes, both saline and fresh.
The basalt lavas the Galapagos volcanoes eject in terrestrial eruptions comes in two basic forms, pahoehoe and aa, which visitors can distinguish by look and feel. The former stems from slow-flowing lava with a well-developed crustal coat and usually manifests as dark, smooth coils and stacks. Aa lava flows more swiftly, and its speed disrupts the formation of a fully-coated crust; half-congealed crust breaks apart in the fast-moving spread, giving the solidified lava a rugged, broken appearance. As it weathers, aa lava oxidizes to a rusted color, lighter than pahoehoe. Aa lavas are particularly prevalent on the big western Galapagos volcanoes. Younger lava flows are often entirely bare, but, given time and agents of geological weathering, soil will develop that can eventually support vegetation.
Some Galapagos volcanoes, like Fernandina, are still active and continue to occasionally erupt, a process that helps increase their bulk. Once cut off from their magma source in the mantle plume, however, the volcanoes die and become simply inactive islands to be gradually diminished by weathering and erosion. The oldest have been whittled down so that they no longer break the ocean surface; these submerged mountains are called seamounts. The Carnegie and Cocos ridges are major seamount chains in the Galapagos. Some of these venerable drowned mountains are as old as eight million years, indicating the Galapagos hotspot’s long existence.