Volcanoes of the World

  Located 1000 km off the South American Coast, the Galapagos Islands are a group of 13 volcanic islands, each consisting of a single volcano, sans one – Isla Isabela (aka Albermarle Island) which consists of six volcanoes including Wolf Volcano, Darwin Volcano, Alcedo Volcano, Cerro Azul Volcano, and Sierra Negros Volcano.  The volcanoes are of the shield variety and are at least 8 million years old, with the method of formation being a mantle plume that is currently located beneath the Nazca Plate.   

  Mantle plumes are responsible for a number of volcanic island chains around the world, including the Hawaiian Islands, the Reunion Islands, and the Azores.    No theory as to the starting mechanisms that produce the plumes has been accepted.  Hot columns of rock rise from deep within the Earth – theorized anywhere from the upper/lower mantle interface (670 km) to as deep as the base of the mantle (2900 km.)  Either way, this mass of hot rock is less dense than the surrounding, cooler rock and begins to rise toward the surface at rates around 10 cm/year.  As the plume nears the surface around 150 km or so, the reduced pressure allows the superheated rock to melt.  The lithosphere intervenes around this point, preventing the plume from rising any further, acting like a cap rock.   Microscopic channels do allow some of the magma to continue the voyage towards the surface and then collect in pools called magma chambers.  From here the magma chamber builds up pressure until it can find a way to the surface and erupt. 

  The mantle plume remains fixed with respect to the lithospheric plates that ride on top of the asthenosphere.  This causes the linear chains that you see in Hawaii, the Galapagos and Reunion Islands.  As stated earlier, the Galapagos Islands are currently located on the Nazca Plate which is moving south-southeast – hence the northwestern island of Espanola is the eldest and the Southeastern island of Isla Isabela is the youngest of the chain.  At one point in history, the Galapagos Spreading Center was located above the plume, producing features on both the Nazca and Cocos Plates. To the northeast, the Carnegie Ridge on the Cocos Plate and is a group of seamounts that were produced up to around 5 million years ago.  The oldest seamounts in both groups are around 8 million years old – older ones are no longer present, having been destroyed in the subduction zone located along the coast of Central and South America.  There are some who theorize that the Caribbean Islands are a remnant of the Galapagos Plume’s activities.  The age of the plume would then be in the range of 90 million years old. 

  On October 22, 2005 the Sierra Negros (“Black Mountain Range”) Volcano on Isla Isabela began erupting.  Located on the southeast corner of the island, Sierra Negros is a 1500 meter high volcano and among the most active of all the Galapagos Volcanoes.  This is the 11^th known eruption of the volcano since 1813, with the last having occurred in 1979.  Lava flows from the October, 2005 eruption flowed down the northeast flank of the volcano and posed no threat to humans or the island’s wildlife.  Preparations were made to evacuate the village of Puerto Villamil (p. 2000) and town of Santo Tomas just in case the flows changed course. 

    Sierra Negros’ caldera is unique amongst the Galapagos.  The caldera is the largest, but is elliptical in shape, 7 X 10km, and rather shallow, 100-140m deep.  It is complex, with signs of several different collapses and young lava flows covering its surface.  Externally the volcano is a typical shield volcano with an “inverted soup bowl” shape and a predominant northeasterly trending rift zone near the caldera.  The southeastern side of the volcano is teeming with vegetation due to being on the windward side of the island – and subsequently receiving copious amounts of wind and rain. 

  This is a brief synopsis on the Galapagos Islands and one of its volcanoes.  Most of the information for this article was obtained from the following sources.  For more information and pictures please go to:

http://earthobservatory.nasa.gov/NaturalHazards/shownh.php3?img_id=13227

www.geo.cornell.edu/geology/GalapagosWWW/GalapagosGeology.html