The Formation and Geological Setting of the Galápagos Islands

Kenneth Leonard
Criminology and Criminal Justice Major and College Park Scholars - Science, Technology, & Society
7/12/2000

Contents:

The Galápagos Platform and Setting
Platetectonics
Hot Spot Theory

The Galápagos Islands are well known for their extraordinary diversity of flora and fauna, while the actual formation of the Galápagos has been overlooked until recently. The creation of the Galápagos is one of the most unique processes on Earth, yet it is still a mystery that scientists are trying to solve. One thousand kilometers (six hundred miles) west off the coast of Ecuador, isolated in the Pacific Ocean, lie the Galápagos Islands. The islands have been formed in the Pacific by the interaction of Plate Tectonics and Hot Spot Volcanism.

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Map of the Galapagos Islands; Image from: http://www.rit.edu/%7Erhrsbi/GalapagosPages/Vulcanism2.html

The Galápagos consists of thirteen major islands and hundreds of smaller rocky outcroppings. A few of the islands lie above or on the equator; however, most are found in the southern hemisphere. A single volcano formed each Island except for Isabela, which resulted from the union of six different volcanoes. The Galápagos Islands lie on a platform of "land" that is higher than the surrounding ocean seabed. This platform is approximately 1,000 meters (3,000 feet) below the surface of the ocean. The area of submerged platform between San Cristobal and Fernandina is approximately 200 meters (600 feet) below sea level. The surrounding ocean in the Pacific is over 3,000 meters (9,000 feet) deep. This suggests that some special processes are taking place to form the Galápagos Islands.

One of the key processes in the formation of the Galápagos Islands involves plate tectonics. Plate Tectonics is an explanatory theory that suggests that the earth's crust is made up of huge plates of mass that move in relation to each other. The crustal movements take different forms and their collisions cause much of the variety of landscapes seen today. There are over a dozen tectonic plates that make up the Lithosphere.
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Map of Lithospheric Plates; Image from: USGS

The lithosphere plates, which extend to a depth of approximately eighty kilometers (fifty miles), move over top of the Asthenosphere.

The movement within the Lithosphere is made possible because it is less dense than the Asthenosphere. The Asthenosphere is denser because it is a semi-liquid and contains higher amounts of Fe (iron) and Mg (magnesium) relative to the Lithosphere, which is a solid, containing smaller amounts of Fe and Mg.. Since the Asthenosphere is molten rock there is internal convection, which is partially explained by the result of Earth's rotation, also known as the Coriolis Effect, and significantly affected by the propensity of liquids to seek thermal equilibrium. This movement within produces various currents causing the plates to either collide, separate, or slide past each other. The movement of the Nazca plate , upon which the Galápagos Islands lie, conveys the motion of the islands in a direction of east-southeast.

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The Nazca, Cocos, Pacific and South American Plates; Image from: http://www.rit.edu/%7Erhrsbi/GalapagosPages/Vulcanism2.html

The Nazca plate is located northwest of South America and contains the Galápagos Islands as its primary landmass. The Nazca plate moves east-southeast at a rate of about five cm per year. This direction of movement causes a collision with the South American Plate , which has formed the Andes Mountains on the western coast of South America. A tectonic process called subduction created the Andes. Subduction is caused by one plate colliding with another and the denser plate sinks underneath the other plate, pushing it upward, thus forming a mountain ridge. The subducted plate melts and produces new magma, which becomes the source material for the chain of volcanoes that are the Andes. The Galápagos Islands are located on the northwestern portion of the Nazca plate and are being ferried towards South America. Two other tectonic plates are located in close proximity to the Galápagos' northern and western most islands. The Cocos plate is located to the north of the Galápagos Islands and the Pacific Plate to the west. Click for picture The interaction between the tectonic plates and a hot spot form the Galápagos Islands.

A hot spot is a region of intense heat within the Earth's mantle. The "Hot Spot Theory" attempts to explain the formation of the Galápagos Islands.
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A Hot Spot; Image from: USGS

The theory states that there is a Mantle Plume that is relatively stationary. This mantle plume is of intense heat and deforms the land above it. The crustal plate rides over the hot spot and is occasionally perforated by the molten rock that is rising from the Earth's mantle. The crust above, is forced upward because of the heat, and volcanoes are formed. The previously formed islands then move slowly away from the hot spot, making room for new islands to be formed by further volcanic activity. The older volcanoes eventually become inactive and slowly erode into the ocean depths. Hot Spots are responsible for the formation of Hawaii, the Galápagos, and other island chains.

The Galápagos Hot Spot is currently located beneath the northwestern region of the Galápagos Archipelago near Fernandina and Isabela The width of the hot spot is estimated to be approximately 150 kilometers (100 miles). Since the Galápagos Islands are moving with the Nazca plate and the hot spot remains stationary, the islands form and slowly drift away from the hot spot, at about 5cm per year, allowing more volcanoes and islands to be formed. Since the Galápagos Islands move with the Nazca plate in an east-southeast direction the older islands are found in the southeast. The newly forming islands are located in the northwest. Since all of the volcanoes are formed underwater, the material forming them builds and accumulates as it spreads out like a sand hill. This process creates the islands in the form of a volcano with gentle sloping sides and a central vent. This type of volcano is known as a shield volcano.

It is calculated that the relative positions and motions of crustal plates have changed in the last thirty million years. These calculations have come from Bathymetric and Paleomagnetic data. This data has suggested that the Nazca plate was first positioned over the Hot Spot about five million years ago. This estimate places a limit on the age of the Islands at five million years. The oldest known Island, Española, is only 3.3 million years old. This age was determined by the potassium/argon method of rock analysis. Rocks collected on Española were tested for their potassium and argon content. The content was measured against the known half-life of the elements. The ratios give an estimate of age. Older islands would have become submerged by now, and there was recently an article published in The Geological Society of America Magazine that explores evidence for the older islands not seen today.

The Galápagos Islands are some of the newest and youngest landmasses known. The majority of the islands were formed less than one million years ago. The most recently formed island is Fernandina, which is about 500,000 years old, making it the nearest island to the Galápagos Hot Spot. Since Fernandina is still very young its volcano has recently erupted. An eruption took place in 1998. In the last 200 years there have been over fifty eruptions throughout the Galápagos Islands. This indicates that the islands are young and still forming. On Isabela an observer can still see columns of steam and gas rising from two of the volcanoes.

In 1954 a huge span of seabed underneath the Galápagos Island Isabela was uplifted. This four-mile span of seabed was lifted over fifteen feet. This uplifting process occurs frequently; however, this specific uplift was very dramatic and uncommon because of its height and that it happened rapidly. A flow of molten rock inside the Earth (magma) through a subsurface fissure or fault, which then exits the fissure and hardens, gradually lifts up the landmass through the ocean surface creating the uplift.

The Galápagos Islands were formed as a result of several of Earth's internal processes. The Galápagos' stationary hot spot formed a chain of volcanoes as the Nazca crustal plate moved east-southeast above the hot spot like a conveyor belt. This process formed a chain of islands that is home to some of the most exotic flora and fauna found in the world. These plants and animals are exotic because of the isolation the Galápagos Islands face, being 1,000 kilometers (600 miles) from a mainland. The Galápagos Islands are a microcosm of Earth's recycling of itself through the birth of volcanoes, growth through eruptions, and death by gradual erosion into the sea.

Additional Reading and Web Sites

http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html
http://www.ms-starship.com/science/Galapagos_geology.htm
http://www.rit.edu/%7Erhrsbi/GalapagosPages/Vulcanism2.html
http://pubs.usgs.gov/publications/text/hotspots.html
http://www.terraquest.com/Galapagos/atlas/geology/geology.html
http://www.oceansonline.com/hotspots.htm
http://www.geo.cornell.edu/geology/GalapagosWWW/GalapagosGeology.html
http://www.darwinfoundation.org/NewFiles/pixlink/volca1.html
http://www.hcrhs.hunterdon.k12.nj.us/science/hspot.html
http://www.geo.arizona.edu/news/Sp98/nazca.html
http://www.Galapagos.com/galmap.htm

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