Understanding the creation and the earthquakes of the Galapagos Islands

Galapagos tectonics
Darwin had part of the answer on the Galapagos islands.
The answer to the origin of the Galápagos  was only available after 1958, when continental drift, or plate tectonics, was discovered.  We now understand that the surface of the earth is divided into massive tectonic plates which slowly drift across the globe.  The formation of the Galápagos is intimately tied to the history of the Nazca plate, on which they lie.

The Galápagos are located on the very northern edge of the Nazca plate, which is bounded by the Cocos (north) and the Pacific (west) plates (see map). The Nazca plate itself is currently drifting southeast, away from the Cocos plate and from the Pacific plate.  This movement of the Nazca plate relative to the Cocos plate is responsible for producing the cluster of volcanic islands we call Galápagos.

Divergent and transform boundaries in between the Cocos and Nazca plates - Graphic ; courtesy NOAA OceanExplorer

There is a large body of geophysical evidence for the existence of enormous plumes of hot mantle material that originate near the earth’s core and rise all the way to the crust. These plumes seem to be stable over many millions of years. and with time, they burn through the crust to form an underwater volcano which may eventually grow big enough to become an island.
But, because the crustal plate is in constant motion, the island will eventually move off of the hot spot. thereby making room for a second volcanic island. And a third, and a fourth…. Thus are archipelagos like the Galápagos formed.

Picture : Armand Vervaeck

In regions of extensive and repeated fissure eruptions, ridges are formed. Often these underwater ridges have substantial height (as much as 2,000 to 3,000 meters) and are considered to include the longest mountain chains in the world. As new oceanic crust forms at the ridges, older crust is progressively moved farther and farther from the ridge, creeping along at a rate of a few centimeters per year. This process is referred to as seafloor spreading. For this reason, we often refer to divergent boundaries as spreading boundaries. As the new oceanic crustal rock moves away from the heated ridge, it cools and contracts, decreasing the ridge height (i.e., increasing the water depth) of the ridge flanks.
Recently, the use of undersea submersibles has provided a window to view the mid-ocean ridges. Scientists have actually observed new ocean floor being produced as red-hot lava extrudes from active fissures, instantly “freezing,” or cooling, in the 2°C bottom water. Associated with the ridges are hydrothermal vents, where super-heated water, gases, and minerals escape from deep within the Earth.

Picture : Armand Vervaeck

Islands farthest from the hot spot are older and more eroded while islands near or on the hot spot are younger and steeper. Thus Isla San Cristóbal, the nearest to the mainland, is approximately four million years old and composed of eroded, rounded cones, while Isla Fernandina dates at less than 7000 years and is considered to be one of the most active volcanoes in the world. Recently former Galápagos islands, now submerged, have been discovered between Isla San Cristóbal and the mainland. This discovery may double the age of the islands. Indeed, several million years from now the present islands may likewise sink beneath the waves only to be replaced by a new set of Galápagos Islands. Who can imagine what course further evolution will take!?

Galapagos Earthquakes.
Although the Galapagos area is mainly a Hot Spot where the Nazca plate and the Cocos plate are pushed away from each other (divergent boundaries) , which allows molten lava to reach the earth’s surface, a small portion of the fault line is a transform fault where the 2 plates are striking along each other with regular earthquakes as a result (see graphic)

Galapagos volcanoes
Mid-ocean islands like the Galapagos are formed from basalt, the most basic of all types of lava.
Basalt has a very different chemical composition from the lavas that erupt from continental volcanoes, and is much more fluid. Consequently, as the lava flows build up to produce a volcanic cone, the island cones have a much shallower slope than those on the mainland.
These shallow-sloped volcanoes are called shield volcanoes and in the Galapagos, they are often compared to over-turned soup bowls.
Such shield volcanoes can clearly be seen in the younger western islands of Isabela and Fernandina. To the east, the volcanoes are lower and more eroded.

Galapagos volcanoes creation, erosion and constant movement - Graphic : courtesy Dr. Robert Rothman

(some text Courtesy – Dr. Robert Rothman, Professor Biological Sciences)
(Pictures and graphics : Dr. Robert Rothman, Armand Vervaeck and NOAA oceanexplorer)

Advertisements

Understanding the Vanuatu M 7.3 earthquake

Which are Nature’s triggers for the massive Vanuatu earthquake from August 10,2010 ?

Courtesy : Wikipedia and USGS

Courtesy : Wikipedia and USGS

In plate tectonics, a convergent boundary also known as a destructive plate boundary (because of subduction), is an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move toward one another and collide.
A
s a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near convergent boundaries.
When two plates move towards one another, they form either a subduction zone.
This depends on the nature of the plates involved. In a subduction zone, the subducting plate, which is normally a plate with oceanic crust, moves beneath the other plate, which can be made of either oceanic or continental crust.

When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it.
The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife).

A deep undersea trench is located in front of such arcs where the descending slab dips downward, such as the Mariana trench near the Mariana Islands. Other good examples of this type of plate convergence would be Japan and the Aleutian Islands in Alaska.
The Vanuatu trench near the island of Efate can be seen very clearly on the satellite maps below.
When both of the plates are made of oceanic crust, convergence is associated with island arcs such as the Solomon Islands.


Main source text and graphics :  Wikipedia

%d bloggers like this: