The Geology of the Galapagos Islands

Background Information

The Galapagos Archipelago comprises 13 large islands, 6 small islands, 42 islets and a number of small rocks and pinnacles, which make up a total land surface of 8,000 km². The Galapagos Islands, located on the equator about 1000 km west of Ecuador, were never originally part of mainland South America. They are a group of submarine volcanoes that grew progressively from the ocean floor until they emerged above sea level about 4.5 million years ago and formed the group of islands. The islands have been addded to and new islands continue to form ever since. Each island is formed from a single volcano, with the exception of Isabel, which makes up 6 volcanoes in close proximity.

Inside the Earth and Magma Formation

By analyzing the seismograms from many earthquakes, scientists have discovered that there are three main levels or shells that exist within the Earth, based on their densities: crust, mantle, and core. Play with themap on left to discover the characteristics of each layer. In the mantle, the lithosphere is the relatively cool and rigid outer layer of the Earth that extends up to 100 km beneath oceans and 200 km beneath continents. Below the lithosphere is the asthenosphere, which is solid, but hot enough to flow. The temperature at these depths are 1400° C and more. By the time the magma reaches the surface, they are cooled to 1100-1200° C.

The Galapagos Hot Spot

Like many oceanic islands, such as Hawaii, the Azores, and Reunion, the Galapagos are thought to be the product of a mantle plume or more commonly known as hot spots. Mantle plumes are columns of hot rock, roughly 100 km in diameter, that rise from deep within the earth. These plumes rise because they are hotter and less dense than the surrounding rock (by as much as 200° C). The rate of ascent of about 10 cm per year. Scientists believe that the mantle plumes form at the the base of the Earth's mantle at a depth of 2900 km, where the rock is heated by the Earth's liquid iron core beneath it. The reason for this belief is that hot spots remain fixed relative to one another over many tens of millions of years, even though lithospheric plates above them move thousands of kilometers in this time. Thus the distance between the active Galapagos and Hawaiian volcanoes have remained fixed, even though the volcanoes are carried off in opposite directions by lithospheric motion.

The Tectonic Situation of the Galapagos

The lithosphere is broken up into about two dozen plates, which move with respect to one another. There is numerous plate motion occuring near the Galapagos Archipelago. A mid-ocean ridge, called the Galapagos Spreading Center, is located just north of the archipelago. This is the result of the Cocos Plate and the Nazca Plate moving away from each other. Also, a major subduction zone is located along the west coast of Central and South America, where the Naza and Cocos Plates are subducting beneath the South American and Carribean Plates. Subduction zones are marked by deep trenches and overlying chains of volcanoes; however the Galapagos Islands are not formed at the junction of two or more tectonic plates like many of the world's volcanoes. They occur within the Nazca Plate, and are the result of a hot spot. The rising magma has pierces through the oceanic crust in a weak part of the plate, where there are fractures, and magma is spewed onto the sea floor.

The Galapagos Archipelago  is a chain of islands. This is not the result of movement of the hot spot, rather, the hot spot remains stationary and the Nazca plate drifts over it to the southeast (at a rate of about 3 inches, or about 6.5cm, per year), taking the older islands with it, while new islands form the to the North west. Thus the oldest island is Isla Espanola in the South west, while Fernandina and Isabela in the northwest are the youngest and most volcanically active