GALAPAGOS GEOLOGY

The Galapagos Islands are a series of gigantic volcanic peaks composed almost exclusively of basalt giving the islands their dark gray color. The archipelago lies near where the Nazca Plate under the South American Plate in an area of great geologic activity. The Galapagos Islands are among the most active volcanoes in the world. Eruptions have taken place in historical times on Fernandina, Isabela, Pinta, Marchena, Santiago and Floreana. The most active volcanoes today are on Fernandina, Isabela, Pinta and Marchena, and the fumarolic activity may be seen intermittently on each of these islands.

Most of the islands have the classic cone shape of a shield volcano created from the build up of layers of lava. These volcanoes have risen from 6500 to almost 10,000 ft (2000-3000 m) above the seabed. The other islands appear like a tilted tabletop and were created from volcanic plateaus. Plateaus create when the eruption of basalt lava poured quickly from fissures rather than central vents.

Plate Tectonics

Have you ever looked at a map of the earth and thought that the continents could be fit together like a jigsaw puzzle? German scientist Alfred Wegner thought this in 1912 and from this simple observation developed the basis of the theory of plate tectonics. During the 1950's and 1960's scientists applied the theory and eventually by 1968 most of the earth's geologic activity was incorporated into the new global tectonics theory or more commonly plate tectonics.

The rigid lithosphere is suspended on a fluid layer of mostly molten rock known as the asthenosphere. On this fluid layer, the continually expanding plates drift across the earth's surface at a rate of 3 inches (75 mm) per year and over millions of years changing the face of the earth.

Ocean crust continually produced in places called mid-ocean ridges. These submarine mountain ranges of volcanic origin are formed at the plate boundary. The excess plate material instead of changing the size of the earth's surface is consumed elsewhere. The oceanic plate material submerges under the crust of the continental plate in a process known as subducion.

At the point of subduction there is a tremendous amount of friction. The two plates are pushing against each other, and the subducted plate forces its way through the upper mantel. When the subducted material reaches a point approximately 60 miles (97 km) under the earth's crust it is melted by the asthenosphere, forming magma. The magma, which is made up of less dense rocks, rises to the surface through cracks and crevasses forming reservoirs near the crust. Eventually the magma reaches a hot spot where it is pushed above the crust in the form of a volcano. The process of subduction has created volcanoes including the Galapagos Islands, the Philippine Islands and Mount St. Helen's.

Hot Spots

Visible in the form of volcanoes or hot springs hot spots are stationary place where magma burns through the earth's mantle to reach the surface. This occurs both oceanic and continental plates; hot spots are located at transition zones between the inner core and outer crust. There for some unknown reason the earth's mantle is hotter than the surrounding mantle. Hot spots activity accounts for the Hawaiian Island chain as well as the geothermal activity in Yellowstone National Park.

In the case of an oceanic hot spot the magma pushes through the crust on the ocean floor. The material is quickly cooled under the water and the m magma is cooled into a gentle slope. As additional material is expelled from the hot spot the slope builds until it eventually reaches sea level forming an island.

As the plate moves at a fixed rate of 3 inches (75 mm) per years eventually a chain of volcanoes is formed. Looking at a volcano's size and its location within a chain indicates the amount of time it was located on the hot spot and when it was created. A sudden shift in the plate will cause the current volcano to be moved off the hot spot and a new volcano to be created. When the plate moves only at its fixed rate the larger volcanoes occur. Looking at the shape of the chain from oldest to youngest illustrates the plate movement at that particular hot spot.

In the Galapagos Islands the Nazca Plate is subducted under the South American Plate in the Pacific Ocean off the coast of Ecuador. The subducted material is melted and recycled in the forming of a new volcano. The hot spot in the Galapagos, where each of the islands were formed is located under Fernandina.