by Benjamin Kase 7/12/00
Sophomore Physiology and Neurobiology Major, College Park Scholars-Life Sciences
The Galápagos Islands are world renowned for the vast diversity of their flora and fauna. What most people are unaware of is the geologic significance of the archipelago. The islands are actually a series of volcanoes whose formation began around 8 million years ago. Many of the islands have since disappeared into the ocean because of plate movement and erosion. The islands remaining above surface ages range in age from 3.4 million to less than 750 thousand years old, with several still classified as active volcanoes (the last eruption occurring on Isla Fernandina in 1998). Though the archipelago was built primarily by one type of lava, there are notable morphological differences (such as contrasting topography) from island to island. On the northeastern end of the archipelago the typical island morphology is called a shield volcano. This type of volcano gets is name from its long flat appearance. On the southwestern end, the typical morphology is classified as an "inverted soup bowl." This volcano also gets its name from its appearance as a turned over bowl. Yet on each of these islands there are many other
geologic characteristics of important significance:
General Volcanic Information
Each surficial structure of the Galápagos Islands is composed of volcanic rock. Volcanic rocks were originally in a liquid state called magma when below the surface and lava when found above the surface. By definition volcanic rocks cooled above ground and contain crystals typically not visible to the naked eye. The principle rock type found on the Galápagos is called basalt..
An alkaline igneous rock that has a dark appearance and contains high levels of Iron, Magnesium, and Potassium. Despite a general similarity in chemical composition there are many other characteristics of lava flows that affect the morphology of the solidified rocks. One major characteristic is the viscosity of lava flows.
Viscosity is a general measure of a liquid's resistance to flow. For example, water has an extremely low viscosity whereas molasses has a high viscosity. Viscosity in lava is chiefly influenced by temperature, and the presence of gases. The higher the temperature of a given lava flow, the lower the viscosity. A particular lava flow may initially flow rapidly, but as temperature decreases, viscosity will increase, and alter the type of flow. The presence of dissolved gases called volatiles also plays a key role in lava flow, and type of eruption.
When these gases (carbon dioxide and water vapor for example) are found in higher concentrations, the viscosity is decreased, which increases the fluidity of the lava flow. Likewise low concentration of volatiles will decrease the fluidity of lava flow, increasing viscosity thus effecting the overall texture of the lava.
Large Volcanic Structures
The type of eruption (explosive or non-explosive) is generally determined by the viscosity, and the concentration of volatiles, within the lava. Basaltic lava flows generally have low concentrations of dissolved gases and usually form the shield volcano morphology typically seen in the eastern half of the Galápagos Islands.
- Shield Volcano
Shield volcanoes are long flat structures that almost resemble a shield laying flat on the ground. Because the lava is of low viscosity, it spreads out over long distances, and progressively builds up on itself. Because of their relatively low viscosity, these eruptions tend to be non-explosive and quiet. Yet, they should not be underestimated. Mauna Kea (in Hawaii) is a shield volcano, and when measured from the sea floor, is the largest mountain on earth. Because of their size, shield volcanoes usually make up the bulk of oceanic islands. This picture, a profile of Isla Pinzon as viewed from the highlands of Isla Santa Cruz, is a perfect example of a shield volcano.
There is also another general type of island morphology found in the Galápagos Islands. The Inverted Soup Bowl morphology is found on the "newer", or younger islands of the Galápagos.
Inverted Soup Bowl
The exact process for the formation of the Inverted Soup Bowl is still a topic of great debate. It is believed by some that this morphology comes from basalt lava flows originating from fissures and vents near the base of the volcano. These lava flows will cause an eventual buildup at the edges of the volcano, giving it the steep sided "soup bowl" appearance. The basic difference in appearance between an inverted soup bowl, and a shield volcano has to do with changes in slope. The shield volcano generally maintains the same slope from the base to the summit. However, the inverted soup bowl has a somewhat level slope at the base, and the summit, but a very steep slope in the central region.
Another commonly held belief says that the bulbous appearance is due to the influx of magma into the central chamber of the volcano. It is this influx, and the pressure the magma exerts that forces the central region out, giving it a rounded morphology. However, superimposed on each island there are a variety of smaller structures that also deserve attention.
Smaller Volcanic Structures
- Cinder Cones
The morphology that often seems to come to mind when thinking of volcanoes is called a cinder cone. Cinder cones are formed from more explosive eruptions where materials called pyroclasts are ejected into the air, cool, and consolidate to form steep mountainous structures. The loosely associated nature of the cinders generally results in a transitory life span for the cones. This picture comes from Isla Floreana, and shows a typical cinder cone.
- Spatter Cones
Spatter cones are significantly smaller than cinder cones and are composed of larger pyroclasts. Spatter cones often form on vents or fissures on the earth's surface. The process for their genesis is quite simple. Spatter cones are formed when gases escape from a lava flow and project pieces of lava into the air. These pyroclasts form the steep slopes typically associated with spatter cones. The process is similar to what happens when you stick a straw into a slurpee and blow. The upper picture comes from the island of Bartolomé, and shows several small spatter cones. The difference in size between a cinder cone and spatter cone is immediately apparent. The picture below is also from Bartolomé, and was taken from the summit looking down at two spatter cones. Both cones have eroded down to sea level. This picture demonstrates the law of cross-cutting relationships. By definition, the inner cone had to be formed after the outer cone, or it would have otherwise been engulfed by the external cone during formation.
- Welded Tuff Cones
Another type of cone formed within the Galápagos archipelago is called a welded tuff cone. Tuff is the name designated for volcanic rock that is formed by the consolidation of very small pyroclasts. Welded tuff is only formed when a particular eruption occurs at extremely high heat. The pyroclasts actually fuse together, to form large sloping peaks. One particularly good example of a welded tuff cone comes from around the island of San Cristóbal, called Leon Dormido (also known as Kicker Rock). The shoe like appearance as seen in this photograph taken from our ship at dusk, helps to explain why the formation is called "Kicker Rock". It is debated that this particular formation may have also formed underwater, in which case it would be categorized as palagonite.
-The specific name given to tuff that cooled underwater. Rapid cooling in an aqueous environment causes a change in chemical composition. Leon Dormido (or Kicker Rock) is argued to be an eroded palagonite cone. Another very good example of palagonite was found on the island of Santiago. In the picture to the right one can clearly see what looks like ripples in the stone. It is plainly evident that the cinders cooled at the water's edge, and eventually fused, preserving wave generated ripples like those of sand in the surrounding area.
Some of the most interesting structures found on the Galápagos are the lava tubes. Lava tubes are formed when the exterior layer of a lava flow cools and solidifies. The lava not exposed to the air remains hot and continues to flow. Once the flow ceases, all that remains is a solid tunnel. These structures can vary greatly in size. The picture to the right comes from the island of Santa Cruz. One can clearly see that this tube is massive in size and can accommodate many grown adults. We were told that this tube extends for more than a kilometer across the island. The picture below was taken on the island of Bartolomé. Though there is no object of reference, the opening is not large enough to accommodate a full grown adult (and none of us tried).
This smaller lava tube, one of the many that we observed on Bartolomé, is far more typical.
Post Volcanic Structures
Fissures are essentially long cracks found on the edges of volcanoes. They are often formed by earthquakes, or other volcanic activity. Fissures frequently serve as location for peripheral volcanic eruptions. The origin of the fissure seen here on the left is believed to be due to the caldera formation on the island of Genovesa. When the caldera was formed, it may have caused the ground to fracture, creating this fissure.
The word caldera comes from the Spanish word for cauldron. Calderas are large circular (or oval) shaped depressions that are greater than one mile in diameter by definition. They are formed when the summit of a volcano collapses. Usually, the magma chamber within the volcano gets somewhat drained from a large eruption, leaving little support for the summit above. In some cases the collapsed area fills with water, and resembles a large cauldron. The panoramic picture seen here is of Darwin Bay on the island of Genovesa. One can clearly see the circular nature of the cove, as it opens up to the sea on the right hand side of the picture.
- Pit Craters
Pit craters are structures that can be found in different areas of the Galápagos islands. They are formed in much the same way as calderas, however, they are not truly volcanic in origin. Calderas are formed after an eruption, yet pit craters simply collapse when the contents of the magma chamber empties out by another means. One of the most famous pit craters is found on the island of Santa Cruz, and is called Los Gemelos. Here one can see what looks like a dry caldera (only much smaller). In order to get this photograph we had to climb a steep hill and walk down an extremely narrow pathway, with a quarry on one side, and the pit crater on the other!
One type of lava flow found in the Galápagos is called A'a (a Hawaiian word). It is said that this name comes from the sound one makes when walking barefoot over the cooled, hardened lava (though I did not test this out myself). The A'a texture is sharp and angular, and is found in places where the lava flow had a very high viscosity, and moved slowly. This lava flow most likely had a low concentration of volatiles. In this picture you can see what a typical A'a flow looks like.
The second type of lava flow, more easily identified, is known as Pahoehoe (pronounced Pah-Hoy-Hoy). This texture is formed from lava flows with low viscosities that moved more rapidly. As the upper layer of lava was exposed to the air, it cooled and began to harden. The continual movement of the lava passing below caused the cooled lava to aggregate and form the ropy texture seen here.
Hopefully this page has helped to organize and clearly explain the seemingly complicated surficial geology of the Galápagos Islands. The analysis began on a massive scale but progressively decreased in size eventually leading to a hand sample scale. Though the analysis could continue to investigate on a microscopic or even molecular level, such a process was regrettably beyond the scope of our expedition.
Monroe, James S. and Reed Wicander.Physical Geology: Exploring the Earth.West Publishing Company.:St. Paul,1995.
Murck, Barbara W. and Brian J. Skinner Geology Today: Understanding Our Planet. John Wiley & Sons, Inc.: New York, 1999
Rachowiecki, Rob. Ecuador and the Galápagos Islands. Lonely Planet Publications.: Australia, 1997.
Galapagos Geology on the Web-Cornell University
Origin of the Galápagos Islands, A Photo Essay