Galapagos Island Geography and Geology - Part Two - Galapagos Volcanoes

Map Showing Galapagos Islands' Volcanoes
from ecuador.us

Shield Volcano, Rugged Lava Rock, Lava Cactus
Photograph taken by Aqua Surround

This is the second of three posts about the geography and geology of the Galapagos Islands. In Part One I talked about the tectonic plates that lie beneath the sea and under the land and explained that without the constant movement of the tectonic plates, their crashing into one another, forcing the hot magma to break through the lithosphere, there would be no Galapagos Islands at all. Here, in Part Two, I discuss the actual formation of the islands from volcanic action that occurs when tectonic plates collide. Each and every one of the Galapagos Islands, their topography, the sheer cliffs that you see and that have become the nesting sites for bird and animal life found no where else on earth, the beautiful outcroppings, marine life, SCUBA sites, black sand beaches, crystalline beaches - all result from volcanic activity. This post should leave you with a clearer understanding of the earth beneath your feet as you stand and examine and feel your environment in the Galapagos Islands. Finally, in Part Three, I will talk about the Pacific Ocean currents, particularly how the hot and cold currents impact and define the plant, animal, bird and marine life throughout the Galapagos.

These remote islands, unlike anything else on our planet, have a dramatic and ever-changing morphography. To give you some perspective on just how dazzling and singular these islands are, I want to share with you NASA's breath taking satellite video. Starting in deepest space, it gradually brings the Galapagos Islands into focus. You will see the entire island system, then a "fly by" of each individual island and its volcanoes before the satellite takes you back into outer space. What an amazing perspective this is!

As I did in Part One, I want to remind you of why I write these posts - what drives me and compels me. It's a very personal journey that I take with you in these pages. These enchanted islands have been my home and that of my family for three generations. They are, to me, a constant source of joy, mystery and delight. From childhood, I recognized that I live in a wondrous and unique place. It is a place I love to share with my guests and, through this blog, have a chance to share with you.

In the Galapagos Islands, each and every land mass that you see - an island, a rock outcrop, a sheer cliff arising from the water as if it were a searing monolith, came from a fire - from a volcano. But, perhaps, these volcanoes are different from what typically comes to mind.

Mt. St. Helens
Photograph from Wikipedia

What do you think of when you think of a volcano?  Perhaps, like many people, you think of Mount St. Helens, Mount Etna or Mount Vesuvius spewing forth deadly molten lava high into the sky, which then flows like fire down the sides of the mountain devastating and destroying everything in its wake. These are all stratovolanos volcanoes and characterized by a steep mountain-like profile and periodic explosive eruptions. They are nothing like the volcanoes of the Galapagos Islands.

Isabela Island with Sierra Negra Volcano in Background
Photograph from Galapagos National Park

The activity here, though equally dynamic, generally is not so dramatic. The formation of the volcanic Galapagos Islands requires study of other types of volcanic action, action that is less obvious and outwardly vivid because it does not take place on the top of a mountain, but underneath the ocean surface and deep beneath the Earth's crust. As I described fully in Part One, the Galapagos Islands were formed as a result of the phenomenal action of the moving tectonic plates over the hot spots. Everywhere you look, every sight you see, every rock, cove, island, and natural formation has been created by underwater volcanic activity and the sea’s constant movement and impact on the resultant geological formations. The result is a harsh rock-based morphography that is hostile to many living things, both plant and animal.

Tectonic Plates Map
from pubs.usgs.gov/publications/text/slabs.html

To summarize, the ocean floor is rock and a mantle plume is a column of hot rock (called a hot spot), deep within the earth. The heated rock is lighter and less dense than the rock surrounding it, so it pushes through the crust and displaces it. This happens at a rate of about 5 cm a year. As the mantle plume rises, one small eruption and accumulation after another, it forces its way to the surface. Successive eruptions resulting in a build up of material over hundreds of thousands of years produce a volcanic island. Visualize, at first, a small mound of lava hardening under the sea. Then see that as the lava builds on top and accumulates, ever so gradually, the mound becomes bigger and taller until it can be seen above the ocean surface. That is the moment when a new island is created. The hot spots stay in one place, but the tectonic plates keep moving. As the mound is carried by the tectonic plate away from the hot spot, volcanic activity stops. The volcanic land mass has an opportunity to cool and contract. Then the slow process of erosion by land and sea occurs, and once again, the mound – ever so gradually – returns to the sea. The island becomes extinct. However, a new island begins to form in the location of the stationary hot spot.

Espanola Island
Land of Blue Boobies and Albatross

In this manner a chain of volcanic islands is formed. In the case of the Galapagos Islands, the oldest visible island is Espanola. As the oldest and most southeastern island, it was formed 3-5 million years ago and its volcano has been extinct for several million years. Like Santa Fe Island to its north and west, Espanola is a remnant of an extinct volcano. Much of these two islands have already eroded. For more information on Espanola Island see my posts about Punta Suarez and Gardner Bay.

Pinzon Island
Photograph by Galapagos National Park Service

Likewise, Pinzon and Rabida - small locales in the middle of the archipelago - have been extinct for about one million years. But, scientists have discovered an 8 million year old area in the eastern most part of the Galapagos that was once an island but is now 1500 meters below sea level as a result of erosion.

In contrast, Fernandina, the youngest and most western island, is just about 1 million years old.

Santa Cruz Island
from Volcano.si.edu

The “middle aged” volcanic islands of Santa Cruz and San Cristobal are still considered by scientists to be active, but barely, and they have evidenced no eruptions in many years. In this photograph, the broad shield volcano forming Santa Cruz Island is seen from its northern coast. The oval-shaped, 32 x 40 km wide island is capped by cinder cones with well-preserved craters that largely bury a shallow summit caldera.

Cerro Azul on Isabela Erupting
from AFP.google.com

Interestingly, the Galapagos Islands are one of the Earth’s most active volcanic areas and more than 200 eruptions have occurred in the last 200 years. Six volcanoes remain active on the western islands - 1 on Fernandina and 5 on Isabela. The most recent eruptions occurred at Cerro Azul on Isabela in 2008 and Volcan Cumbre on Fernandina in 2009. Historic eruptions have occurred on many of the Galapagos volcanoes, including Fernandina, Volcan Wolf, Alcedo, Sierra Negra, Cerro Azul, Santiago, Pinta, Floreana, and Marchena. Eruptions in the recent geologic past (the last 10,000 years or so) have also occurred on Volcan Darwin, Volcan Ecuador, Genovesa, San Cristobal, and Santa Cruz. A number of submarine volcanoes many have also been active in this time.




Types of Galapagos Island Volcanoes

There are three basic forms of volcanoes: stratovolano, cinder cone and shield. The Galapagos Islands are primarily shield volcanoes that have taken two distinct shapes. First are those islands that have the typical shield volcano geological formation. The others are large volcanoes that started as shield volcanoes but have, over time, taken on the appearance of inverted soup bowls with deep calderas.

 

Shield Volcanoes

La Cumbra on Fernandina Erupts in 2009
Photograph from National Park Service

Most of the Galapagos Island volcanoes are known as shield volcanoes. Lava deposits over time create a volcano, as described above. In the case of a shield volcano, instead of erupting from the central vent and shooting high into the atmosphere as a stratovolanos volcano (like Mt. Etna), the magma pours out in all directions and the flow is adequately hot and fluid that it flows out and downward to form a gentle sloping cone of flat domelike shape. There is a steady accumulation of broad sheets of lava. The name “shield volcano” is derived from the image of a warrior's shield.

Volcan Wolf  Shield Volcano
Photograph from Creative Commons by Acme

Isabela Island, which is the largest of the Galapagos Islands, is actually the result of six shield volcanoes fusing together under the ocean surface. Isabela’s six shield volcanoes have resulted in the island’s distinctive sea horse shape.  The volcanoes that form Isabela are Alcedo, Cerro Azul, Darwin, Ecuador, Sierra Negra and Wolf. With the exception of Ecuador, all are still active. Fumes continue to rise from Sierra Negra and Cerro Azul.

San Cristobal - a fusion of about four volcanos
Photograph by Ed Vicenzi of Smithsonian Institution

The islands of San Cristobal, Sante Fe, Santa Cruz and Santiago all are identifiable as shield volcanoes. San Cristobal is a magnificent and varied destination because its topography resulted from the fusion of at least four separate volcanoes. As a result it is one of the most diverse and interesting islands with many very special and dramatic visitor sites both on the island itself and around the island.  I have written several posts about San Cristobal: Kicker Rock, Visitor Sites on San Cristobal Island itself, and Visitor Sites nearby the island of San Cristobal.

Large Volcanoes with Deep Calderas

Volcan Darwin Shield Volcano on Isabela
from Volcano.si.edu

Some of the Galapagos Island volcanoes have the appearance of inverted soup bowls, with large indentations in the middle. These indentations, known as calderas, are usually formed by the collapse of land after the volcano erupts. When the magma pours out of the earth from beneath the crust, an empty area or pocket is created where the magma was once beneath the surface. Naturally, surface rock is heavier than the pocket so the crust collapses into it in a ring shape, causing the geological formation known as a caldera. First the center collapses, then the sides fall in around it, resulting in the inverted soup bowl morphology.

Fernandina Eruption Satellite Photograph
from Earth Observatory NASA 2009

Though not the only, Fernandina Island may be the most dramatic, example of this volcanic formation within the archipelago. Its caldera is relatively large in comparison to the size of the volcano itself.  Fernandina’s caldera is over 1100 feet deep and in the past sometimes contained a small lake. Fernandina has erupted 24 times since 1813. In 1968, after a brief eruption, a large part of Fernandina’s northern caldera wall collapsed.   Fernandina erupted again in 1988, collapsing part of the southeast caldera and depositing a 250 meter thick pile of debris at its base and into the lake, which then disappeared. Fernandina also erupted in 1991, 1995, 2005 and 2009. For more information about Fernandina, visit my post about the island and its remarkable visitor sites.

Google Earth Satellite Picture of Marchena's Caldera

Marchena and Genovesa are also examples of caldera volcanoes. Marchena's caldera is one of the largest in the Islands, but it is nearly filled with lava. In contrast to other Galápagos volcanoes, the 6 x 7 km caldera and its outer flanks have been largely buried by a cluster of pyroclastic cones and associated lava flows. Its last recorded eruption was in 1991. Other young lava flows, some of which may be no more than a several hundred years old, filled the caldera and flowed down its outer forested flanks, in some cases to the sea.

Genovesa Island
Photograph from Galapagos National Park Service

The shield volcano known as Genovesa, a small exquisite treasure of an island, is just 64 meters above sea level at its highest summit. Moreover, its caldera is now below sea level. A break in the caldera on the south side forms Darwin Bay. Another lake, formed by a crater at the center of Genovesa, is less than 6000 years old. For more about Genovesa Island, go to my post about Prince Philip Steps and Darwin Bay.

Volcanic Lava Features

Lava Fields and Flows

Volcan Chico Lava Flow Pattern
Photograph from Creative Commons by Arkintina

When a volcano erupts, a liquid stream of molten rock (called lava) pours to the Earth’s surface from an erupting vent. The igneous rock that remains when the molten material cools and becomes solid is also called lava. A lava flow is the actual movement of the lava. The flow of lava that erupts from every volcano forms the island’s surface. In fluid form the flowing lava can extend many kilometers from the point of the eruption itself. The lava is usually slow moving - about 1/2 mile/hour - but it can move up to six miles an hour. How rapidly the flow occurs depends, in part, on the steepness of the slope of the volcano.

Dramatic Flow of  Lava in Hawaii
Photograph from Wikipedia

Throughout the Galapagos Islands, we see primarily (but as I explain below, not exclusively) two types of lava formation: Pahoehoe and Aa. The words have a Hawaiian derivation. Pahoehoe refers to smooth, unbroken ropy lava and Aa means hurt. When you say the words out loud, they actually evoke the characteristics that they imply.

Pahoeho Lava Formation
Photograph from Wiki

Pahoehoe lava is gorgeous. It can look like a weaving, a rope, a piece of surreal art, or just about anything your imagination can conjure. It has a smooth ropy and undulating surface and is formed from slower moving lava. As the lava proceeds slowly snaking its way down the volcano walls from the eruption site, its outer layer cools and hardens in the air, forming an outer crust, while the internal lava stays hot and continues to flow inside the core. The hot lava carries and stretches the cooled outer layer with it. The outer layer starts to assume a ropy appearance and begins twisting in on itself. The hot lava inside continues to flow, sometimes leaving a hollow inside the rope. At the ends of the lava flow, in all directions, small areas break out, then other areas break out beyond that point. Pahoehoe lavas typically have a temperature of 1100 to 1200 °C. The result is nature’s modern art! The forms take many shapes and are very sculptural in nature.

Photographs of Aa Lava
Both from Creative Commons by HBarrison

Aa (“ah ah”) is sharp and rubble-like. When I guide a hike on Aa lava, I generally suggest that you use a walking stick for balance because it can be dangerous. Aa lava is difficult to walk on and would be painful to walk on with bare feet. It forms from more viscous lava, usually flowing more rapidly over a steeper landscape.

Like pahoehoe lava, the outside lava of Aa lava cools more slowly than that inside. But Aa lava is more explosive in its nature as little gas explosions bubble up from the inside and erupt through the crust, instead of flowing through it. The small explosions then cool into a dense and sharp outer core. The clinker-like surface actually covers a massive dense core at the most active part of the volcano’s flow. New lava carries cooled, edgy and rough fragments tumbling down and forming the sharp irregular topography.

Birds Perched on Aa Lava on Isabela Punta Albemarle
Photograph from Creative Commons by Philip Marsh

Pillow Lava

Pillow Lava at Isabela's Volcan Alcedo
Creative Commons Photo 

Less common in the Galapagos Islands is the phenomenon called pillow lava. These formations contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Upon hitting the water, the outside cools very quickly compared to the inner core. The inside continues the build up of hot lava which then bubbles up again, blows another hole in the core and extrudes another pillow-like shape. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter. Pillow lava can be found at Punta Moreno and Volcan Alcedo on Isabela and on North Seymour Island.

Volcanic Plateaus

Baltra Island Plateau
US Navy Image from
Terraneanalysis.com

Another volcanic geological formation that is present throughout the Galapagos Islands is known as a volcanic plateau. You will see plateau formations on South Plaza, Baltra and North Seymour Islands. These formations differ from the shield formulation because the lava pours from fissures throughout the Earth’s surface rather than from a central vent. As there were many points from which the lava flowed, there was no central build up point, rather many smaller ones, thereby resulting in a flattened, rather than domed, appearance. This aerial photograph from the US Navy graphically demonstrates the flatness of a plateau as compared to a shield volcano.

Tuff Cones

Pinnacle Rock with Shield Volcanoes in the Background
Photograph by Aqua Surround

Occasionally, a dramatic rock formation will be created from a volcano’s hardened ash. This occurs if extremely high temperature lava spurts into the ocean and explodes. The particles may splatter down into a cone shape. The best known and most widely recognized example of the tuff cone phenomenon is Pinnacle Rock on Bartolome Island. For more information about Bartolome and all it has to offer, and particularly about Pinnacle Rock, see my blog post on the island's visitor sites.

Summary and Conclusion

Devil's Crown
Photograph from Haugan Cruise Lines

Like so much else one encounters in the Galapagos Islands, its volcanoes are distinctive and rare. They enrich us, teach us and inspire us. Each and every volcano is different. Each island has its own texture and history. Each rocky inlet, each tuff cone, each plateau and each lava formation is an environment and story unto itself.

Darwin's Arch from Wikipedia

The best way to experience the Galapagos Islands is to come here - feel it, smell it, see it, swim with the sea lions, observe the prehistoric looking land iguanas and the unique marine iguanas. See how the wild life reacts to you without fear or intimidation. Just allow yourself moments to settle comfortably into your surroundings and experience the new world around you. But, until you can do that, try a video to at least get the sense of what this world offers that is unlike anything else on earth. A series of videos by the BBC, called Born of Fire, is just one of many videos that will give you a sensation of stepping onto this enchanted ground I call Paradise and my home. Enjoy!