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Galapagos Geology


Galapagos sea lion

The Galapagos Islands are volcanic in origin and appear lo have been formed about 3-5 million years ago, geologically very recent. These volcanoes formed under the sea, broke through the ocean floor, grew in size, and eventually emerged, rising from the surface of the water to become islands. Each island formed from a single volcano with the exception of Isabela, a lava welded union of six volcanoes. The eastern islands (the oldest is Española) seem to be significantly older than those on the west, with the rocks on Isabela and Fernandina islands being less than 200,000 years old. This is consistent with the "hot spot" theory of Galapagos Island formation.

In this case, the term "hot spot" doesn't refer to the popularity of the islands. Almost all the world's volcanic areas are over the boundary lines (called margins) between the relatively thin plates that make up the surface crust of the earth. Not so for the Galapagos, which instead seems to be directly over a "hot spot," a region of intense heat in the earth's mantle (the transition zone between the inner core and the outer crust), which is hot enough to burn through the crust, forming a volcano. As the plates are moving at a fixed rate (about 3 inches per year), a string of volcanoes is formed. Because of their underwater origin, the material forming the volcano builds and accumulates, spreading out like a sand hill. This gives rise to a gentle sloping shape with a central vent, which is characteristic (and the name origin) of what is called a shield volcano. Today the Galapagos are among the world's most active volcanic areas; there have been over fifty eruptions in the last 200 years, some quite recently. These events have been on the two westernmost islands, Isabela and Fernandina; their six active volcanoes are still being fed by the "hot spot" (only the Ecuador Volcano on Isabela Island is no longer active). That these are active volcanoes is evidenced by the columns of steam and gas [fumaroles] which the western Island visitor can see rising from the Alcedo and Sierra Negra Volcanoes on Isabela.

Part of what makes visiting the islands so fascinating is that they are in the early phase of their development. Aside from the eruptions, the recent activity list is long and continues to grow. In 1954, almost 4 miles of the coastal seabed of Urbina Bay, Isabela Island were dramatically and suddenly uplifted about 15 ft. These uplifts, as they are called, occur frequently, but not as dramatically. Many of the islands themselves are uplifts, formed by the flow of molten rock inside the earth (magma) through a subsurface geological fissure (or fault). The magma exits the fissure as lava, hardening and gradually lifting the "land" mass through and past the ocean surface. Uplifts are typically associated with a previous or impending eruption. As the magma beneath the summit of a volcano cools and contracts, the entire peak may collapse inward to form a large, bowl-like depression called a caldera. In 1968, the Fernandina Island caldera collapsed, dropping about 1000 ft in 2 weeks, an event accompanied by hundreds of earthquakes.

The volcanic features are fascinating. On James (Santiago) Island, two distinct lava patterns can be seen - the smooth, rope-like. Also, across from James on Bartolome Island, is probably (he most famous (and most photographed) landmark in the Galapagos - Pinnacle Rock, a remnant of what is known as a tuff cone. Tuff cones are interesting vertical rock formations of" volcanic origin which are a consolidation of hardened ash particles (tuff). During an eruption, as the hot lava poured into the cold water, explosions would occur. Pockets and fragments would be sent flying in all directions and come spattering down to the ground, hardening into a cone shape. Not surprisingly, some of these formations are termed cinder cones and spatter cones. Often, as the lava flowed, the exterior portion cooled and hardened even as the molten material continued to flow in the interior. When the lava flow abated, there was not enough liquid to fill the inner cavity, and a tunnel was formed (called a lava tube or lava tunnel). Several excellent examples exist in the Highlands of Santa Cruz Island, where a "climb down and walk through" tour is available; miner's lanterns are provided for these.

When a lava flow subsides, the surface features that are directly over pockets of magma are somewhat unstable, causing depressions called pit craters; these are formed in the same fashion as a caldera, but are not located over a central vent.

Most visitors to the Santa Cruz Highlands see the pair of giant pit craters called Los Gemelos (The Twins). A good place to see the Galapagos hawk and barn owl.

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