The Galapagos Islands
Geology
"Geologising in a volcanic country is most delightful; besides the interest attached to itself, it leads you into most beautiful and retired spots… Some of the craters, surmounting the larger islands, are of immense size, and they rise to a height of between three and four thousand feet. Their flanks are studded by innumerable smaller orifices. I scarcely hesitate to affirm, that there must be in the whole archipelago at least two thousand craters. These consist either of lava and scoriae, or of newly - stratified, sandstone - like tuff. Most of the latter are beautifully symmetrical; they owe their origin to eruptions of volcanic mud without any lava".
Charles Darwin,
Voyage of the Beagle, 1835.
The exact age of the first Galapagos island is still a topic for debate. Geologists refer to the Carnegie Ridge, which is composed of seamounts, (thousands of feet below the surface of the Pacific ocean), now to the northeast of the present islands, that may have emerged some 10 – 12 million years ago, and some would say as far back as 15 million years ago.
What is more certain though is that the present islands have been in existence for about 3 – 5 million years. In geological terms, this makes them extremely young! The islands, 16 major and 43 smaller named islets or rocks, are all volcanic in origin, formed directly as a result of volcanic eruptions or in the case of many smaller, apparently flat and low lying islands, the result of blocks of volcanic rock being uplifted from below the surface of the ocean. Only in a few rare sites can sedimentary layers be observed, for example North Seymour and Baltra Islands, and these are composed of recently uplifted beach sediments that are sandwiched between layers of basaltic lava.
The type of activity that has created the Galapagos is known as a "hotspot" or mantle plume. This refers to an area beneath the earth’s crust where intense heat rising up through the upper mantle breaks through the oceanic crust thus creating a submarine volcano. Over the millennia, the volcano continues to erupt and grow until it reaches the surface of the ocean and an island is born.
The earth’s surface is comprised of a jigsaw of plates that move relative to each other, some parting, and some colliding. The oceanic plate known as the Nazca Plate, below the islands, is moving slowly eastwards but the hotspot in the mantle remains constant, so one volcano emerges behind the other and separate islands appear. If the eruptions continue for long enough periods above the surface of the ocean two or more volcanoes may join together to form larger islands. This is the case of Isabela Island, which is comprised of no fewer than six volcanoes, five of which are still considered to be active.
Due to the presence of faults or fissures in the crust below the islands, their movement is not uniformly to the east, but may be north – south as well. For this reason the Galapagos do not form a chain of islands in a straight line as do the Hawaiian Islands, another example of hotspot activity, but are juxtaposed. The reason for this erratic movement is due to the proximity of the Cocos plate, adjacent to, and laying north of the Nazca plate. Although moving eastwards the Cocos plate is also moving away from the Nazca plate at an angle of approximately 10 degrees. This results in a tearing or fracturing of the sea floor, which is then refilled by molten lava from below, a process known as ocean or seafloor spreading. Logically therefore, the older islands are to the east of the archipelago whilst the youngest are to the west. This is the case of Fernandina Island which is thought to be only 500 000 years old, approximately.
The present day location of the hotspot is just to the southwest of Fernandina, but the volcano is still one of the world’s most active. Such volcanoes are termed "shield volcanoes" due to their low lying, broad based, almost "Chinese hat – like" appearance. This is a consequence of the origin and mineralogical content of the magma from the hotspot. As the material is derived from upper mantle and re - melted oceanic crust, it has paucity in silica, SiO2. The more silica in the melt, the more viscous and slow – moving the lava. Being low in silica, the lava in Galapagos flows rapidly away from the point of emission, giving rise to the shield volcanoes.
Volcanic activity in the islands is mostly restricted to Isabela and Fernandina, but minor eruptions have also taken place on Marchena, (1991, personal observation), and during the mid to late 1880’s, (recorded by Capt. Sullivan), at Sullivan Bay on the island of Santiago.
Underwater fumaroles, (gaseous release), are common at Roca Redonda and more recently have been reported at Wolf Island, (personal observation).
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