Galapagos – Evolution – Travelogue 1
The active volcanoes of the Galápagos Islands are oceanic shield volcanoes that are initially built up undersea by basalt magma. Due to constant lava flows, a rather flat cone is formed layer by layer, which rises steeply only at its highest point and culminates in a so-called caldera (Spanish: cauldron), a circular crater-like depression that is created by the sudden collapse of the crater floor into an emptied magma chamber. The largest caldera can be found on Isabela, it has a diameter of 10×8 km.
Typical lava structures on Galápagos are the black Pahoehoelava (Pahoehoe = Polinese: rope), also called knitted lava, and the gray or red Aalava (A’a = Polinese: pain), also called block lava. Pahoehoelava is formed from very low-viscosity basalt lava, which cools down on the surface due to the air and causes only a very thin film to solidify. The liquid lava underneath continues to move, folding and twisting the surface skin more and more until its structure resembles vast numbers of adjacent and interwoven ropes.
Over a period of about 1 million years, small ash cones consisting of thicker lava form on the surface and in the caldera. After that, gradual erosion begins, in the course of which individual eruptions can still occur on the flanks of the shield volcanoes. The Aalava is formed by slow-flowing lava, in which a thicker layer cools down quickly, which then breaks up into sharp-edged floes. They are transported by the lava flowing below and piled up at the end of the lava flow.
The low-viscosity and slowly cooling lava often forms so-called lava tunnels, which are formed by the fact that the inner liquid lava continues to flow under the solidified surface skin, gradually dries up and leaves elongated, often kilometer-long cavities. Such a large and accessible lava tunnel can be seen on Santa Cruz. In volcanically active areas, earthquakes and tectonic movements occur again and again, caused by magma movements in the depths. They sometimes cause uplifts that raise submarine-formed lava blankets with fossil-bearing layers above sea level. This phenomenon is found on Isabela (Urbina Bay) and San Cristóbal and is responsible for the formation of some islands, such as Baltra and Española , which were not formed by volcanoes, but by submarine lavas raised by tectonic movements.
Galapagos – extinguished lava from an explosion crater
Over a period of up to 10 million years, the surface weathers more and more and gradually submerges in the water due to drifting on the Nazca Plate, which slopes more and more towards the mainland.
