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Monday, August 15, 2011
Everything Begins with Volcanoes
Spatter Cones and Lava Field
All the books I've read about the Galapagos begin at the beginning. How did these islands come to be in the middle of the ocean? The answer, of course, comes from a molten hot spot in the earth's crust, and tectonic plate movement.
A mid-ocean ridge to the west moves the Nazca plate in an east-southeasterly direction towards the continent of South America. A plume of hot magma seeps up through the crust, and breaks through to create volcanoes. As the Nazca plate continues to move, new volcanic islands are created. When the islands move away from the plume, the ocean begins to erode the rock, and the island may sink entirely below sea becoming a sea mount. At least one island sank and rose again, because we saw a layer of limestone and big chunks of coral on the beach.
River of Lava
Sometimes the rocks are red and rough as a result of being blown out the mouth of the volcano. Iron and sulphur make the reddish color. The black lava flowed from the volcano like a river at a higher temperature, and silica gives it a shiny surface.
We saw examples of each at Sullivan Bay, where the last eruption was 100 years ago. We think of volcanic regions creating rich fertile soil, as in Hawaii or Italy, but this area is still very infertile. The only plants we saw were lava cactus and a small grass called carpetweed.
Fortunately, we went to the lava fields on a cloudy cool day. When the sun shines, the temperature must become unbearable. With little rainfall, only the most tenacious plants can survive, yet we were surprised to see bees and locusts.
At Sullivan Bay, the we see the pahoehoe variety, a Hawaiian term. The lava emerges at high temperatures, and as the top cools and crusts over the lava continues to flow underneath, causing the crust to wrinkle into ropey shapes. Imagine sliding into a throw rug - the top pushes into ridges. Different eruptive events leave their trace with lava rope going in different directions.
Frozen Lava Ocean
As far as the eye can see, there is nothing but an ocean of frozen lava in all directions, reflecting the sun. Occasionally, a bubble of lava would collapse leaving a hole large enough for a person to disappear in. One of our group slipped on the unsteady surface and cut her arm pretty badly.
A more explosive eruption creates spatter cones around the main vent. We climbed the 375 steps to view Pinnacle Rock - the most famous view in the Galapagos, with a chance to peek inside these cones on the way. The most recent eruption was at Sierra Negra on the island of Isabela in October 2005. It is the largest volcano in the archipelago, 11 km in diameter, erupting with fountains of lava jetting over 200 m high, and a cloud rising 12 km into the sky, producing well over a million cubic metres of lava per hour on the first day. I told Dick I'd like to see an eruption, and he cautioned me to be careful what I wished for.
Devil's Crown is a volcanic caldera which sank into the ocean, so that only the tips are left surrounding a watery center. The group went snorkeling there, but when I saw the strength of the currents, I wisely declined. I don't like to swim in water that takes me somewhere I don't intend to go. Eventually, the Nazca plate will carry the islands to a trench to be subducted beneath the South American plate. At that point, it may give rise to more volcanoes in Ecuador, as we saw later in the week. I love geology and enjoyed getting a chance to see something other than limestone here in Kentucky. I would dearly have loved to pick up a small piece of Galapagos lava to add to my rock collection, but I didn't want to get into any trouble with customs, so I behaved myself.