Coastal Geology of the Galápagos Islands

Eugenia Leone

July 5, 2004

There is so much to coastal geology on the Galápagos Islands, that the easiest way to start is by going chronologically. While flying from San Cristóbal (the island furthest East) to Baltra (one of the central islands) I noticed that most the islands looked like they were outlined in black. This makes sense since the islands are volcanic; the black outline is the basalt sticking out. There were few places were the black would be replaced by beach or forests (which I later found out were Mangroves). The day we landed on the islands, we visited this beach on Santa Cruz called Bachas. The sand was white, but unlike the beached here, it wasn't made of quartz. It was made of carbonate. The beach also featured a few lava outcroppings (made of basalt) that extended into the ocean and provided platforms for Sally lightfoot crabs to relax on.

The next morning, I had woken up before the guide, Luis, knocked on the door. I looked out the small window, and through the curtain I gazed at a spectacular sight: Pinnacle Rock. We had reached Bartolomé, a small island off the shore of Santiago or James Island. I got dressed and grabbed my camera. It was a little chillier than I had expected, but the sun was under the clouds, so the light made the island glow orange. It was stunning. I took a few pictures, and then Dr. Merck came over to test my geology knowledge. In short, he asked me what the sediment on the island was. After I responded tuff, he pointed out the angle of repose of one of the hills, and all my knowledge (all 1 semesters of geology) clicked into place. It was a good feeling. After breakfast, we took a dingy around Pinnacle Rock, where we saw sea lions and penguins perched on heaps of dried basalt. We then landed on an outcropping of lava that lead to the path and a set of wooden stairs. Shortly after landing, Luis told us that Pinnacle Rock was man made. The US army in the 1940's had tested missiles by blasting away rock, and left the pinnacle the way we now witnessed it. I was slightly upset by that news, since I had been so excited to see it in the morning. Other things I noticed were that besides crabs, birds and iguanas use the coastal lava as a perch. Cliffs on the coast are also used as perches for birds. The coast on Bartolomé also featured two submerged crater-looking things. They looked as if they were smaller volcanic peaks, that eroded, and left circular depressions under water, but I don't think that's what they are.

In the afternoon we visited Santiago (James) Island. Specifically, we saw a 100-year-old lava flow (this is basically brand new in geologic time). The coast here was all basalt, but because of the tides, water gets trapped in depressions in the lava leaving tide pools. If you are ever in need of pahoehoe, all you need to do is go there. Interesting side fact, Santiago and Bartolomé are so close to each other, that maybe as soon as the next eruption, they will be the same island.

On the 19th (day 3) we visited part of Isabela Island. There were black and brown sand beaches, and basalt cliffs that were pounded on by rough surf. We visited a mangrove forest, where I noticed that the mangroves really only use the underlying basalt as a means of support (perhaps to get some minerals as well). We also saw 2 volcanic formations sticking out of the water. They were basically large rocks that were home to crabs, marine iguanas, Blue-footed boobies, flightless Cormorants, Pelicans, Penguins, and other birds. In the afternoon we went to a black sand beach that we snorkeled from. I was a bit more preoccupied with putting my fins on and getting into the water then with the sediment of the beach, but the fact is that the beach was black and that means that the sediment came from ground up basalt.

The next day we were on Fernandina, at Punta Espinosa. People had built a jetty on the coast, but three weeks after they finished, there was an uplift of one meter and the jetty was left high and dry. The rock that this uplift exposed became one of the more important nesting grounds for marine iguanas. Flightless Cormorants also used these rocks for their nests. The intertidal pools provide food for finches, young iguanas, crabs, and lava lizards. There are also large cracks made as the lava cools and condenses, that sea lions use to retain heat. The black basalt absorbs lots of heat, so the marine iguanas bask on it. That afternoon we went to Punta Vicente Roca, which is a bit further North on Isabela. We only went snorkeling, but from the boat we got to see sheer cliffs with dikes, sill, eroded tuff, and caves produced by the pounding surf. You could tell that the top of the cliffs was being used for birds nesting, since the edges were white. The water here was cold, and murky, but it was fun to snorkel anyway.

On the 21st, we went to Punto Egas on Santiago Island. There was so much here to comment on. We landed on a black sand beach that featured walls of layered tuff. The walls had names carved into them, which I hoped were not recent. One of the walls led into the ocean, where it had been partially eroded, since it had become an arch. Just like many other islands, basalt rocks stuck out into the ocean. On the other side of the island, the geology got very interesting. There was a spot where lava and tuff intertwined, showing that the eruption and the spewing of ash happened at the same time. There were also rounded basalt rocks lodged into a more recent lava flow. This meant that after the lava rocks were rolled around in the surf (making them rounded), another eruption occurred that flowed into the ocean. The older, rounded rocks rolled onto it, and got stuck. Erosion exposed it again. I tried to take in as much as I could, while we walked on the beach. Drs. Merck and Holtz, along with Luis, kept discussing these pothole looking structures in the tuff. I had seen what they were discussing earlier in the walk, but with my limited knowledge I thought they were potholes. When they finally decided what the structures were, they quickly informed me. They had evidence, such as tube worms (only form in water) and ripple marks, that the ash had originally landed in water. When this happened, air pockets in the ash probably tried to escape as the ash solidified into tuff, leaving circular depressions in the rock as the bubbles left. What finally sold them on this idea was the lip around the depressions. In potholes, there is no lip. There's just a depression made by the grounding of other materials against the rock in a circular shape, but here, it looked like something had caused the surrounding rock move out of the way. It was very cool to look at. On this beach there were also small lave bridges, where the ocean had eroded away under it, and intertidal pools. That afternoon we went to a red sand beach that featured small cobbles of rounded basalt, and sand that was extremely fine.

On the next day, we visited the highlands and the Darwin Research Center, so I had the day off. It was nice.

On the 23rd, we visited Española. This island featured large rounded boulders as the coast and some of the inland sediment. Nazca and Blue-Footed boobies, as well as Albatrosses and other birds used these boulders as nesting grounds, and take off points. This island also had a blowhole, which is a lava tube that has 2 ends exposed to water and air, so every time the tide rushes in, the water gets pushed through the tube and shot out the end exposed to the air, making a pseudo geyser. It was fun to watch. In the afternoon we went to Gardner Bay. We landed on a white sand beach, which was composed of calcium carbonate. There was also a piece of pumice on the beach that Luis said had come from Alcedo, a volcano on Isabela.

The last day of the trip we visited North Seymour, a very small island in between all the central isles. It hosted a nesting colony of frigate birds, which I could not photograph since I had been too lazy to bring my camera along. It turns out that I'll still get the pictures, since the whole group is sharing all the pictures with each other. In terms of geology, this island had pillow basalt and sandstone. A bit off the coast there was another island. From North Seymour you could see where the old seabed used to lie, but had been pushed out of the ocean by, as Luis put it, "geologic action." It was a cool sight, and even though it was 6:30 in the morning, the geologist in me still appreciated it. It was very sad to think that these were the last things we were going to see before our return to the mainland. None of us were very upbeat. We each said goodbye to the sea lion on the dock, and then we saw a scorpion. It turns out our luck had not yet ended. When we had sat on the plane, one of us asked, "How many people miss Luis?" Everyone raised their hands. We all agreed that this had been an awesome trip, and we thanked the teachers for it. A few days later, when we arrived at National, we all said our goodbyes, and agreed to share our pictures and get back together. And with that, we returned to our normal lives in the states.