I had it in mind that I wanted to visit Siberut Island, one of the Mentawai Islands off the west coast of Sumatra, because I believe these islands are the accretionary wedge of the Sumatra subduction zone. My wonderful host and guide, Sauidi, humored me and arranged for us to fly to Sipura Island, the island south of Siberut, where there is a little more of infrastructure.
We woke up at 5 am, drove to the airport, and boarded a Cessna 208B Grand Caravan, a one propeller plane that can carry eight passengers. It was thrilling for me to follow the procedures of the Captain as he got ready for departure, the takeoff, and the climb to 10,000, which is a lot higher than the 3,000 ft I normally fly at. There was an overcast on takeoff, and I thought we would break over the cloud cover, but we didn’t, so the Cap zig-zagged between clouds as much as he could, but it was inevitable that at some point we would have to go through a cloud and feel the turbulence. Then we hit rain, which on a small plain can feel quite intimidating. The Cap and his co-pilot were cool, however, taking the weather in stride, and after 40 minutes in the air we started our descent to Sipura. I could see the runway to our right, and it seemed to me that our approach was getting too low for us to make the incoming turn safely. What I didn’t see was the other runway ahead of us so I was quite surprised when we landed with a bump.
Sauidi had arranged for a van to pick us up, and on the drive to town I looked eagerly for outcrops of blue schists, pillow lavas, wacky sandstones, or even serpentinites. Nada! Nothing to be seen but bloody vegetation. We stopped to say hello to the folks of the Disaster Relief Agency, and they assured me that not a rock was to be seen in the whole island (or in Siberut Island for that matter). What a great disappointment :(
The geomorphology of the islands, as gathered from Google Maps in my cell phone, lends some credence to the idea that they are under thrusts of rocks accreted unto the edge of the Sumatran mainland, but it would have been useful if I could have found rocks typical of an accretionary wedge. Mind you, I am not the only one who thinks these islands are the accretionary wedge, and there is evidence from microtremors that the islands are undergoing internal deformation, but the visit itself was … a bust.
Lunch was good, followed by a nap and reading time, and in the afternoon we went to the beach, where Irvan took the opportunity to quiz me about sandstone and limestone classifications.
We are staying here one more day, so either I learn to relax or I will go bonkers.
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