We spent the night at a resort by the lakeshore. It was a modern beautiful resort, with more walkways and rooms than Hogwarts, but every room had a beautiful view and the swimming pool and the open veranda afforded us great views of the sunset over the lake.
The following morning we crossed the bridge that joins the island to the west side of the caldera, and following the lakeshore a couple of kilometers to the north reached a hot spring area where the rocks have been hydrothermally altered. One of our youngsters kept calling it “a karst” (or maybe I misunderstood), but relict textures showed that we were dealing with a rhyolite dome. Perhaps one of the ring fracture rhyolites intruded after collapse. Clearly the place enjoyed some past glory as a spa, but I think it needs a facelift to cater to the modern tourist.
Then we started climbing out of the sheer wall of the caldera. The lower part of the wall was formed by andesites and basalts, and about half way up I clearly saw the welded base of the Toba Ignimbrite, marked by several large discharge springs that cascaded down unto the cliff. But we couldn’t stop because the road was so steep and narrow. Finally I told Michael, who was driving, that I needed to see the rocks, and with some dangerous maneuvering he stopped a few hundred meters later by the side of a rolled boulder. My bad, because I have not been able to explain to them what “outcrop” means to a geologist. The view from up high was spectacular.
Once we made it to the tableland formed by the upper surface of the Ignimbrite I started to panic. I spotted a long road cut ahead and yelled “Stop!” To Michael, who adroitly managed to half pull off the road. And there it was, in all its unwelded glory, the famous Toba Ignimbrite. I went all mushy and reverently picked small lumps of pumice, and with them in my hand I proceeded to amaze my audience with an emotional speech about the way ignimbrites form, their significance, how we recognize them, and finally—after breaking a piece of pumice in half—told them how this was a vesiculated sample of rhyolitic magma with a fair amount of crystals of biotite, sanidine, and quartz. My good friends didn’t disappoint me, reverently collected a piece of pumice, and we got under way.
For the next 30 km we saw nothing but Ignimbrite, with many sand quarries that would have made a much better place to stop, but I figured that by that time my companions had had enough volcanological rapture from me.
At the edge of the tableland we plunged into the real landscape of Sumatra, with jagged mountains and many intervening valleys. This landscape continues all the way to the western shore of Sumatra, which brings with it the challenges of moving through a mountainous region (lost of narrow twisting road ahead of us), and for the locals the challenge of moving and farming in small plots, scratching as best they can just enough corn, coffee, and vegetables to make it to the next season. It reminded me a lot of southern Mexico, where the mountains of Oaxaca and Chiapas, and the variety of climates, have been a serious deterrent to social and economic development.
Our next geologic stop, 100 kilometers from Toba, was at another hot spring. My newly trained geologists were eager to find the hydrothermal alteration, and were very disappointed when I told them that no, this was a carbonate spring, and all the white they saw around them was travertine limestone (I wouldn’t call it “karst” because I don’t want them to start a new rumor), beautifully decorated with stalactites, stalagmites, and travertine fountains. “So why is it here?”, they asked, and I turned to Suaidi for him to explain that we were on the West Sumatra right-lateral fault, which cuts the island in the same way that New Zealand is cut by its own Alpine fault. The hydrothermal activity, in this case, is caused by deep circulation of meteoric water through the fractured rocks, a phenomenon that repeats itself at the Seraille geothermal field 50 km to the south (along the fault), and a disseminated gold ore deposit another 50 km south (this time a bit off the fault).
After this little adventure it was time for our friends from the meteorological station at Silangit to depart back home. I am going to miss them greatly. Kris, Michael, Cindi, and Andre have become good friends and fabulous traveling companions, displaying great curiosity and interest in all that we saw, and stretching their command of English to make sure I felt welcomed at all times. I hope to see them again soon, either here in Indonesia or when they come to visit me in California. Good friends always meet again!
Suaidi, Irvan, and I continued south along the fault, and with the help of Google Earth located the different segments, and a great example of a pull-apart basin in the valley of Tarutung, and figured that it is a right-lateral fault. The fault is also a cultural and culinary watershed, and as night fell we started hearing the baritone voice of the bilal chanting and calling the faithful to prayer. Indonesia is largely a Muslim country, but in the highlands of North Sumatra we had found a good number of Protestants and Catholics (the missionaries probably enjoyed the more salubrious climate and the beautiful Toba Lake), but now we are in Muslim Indonesia.
I was by now pretty tired of being in the car, and was just thinking of falling asleep at our hotel in Padang Simuenpan, but could not very well say no when Sauidi suggested that we should have dinner before going to the hotel. He knew just the place. It was a large restaurant complex, with large, open-sided “booths” where you “crouch” on rattan mats around a low but very large table. Then you order enough dishes to cover said large table with plates. Seemed like a lot of food to me, but the aromas wafting from it were very enticing. Three types of fish, each braised on a different but delicious sauce with piquant spices that were quite unbeknown to me, a stew of leaves of cassava that was to die for, two or three types of salad, white steamed rice (of course), and my rather spartan beef broth. The place is famous for its fruit juices, but I am still being cautious about not drinking water of unknown origins, so I missed on that part of the treat. Otherwise, it was a heavenly feast!
Random notes on drink and food: For lunch we had horse meat in a savory sauce (plus many other dishes), and I drank a Badak cream soda (rhinoceros soda).
Crossing the West Sumatra fault we also came into durian country. This smelly fruit is much feared by foreigners, hotel managers, and taxi drivers because it stinks. I am being discouraged to try it, but remember from when I tried in Malaysia that if you can pinch your nose it can be quite tasty.
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