Indonesia 2023. Day 5. We cross the equator!

 

A long day of hard driving, always following the strike of the West Sumatra fault, but twisting and turning a thousand times. In the distance we saw another large geothermal development along the fault, and we stretched our legs in a small hot spring that issues from the floor of a small tributary valley. Clearly the fault has created suitable conditions for lots of hydrothermal systems to form.

By now I have had an annoying head cold and sore throat for four days, and I am sick of being sick. I am at the stage, as my father used to say, that if you tell me that chest rubs with monkey shit will cure me I will start looking for a monkey. Speaking of which (the monkey, not the shit), I was reminded that I am on a remote island by a family of monkeys, peacefully munching some plants by the side of the road. This is also the place where the Sumatran rhinoceros and tigers are fighting the threat of extinction, where small Sumatran elephants are to be found, as well as boas and rather innocuous crocodiles.

We went through the fabulous gorge of the Lintas Sumatera river (near Sunpadang), where the fault makes a "bend", not unlike the bend of the San Andreas fault as it crosses the Transverse Ranges. Formally is where one right-lateral fault segment has taken a step to the left, causing a transpressional regime, the uplift of the mountains that caused the gorge, and the clockwise rotation of the intervening block. One can see quite clearly the beds on Google's digital elevation model, and making some quick and dirty calculations, and a whole lot of assumptions, it seems to me that the displacement rate of the fault could be from 1 to 0.1 mm per year.

I am curious about what caused a major right lateral fault to form above the Sumatra subduction zone. I think it is because of the change in the elongation direction of Sumatra and Java, such that Java experiences subduction perpendicular to the trench, while Sumatra experiences subduction at an oblique angle, a component of which is accommodated by the right-lateral fault. Poor guys the Sumatrans, for they get their earthquake risk from two different and powerful sources.

With a lot of fanfare we crossed the equator! This was Irvan's first time crossing it, so we were ready to throw him unto the river but had to stop because he is still learning to swim. Pity. Instead he got a celebratory T-shirt from me.

We finally made it to Bukittinggi, where we will spend the night. It is a very nice city with beautiful buildings, ample boulevards, and lots and lots of people! Today is Saturday, so everybody is out for a stroll, meeting with friends in a cafe, or just hanging out at the Central Park, where there is a big clock tower, and where children can run around, buy shiny little toys, or meet their favorite cartoon characters (once again it is confirmed that children around the world are all alike). Bukittinggi is very close to the even larger city of Padang, and because of its beauty and cooler climate the Saturday crowd included many visitors from "the city". 

And we met new friends! Just by chance we bumped into three young associates of Sauidi, and tomorrow we will travel with them to see the Little Grand Canyon of Sumatra. 

Looking to the west, southwest, and southeast I can see that we are surrounded by volcanoes like the Siamese twins of Mounts Manindjau and Pandan, with their beautiful co-joined crater lakes, Mount Singgalang, and Mount Marapi (not to be confused with Mount Merapi, in Java).