The Barisan Mountains: Suoh


Would you call this interesting little place in Sumatra’s Barisan Mountains a volcano or hot springs?



Lake Suoh is not easy to get to. This video is an excerpt from what I think is part of the application to make Bukit Barisan Salatan National Park, which includes Lake Suoh, part of a UNESCO World Heritage site. It was accepted; we’ll look at that in more detail on Tuesday.


Don’t ponder overmuch. Even volcanologists send mixed signals about this.

The basin containing Suoh is bigger than your bathtub but tiny compared to many other geologic basins, including this one in North America. (Image:
Jay Galvin, CC BY 2.0)
  • Suoh has a page in the Smithsonian’s Global Volcanism Program, as befits the site of a VEI 4 steam-driven eruption in 1933 and a much smaller explosion in 1994.
  • Yet the first thing they point out on that page is that Suoh formed through ground movements (tectonics), not by volcanism — in other words, it’s a basin.

In such settings, geological features are typically either volcanic (like Toba, in northern Sumatra) or tectonic (like the subduction trench off western Sumatra’s coast). You seldom see the two combined.

Volcanism and tectonics do often work together, though.

One example you might remember is the magnitude 6.9 earthquake at Kilauea during that volcano’s 2018 East Rift eruption. Magma movement through the plumbing system caused tectonic activity in one of Kilauea’s rocky faults.

So, let’s take a cue from Kilauea and look for a fault near Lake Suoh.


Aha! That’s why they call the Barisan Mountains “The Backbone of Sumatra.” Suoh (the little green dot at the end of that longer green valley leading into the Sunda Strait) looks much bigger in the video. (Image: Sadelmelik via Wikimedia CC BY-SA 3.0 (see full license at link)

There is a fault. And it’s not your imagination — this 1200-mile-long tectonic structure is broken up into segments that sort of resemble vertebrae in the backbone.

Why?

Part of the reason is that this is a subduction zone, where one tectonic plate is diving into the planet underneath the leading edge of another plate and melting as it goes down.

That’s why Sumatra has infamous volcanoes and such huge, deadly earthquakes.



See how the seafloor plate and the Sunda plate are approaching each other at slightly different angles?


Since the seafloor plate is made out of a denser material than Sundaland’s continental crust, there’s no full-on collision at Sumatra, like the one where India is ramming into Asia and raising the Himalayas and other high ranges.

Like this, though he doesn’t give his sources. Note that the volcanoes are on the back-arc side of the fault. (Image: Herman Darman via Wikimedia, CC BY-SA 4.0)

Instead, the seafloor grinds down underneath the Sunda plate, melting as the subterranean temperatures and pressures increase. Some of that melt rises, and eventually volcanoes form at the surface.

On Sumatra, those volcanoes are the Barisan Mountains, the island’s “backbone.”

This happens in subduction zones all over the world. Such a process built the Cascade Range in the Pacific Northwest, for instance, and the Andes in South America.

There’s sometimes even an eerie similarity to the landscape. Those islands off the west coast of Sumatra are the geological equivalent to Oregon’s Coast Range (a wedge of material scraped off the seafloor plate as it subducts), while the narrow channel between those islands and Sumatra is a forearc basin just like the Willamette Valley where I live — the only difference is that it’s submerged.

And the Barisan Mountains are Sumatra’s volcanic arc, just as the Cascades are the Pacific Northwest’s arc.

But at Sumatra, there’s also some shearing going on. That is, the seafloor plate is also coming into the Sunda Plate at an angle.

To make things even harder for geologists, there’s also been some rotation of Sundaland going on since at least Miocene times.



“Got Dramamine?”–Earth. (The light blue section is Sundaland, including Sumatra and the rest of Indonesia).


During this intricate dance in which land and seafloor shuffle along just a few millimeters each year, the Indo-Australian and Sunda plates rub against each other for a while — the way the Pacific and North American plates do at the San Andreas Fault — before the Indo-Australian plate finally takes its dive.

A “strike-slip” fault makes this possible. Rocks on either side of the fault zone move horizontally in opposite directions. And that’s exactly what the Great Sumatran and San Andreas faults are: enormous strike-slip movement enablers.

But throw some shear and rotation into it, and the Sumatran fault kinks up or bends until it gives way like this:



Geologists call that a pull-apart basin.

These can be found along many big strike-slip faults, and Suoh — remember our volcano/tectonic mystery? — is a classic example.

The ground here has pulled apart to release some of the horizontal stress on the fault from all that shear and rotation we mentioned up above.

Since this is a subduction zone, there’s a lot of heat close to the surface here. It’s not surprising that groundwater simmers in these sunken areas, occasionally flashing into steam explosions.

There are at least 12 more pull-apart basins in the Barisan Mountain range, generally located near a cluster of volcanoes but not within the cluster.

Suoh, for instance, is near the Pesagi-Sekincau cluster in southern Sumatra, but not a part of it. (Muraoka et al., Figure 4)

Wait. There are so many volcanoes in the Barisan Mountains that they cluster together?

Yes.


Monitoring natural hazards in Indonesia is challenging. Note how few green (“normal” aviation code) volcanoes there are on Sumatra. The red volcano is Sinabung, which has been erupting for years now. (Image: MAGMA Indonesia, current as of 6:22 p.m., Pacific, June 29, 2019)


With Toba near its northern end and Krakatau to the south — not to mention the record-breaking Marapi (most active) and Kerinici (tallest) volcanoes in between, and fiery Sinabung — the Great Sumatran Fault could be called the most hardcore strike-slip fault in the world.

That is, you could call it that if these volcanoes belonged to it. But they don’t.

The line of fire mountains crosses the Sumatran Fault system now and then, but most of them sit a little east of it, on the back-arc side. They’re only there because of the subduction.

The fault, on the other hand, only exists as the result of complex tectonic plate motions.

Certainly the two interact in complex ways. But it’s not a simple relationship, and some of the best minds on the planet are hard at work, trying to figure it all out.

It’s not just Suoh. Sumatra is one of the few known places on Earth where volcanism and tectonics coexist.

Next week, we’ll take a look at Krakatau, the Sumatran volcano that most recently has raised questions about how well people and volcanoes can coexist.


Sources:

Hidayatika, A.; Utami, P.; and Suharno, R. Analysis of Thermal Manifestation in Suoh West Lampung Indonesia, in Proceedings World Geothermall Congress 2015. Retrieved from https://pangea.stanford.edu/ERE/db/WGC/papers/WGC/2015/14047.pdf

Muraoka, H.; Takahashi, M.; Sundhoro, H.; Dwipa, S.; and others. 2010. Geothermal systems constrained by the Sumatran fault and its pull-apart basins in Sumatra, western Indonesia, in Proceedings World Geothermal Congress 2010, 25-29.

Putra, A. F., and Husein, S. 2016. Pull-apart Basins of Sumatran Fault: Previous Works and Current Perspectives, in PROCEEDING, SEMINAR NASIONAL KEBUMIAN KE-9 PERAN PENELITIAN ILMU KEBUMIAN DALAM PEMBERDAYAAN MASYARAKAT 6-7 OKTOBER 2016; GRHA SABHA PRAMANA. Departemen Teknik Geologi. Retrieved from https://repository.ugm.ac.id/273463/1/5.%205DPB-01-Pull-Apart%20Basins%20Of%20Sumatran%20Fault%20Previous%20Works%20And%20Current%20Perspectives-Putra%2C%20A.F.%2C%20%26%20Husein%2C%20S.pdf

Sieh, K., and Natawidjaja, D. 2000. Neotectonics of the Sumatran fault, Indonesia. Journal of Geophysical Research: Solid Earth, 105(B12): 28295-28326.


Edited June 30, 2019.



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