The Taiwan Badlands

This weekend I had the opportunity to spend some time in the Taiwan Moonscape, the area of badlands in southern Taiwan. I had never seen the area up close and personal, merely ridden through it on a bike. One thing I hadn't understood was how big they were. The map below should give some idea of the extent of this 100,000 hectare formation which crops out as badlands in the area around Tianliao, with a thickness variously given as 3-4 kms (source), spreading across southern Taiwan from Chiayi to Kaohsiung. Mudstone formations are also found in Kenting near Hengchun and in Taitung. Walking around in the area naturally caused me to wonder: what was its origin?

Extent of the badlands in southern Taiwan, the Gutingkeng Formation (source).

Taiwan's badlands are unique, being the only badland formation on earth in a tropical area with good rainfall. The others in Italy, the US, and elsewhere, are all in arid or semi-arid regions. Badlands typically consist of weak sedimentary rock and fall into two basic types: calanchi, with pinnacles or sharp peaked ridges, as in Taiwan, and biancane, which are more rounded.

This paper on the history of the rivers in the area observes that the badlands are undergoing uplift. As the Philippine Plate shoves its way northwest toward China, it pushes the crust up, causing the area to tilt toward the ocean and lifting it up. The effect of this uplift can be seen in this picture below, which shows how the rock layers lie on their sides perpendicular to the ground, instead of parallel to it:

The mudstone exposed in the foothills area of southwestern Taiwan is a sedimentary rock formed several million years ago (source). It is usually referred to as Pleistocene in origin. It is cut through with layers of limestone in places, which are quarried by Taiwan's ubiquitous gravel industry. The mudstone area is also the location of mud volcanoes, which track one of the region's major NE to SW running faults. At the famous hot springs site of Guanziling the hot springs crop out through this deposit of mudstone.

The mudstone layer itself, as far as I was able to ascertain, is a deposit of marine sediments that formed during the late Cenozoic period (Pliocene/Pleistocene) as a giant sedimentary basin was gradually filled in with silt and clay and then uplifted to its current position by the action of the Philippine Plate.

One interesting idiosyncrasy of these badlands is that the bare slopes are mostly south facing. The reason for this is solar radiation (source), not monsoon rains or typhoons as one might think. During the long dry season, the sun heats the slopes, making them inhospitable to vegetation, and causing them to crack, speeding weathering. The result is the south-facing bare slopes so typical of the badlands area. According to this article, because of the massive precipitation and sedimentation, the Erhjeng River, which drains the area of mudstone, yields 1.3 x 10⁷ tons of sediment annually, the highest rate of sediment discharge in Taiwan.
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Siaolin Village Geological Probe Report

Typhoon Morakot buried Siaolin Village, killing hundreds. Many victims claimed that tunneling for a nearby diversion project was a cause of the disaster. This week the geological assessment was released....

“More than 30 million tonnes of mud rushed down from nearby Mount Siandu [獻肚山] at a speed of 180kph when the mountain collapsed,” Chen Chien-chih (陳建志), a professor at National Central University’s Graduate Institute of Geophysics, told a press conference yesterday. “The massive mudslide was divided into two ... only 30 seconds after it began — the larger mudslide continued on until it hit the Cishan River [旗山溪] valley, wiping out the northern part of Siaolin. This explains why some survivors said they heard two large bangs that morning.”

He said it was only 110 seconds from the beginning of the mudslide until the northern part of the village was wiped out.

The smaller mudslide went down the valley of a smaller, unnamed creek and formed a barrier lake, Chen said.

“As the mud, sand and stones built up, the barrier lake eventually collapsed 30 to 50 minutes later, burying the southern part of the village,” he said. “Most of the survivors came from the south of the village — they made it out in time, before the second mudslide buried the southern part of the village.”

National Taiwan University geosciences professor Hongey Chen (陳宏宇) said that long-term unstable geological composition was one of the causes of the tragedy.

“Field research found the geological composition of Mount Siandu to be quite unstable as it’s located on the border between shale and sandstone composition, and many geological layers there are actually leftover debris from ancient mountain collapses that occurred thousands of years ago,” Hongey Chen said. “If nothing happens, nothing happens — but if exceptionally heavy rainfall occurs, unstable layers can be washed down at any time.”

He said that as many as 92 other communities in Nantou, Chiayi, Tainan, Kaohsiung, Pingtung and Taitung counties are also under threat from unstable geological compositions.

I posted on Siaolin last year, translating a presentation on the diversion project many villagers claim to be the cause of the disaster. The Public Construction Commission reported earlier this year, saying that rainfall, not construction, caused the disaster. To wit:

Lee said the report’s conclusion was also based on the fact that the amount of explosives used to blast the tunnel was insufficient to have caused tremors large enough to lead to disaster.

The largest amount of explosives used at any one time was 182kg, less than the tremor caused by the 2006 Hengchun Earthquake, the equivalent of 475 million kilograms of explosives.

Commonwealth Magazine had an in-depth piece on the region's complex geology shortly after the disaster....

When there is too much precipitation, the environment's capacity to absorb it becomes overwhelmed.

"That's especially true in the Gaoping River's midstream and downstream areas, where the geological structure is a mix of sandstone, mudstone and shale. The terrain is not only easily loosened and vulnerable to collapse, it features dip slopes and multiple faults and is traversed by a fracture zone, conditions conducive to the entire side of a mountain sliding down," Chen explains. The result was widespread flooding, from the Cishan River Valley, encompassing Namasia Township and Siaolin Village, where hundreds were buried alive during Typhoon Morakot, to the Laonong River Valley, including Liouguei Township and the hot spring resort town of Baolai.

And a warning for the future....

Dr. Huei-long Wu, the director general of the Soil and Water Conservation Bureau, estimates that the flooding last August created about 1.2 billion cubic meters of debris in the disaster area that stretched south of Jiayi County and into Taidong County. But only about a third of that, or 400 million cubic meters, actually made its way into rivers, with the rest remaining upstream or at midstream. Thus, the next big storm could trigger even bigger debris flows, Wu believes.

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Daily Links

• Cilantro, that ubiquitous spice in Taiwan: why you might hate it
• The Economist asks what makes a country
• China is probing possible iron ore supplier monopoly. Because there is nothing like a one-party state complaining about monopoly behavior.
• Big anti-ECFA rally slated for June 6
• Taiwan should welcome Chinese students to boost competitiveness, says the MOE, even though it is unlikely that Taiwan students will study in China, says MOE.

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[Taiwan] Don't miss the comments below! And check out my blog and its sidebars for events, links to previous posts and picture posts, and scores of links to other Taiwan blogs and forums!