What is Karst Topography?
by Andrew Lees, on april 27, 2023
What is Karst Topography? A visit to Batu Caves, Malaysia
In our recent blog we looked at sinkholes and how they are formed: in most cases, by the dissolution of limestone bedrock. The topography of caves, sinkholes and other solution features caused in this way is referred to as karst. In this blog we look at one spectacular example of a karst topography in Kuala Lumpur, Malaysia.
What exactly is Karst?
Rainwater absorbs carbon dioxide from the atmosphere to form a mild acid. As this acidified water collects and is then passed down through fissures in carbonate rocks – such as limestone – it dissolves the rock and washes it away. On exposed surfaces, small solution features such as clints and grikes then begin to form, followed by larger features such as vertical shafts. In mature karst landscapes the majority of rock may have been removed leaving tall limestone pinnacles, while below the surface will be interconnected subterranean passageways and caves. Some of these may collapse to form sinkholes and thus a karst landscape is developed.
Over geological time, karst landscapes can become overlain with new deposits, burying the high pinnacles and sinkholes. The passageways and caves may become filled or partially filled with minerals or sediment, often becoming invisible from above.
What are the construction problems caused by buried karst?
From the surface there may be little or no evidence of a buried karst topography. Just below the surface, the bedrock may vary in depth from a few metres to a hundred metres by moving location slightly – even just the width of a house. Boreholes may reveal shallow bedrock although that may be underlain by large, interconnected caverns - and without a detailed geological study, the hidden problems may go undetected. Foundation design, particularly for high rise structures, poses a very real challenge in these areas.
The karst topography buried below Kuala Lumpur
The low-lying plain of the Kuala Lumpur area lies over extreme karst limestone bedrock, laid down over 400 million years ago in the Silurian period. Buried karst features include pinnacles, caves and sinkholes, and the geology of the area is further complicated by faults and folds in the bedrock: the result of large tectonic movements. Yet the ingenuity and skills of geotechnical and structural engineers have enabled construction to take place over these extreme geological features, including the high-rise Petronas Towers, the tallest buildings in the word up until 2004.
Batu Caves – visible karst topography
Misleadingly perhaps, Batu Caves is the name given to a limestone hill rising almost 213m above the surrounding area, 13km north of Kuala Lumpur. Originally called Kapal Tanggang, this hill is an outcrop of the same extreme karst limestone bedrock that lies below Kuala Lumpur. It provides us with an ideal opportunity to see the cliffs, caves and sinkhole features that lie buried below the surrounding region. Batu is the Malay word for rock, and also the name of the nearby Sungai Batu (Batu River) that flows past the hill.
Within the outcrop there are a series of caves - three main caves and several smaller ones - collectively known as Batu Caves, all of which were formed by dissolution of the limestone rock. The cave system today is an important Hindu temple complex. The largest cave, known as Temple Cave, is accessible after a climb of 272 steps. It has a high arched ceiling and houses very ornate Hindu shrines. Beyond Temple Cave and up a further 56 steps, the chamber opens up to the sky in a large solution feature. Standing at the bottom of the open chamber gives a good idea of what it would be like to look up from the bottom of a sinkhole after collapse. Take a look at Andrew Lees’ video from Batu Caves, to view this unique perspective.