Some of the most beautiful and distinctive art from Southeast Asia depicts a seemingly magical landscape of magnificent rock pillars that rise above alluvial valley floors and utterly dwarf any manmade features that may appear in the work (Figure 1). Such artwork conveys the sublimity of the natural world and an idealized state of harmony between humankind and nature. The landscape is known as tower karst and because of its striking beauty (Figure 2) it has served as the backdrop in such movies as the James Bond classic, “Man with a Golden Gun” and more recently, “The Beach”. The recent episodes of the TV series “Survivor” are also set among tower karst islands in Thailand. This article discusses the origin of tower karst in general and focuses on that which occurs in Southern Thailand near Phang Nga Bay in the Andaman Sea (Figure 3a and 3b).
Any landscape whose characteristics derive primarily from the dissolution of limestone rock is termed a karst landscape. Limestone is a type of sedimentary rock that forms in the ocean by chemical processes that result in the precipitation of calcium carbonate. Over thousands to millions of years, the calcium carbonate can accumulate to form layers of limestone rock thousands of feet thick. Once this rock is uplifted above sea level by geologic forces, ground water percolates through it and slowly dissolves it, creating a variety of solution features. Thus, if water is present to dissolve the limestone, karst landscapes can develop in almost any climatic region, including arctic and arid areas. However, karst landforms are most likely to develop in temperate and tropical climates because of the greater water availability.
The dissolution of the limestone often creates sinkholes and caves, and it is these solution features that people in the United States and most other midlatitude countries usually associate with karst. In the mid-latitudes, these features are common where thick, hard, porous, and pure (greater than 60% calcium carbonate) limestone layers are present. For example, Mammoth Cave in Kentucky and Carlsbad Caverns in New Mexico are cave systems developed in such layers. Thick, resistant limestone is also necessary for tower karst to develop and caves are also common in tower karst regions; however, the towers that characterize tower karst can only develop in the tropics because it is only there that the right conditions of rock and climate interact to create these magnificent features. The most spectacular examples of tower karst occur in the monsoon area of Southeast Asia, including the Malayan Archipelago and Indonesia. China contains what is perhaps the most beautiful and extensive tower karst and it has significantly influenced the culture of this region. Tower karst also occurs in Papua New Guinea, Australia, Honduras, Cuba, Jamaica and Puerto Rico.
The towers that define this type of topography can reach heights of more than 1000 feet and are very steep-sided, with slopes ranging from near vertical to overhanging (Figure 4). Individual towers can range in extent from small pinnacles (Figure 5) to blocks that are several square miles in area. They often occur in groups, and are usually surrounded by a river, alluvial plain, or, in the case of Phang Nga Bay, by a mangrove swamp (Figure 6). These moist conditions are largely responsible for the creation of tower karst.
In Thailand, tower karst occurs predominantly in the southern, peninsular region of the country, which is 500 miles long and has a maximum width of 124 miles. This part of the country is bordered on the east by the Gulf of Thailand and on the west by either Burma or the Andaman Sea (Figure 3b). The fossiliferous, light gray limestone (Figure 7) that forms the tower karst is part of the Ratburi Group, a geologic unit that was deposited during the Permian Period between 286 and 245 million years ago. This unit also contains some sandstone and shale, and ranges in thickness from 2467 feet to 3000 feet in peninsular Thailand. This unit is more than 6500 feet thick in other parts of Thailand. Wherever it is exposed, it forms prominent ridges and towers.The ridges develop because the limestone outcrops occur in long narrow belts that follow the lineation of the mountain chains on the peninsula. This arrangement facilitates ridge development as surrounding rock layers are removed by weathering and erosion.
Consistent with the fact that tower karst can only develop in the humid tropics, one researcher has determined that tower karst development requires a minimum of 47 inches of precipitation per year and an average temperature of 64°F. Peninsular Thailand, with a minimum rainfall of 51 inches per year and an average annual temperature of about 82°F clearly meets these climate requirements. Consequently, the tower karst in this area is well-developed and spectacular.
Given that limestone dissolves in water, one wonders how giant towers of limestone rock could not only form, but grow in height in a hot, wet, monsoon climate? The answer to this question has to do with vegetation, microorganisms, the amount of carbon dioxide (CO2) dissolved in water, and the porosity of the limestone. In order for water to dissolve limestone, it must first react with carbon dioxide to form carbonic acid. Thus, it is actually the carbonic acid that dissolves the limestone, not water per se. The carbon dioxide that reacts to form carbonic acid occurs naturally in the atmosphere, which is why rainfall is naturally acidic with a pH of about 5.6.
The acidity of the rainfall is also partly dependent upon its temperature; specifically, the warmer the water, the less carbon dioxide It can dissolve. This means that, all other factors being equal, warm, tropical rainfall is usually less acidic than cooler rainfall in temperate regions. So, rainfall onto bare, unvegetated limestone in the tropics is not particularly effective at dissolving it. However, because of the warm, wet climate of the monsoon tropics, lush vegetation covers virtually all surfaces that are not too steep to prohibit its growth (Figure 8). The roots of plants release carbon dioxide to the soil and this elevates the concentration of this gas to levels that are higher than that found in the air. In addition, microorganisms decompose dead plant and animal material in the soil and this also results in the release of carbon dioxide. Because of these processes, the carbon dioxide content in tropical soils is up to 15 times greater than that of the atmosphere. As a result, the soil water becomes far more acidic than rainfall and aggressively dissolves the limestone below the surface. In some swampy, karst areas, water with a pH as low as 3.0 has been recorded. This level of acidity is equivalent to that of vinegar.
Karst towers are surrounded by rivers, alluvial plains, or mangrove forests (Figure 9). These environments provide not only abundant water, but high levels of carbon dioxide as well because of the abundant vegetation that grows there. It is for this reason that tower karst occurs in these areas. These environments result in the dissolution of the limestone along the base of the towers and, as the soil surface is eroded, the dissolution of the limestone layer proceeds downward.
Because the manner in which the towers develop, the sides of the rock towers become very steep, so very little soil or vegetation can develop on them. Consequently, the water that runs across their surfaces is not very acidic, and so the sides of the towers do not dissolve as rapidly as the bases. In addition, in the hot tropical sun, water evaporates rapidly. When it does, any calcium carbonate that was dissolved in the water precipitates. The precipitated material plugs cracks and pores in the limestone and this reduces the amount of water that flows through it. This process is called “case hardening," and it further limits the amount of dissolution. In so doing, it greatly increases the tower's lifetime. Reprecipitation also results in the formation of stalactites all along the sides of the towers (Figures 10a and 10b) and in the creation of speleothems inside the caves that frequently form in towers (Figures 11a and 11b). As a result of this combination of processes, the base of a tower dissolves more rapidly than its top and sides, and so it grows in height as the surrounding land surface is lowered by erosion
In the early stages of tower karst development, solutional surface depressions known as dolines develop into steep-walled depressions known as cockpits. If the solution progresses, steep-sided hills will form between the cockpits. If alluvium that is deposited either by a river or by runoff from hillsides fills the cockpit depressions, then the contrasting weathering rates described above will lead to the development of tower karst.
The contact between the alluvium and the base of the towers is a locus of spring development and cave formation. Because of the cracks and pores in limestone, water readily enters it and flows downward until it either meets an impermeable layer or the rock becomes saturated. When this occurs, the water will emerge from the rock as a spring. An excellent example of this can be seen at Waterfall Cave in Sa Bok Koronee National Park. Here, a small stream emerges from a cave at the base of a rock tower, whereupon some of the calcium carbonate in the water precipitates forming small "waterfalls" (Figure 12).
Unlike their counterparts in the temperate mid-latitudes, caves in the tropical karst regions are shallow and not well developed. They usually consist of a network of small tunnels (Figure 13) rather than an extensive system of caverns such as that which comprise Carlsbad Caverns in New Mexico. The difference results from the differing rates at which soil water becomes saturated in these climate regions.I n the tropics, the water dissolves the limestone more rapidly near the surface because of its greater acidity. When water becomes saturated with calcium carbonate, it cannot continue to dissolve limestone at greater depths. So, deep caves cannot form in tropical karst. As discussed previously, the saturated water redeposits the calcium carbonate, thereby reducing the porosity of the limestone and further limiting cave development. Finally, because the tower karst landscape is fragmented into towers, this also hinders the development of substantive cave systems. Nonetheless, some larger, shallow caves do occur and, when they do, they are associated with through-flowing rivers such as that which forms Waterfall Cave.
One important cave in southern Thailand occurs on the tower karst island of Ko Phi Phi Lay in the Andaman Sea (Figure 14 and Figure 15). This island is the home of a type of bird that produces nests using its own saliva. For generations, indigenous people have risked their lives to build bamboo scaffolding to reach the nests at the top of the cave. Without any protection or specialized climbing gear, these people climb over a hundred feet up the scaffolding, the tops of which are secured by ropes to stalactites and ledges in the dark heights of the cave. The motivation for doing this is, in modern times, purely financial. The nests are used to make a soup that is believed to invigorate those who eat it. In Japan, the soup will sell for as much as $100 a bowl. The people who harvest the nests claim that the nests can be harvested up to three times a year without overly stressing the birds.
If the alluvium at the base of a tower remains in contact with the base at a given elevation for an extended period, overhangs may develop as soil water erodes the base laterally. Wave action and salt weathering also erode the base of towers that rise from the sea (See Figures 7, 10b and 16). These overhangs can be as much as 20 feet high and 66 feet wide in some areas and, were it not for the natural strength of the limestone, they would quickly collapse. Eventually, however, the strength of limestone will be exceeded and huge slabs along the sides of towers will sheer off (Figure 17). By this process, as well as gradual dissolution, the tower will shrink in size until nothing of it remains. The sheering of the side of a tower is undoubtedly a spectacular event, though quite dangerous to anyone who happens to be nearby.
At Tiger Cave Monastery in the province of Phang Nga, Buddhist monks have established a temple, part of which extends under a karst overhang. Access to this part of the monastery is by a stairway that ascends a tall, nearly vertical ridge of limestone and then descends on the other side into a closed karst valley that is lush with the dark green vegetation of a tropical rain forest. In this natural sanctuary, in which trees more than one thousand years old can be found, monks have placed a larger-than-life Buddha image under an impressive overhang. A series of small caves extend into the limestone walls under the overhang and the entrances to some of them are occupied by Buddhist monks who have built small shelters there to protect them from the daily rains. That monks consider this valley and its caves to be sacred is further evidenced by their placement of numerous Buddha images and offerings on formations within the caves. In this same area, a large reclining Buddha has been placed under an overhang (Figure 18). Veneration of caves by the Thai people is a common practice, as it has been for most peoples throughout the history of humankind.
The overhangs and caves that develop along the contact between the tower base and alluvium can be used to trace the developmental sequence of alluvial valleys. This is because their development represents a period during which lowering of the alluvial plain slowed and enough time was available for lateral solution to occur. Conversely, the lack of overhangs indicates a more rapid rate of lowering of the alluvial plain-one that did not permit sufficient time for overhangs and caves to develop. If human artifacts or other dateable materials can be found within the caves, they can be used to date the stages of development of the towers.
Using paleomagnetic data obtained from caves at different levels in a tower in China, one researcher found that the tower had grown by the lowering of its base at a rate of less than 0.91 inch per 1000 years during the last one million years. Based on this rate, it would take approximately 1,318,700 years for a 100-foot tower to develop. The tower in China, from which this rate of development was determined, has been developing for at least 2.5 million years. Clearly, these towers develop slowly and last for great periods of time. Researchers believe that, in other parts of China, some tower karst began forming at the end of the Cretaceous Period 65 million years ago. If this estimate is accurate, this is remarkable because few landforms on earth, other than mountain ranges, are anywhere near this old. So, as these examples show, tower karst features can represent truly ancient landforms and can be of great use to geologists, geomorphologists, and archaeologists who seek to understand the past.
Because of on-going crustal plate collisions in Southeast Asia, the Malay Peninsula is experiencing a slow deformation that is causing the east coast of the peninsula to emerge from the Gulf of Thailand, while the west coast is slowly dipping and subsiding beneath the Andaman Sea. The result of this gradual deformation of the region has been to create an emergent coastline along the east coast that is characterized by wide, sandy beaches. The west coast, however, has few beaches and is characterized by drowned river valleys, prominent headlands, mangrove forests, and isolated islands of partially submerged tower karst (Figure 19). The island of Ko Phi Phi Lay, mentioned above, is one of these islands. Submergence was also facilitated by the rise in sea level that occurred at the end of the Pleistocene epoch ten thousand years ago. At that time, the great ice sheets that had covered much of North America and Europe melted and raised sea level about 350 feet. Because of this submergence, the west coast of Thailand, especially in the region of Phuket, is especially fascinating and beautiful, and attracts tourists from all over the world (Figure 20).
Rising abruptly and dramatically from the sea, the rock towers provide scenery of unparalleled beauty. These tower islands can be accessed by boats, and around many of the towers, small, sandy beaches have developed that make it possible to stop, have a picnic, and do some snorkeling on the magnificent coral reefs that grow on the submerged portions of the towers. Amazingly, several of these islands are inhabited, with the local population being confined to a narrow strip of sand along the base of the vertical towers. Imagine what life must be like on these islands. The scenery is one of vast horizontal expanses of open ocean, punctuated by vertical walls rising several hundred feet above the water (Figure 21). Movement is severely limited by water on the one hand and rock cliffs on the other. Hurricanes, or typhoons as they are known in this part of the world, would easily submerge the only level land around, so life is delicately balanced between extremes.
The inhabitants of one village, known as Panyee, dealt with their space limitation by building the entire village on stilts (Figure 22). Even the school "soccer field" is nothing more than a "pier" between buildings (Figure 23). The villagers use the rock tower next to their perched village exclusively for burial of the dead and for the siting of a mosque (Figure 24). Tourists can reach Panyee by boat, but tourism is changing this community. Selling cheap souvenirs has replaced fishing as the primary source of income for the people of Panyee (Figures 25a and 25b).
tower karst of Thailand, like that found elsewhere in the tropics, is unlike
any landscape to be found in North America or Europe.
For those who have the opportunity to see it, its grandeur is unforgettable
Learning of the origins of this beautiful and fascinating landscape only
enhances one's appreciation of it.
1. Chinese painting depicting a tower karst landscape.
2. Sunrise at Phang Nga Bay on the Andaman Sea.The high humidity and salty sea air create perfect conditions for beautiful sunrises.
3a. Maps of Thailand and Southeast Asia.Phuket, Krabi and the Andaman Sea are shown on the southwest portion of the peninsula.Sources: http://www.amazing-thailand.com/Images/MainMap.jpg and http://www.cia.gov/cia/publications/factbook/
3b. Larger-scale map of peninsular Thailand. Source: http://www.amazing-thailand.com/Images/South.jpg
4. Tower karst at Phang Nga Bay exhibits the tall and steep sided character of karst towers.
5. This remnant rock pinnacle is called Nail Head Island.With continued salt weathering along its base and wave action, it will eventually topple.Dozens of tourists visit this site by boat each day from resorts located along the coast.
6. The acidic waters of a mangrove swamp create ideal conditions for the development of tower karst.The tidal fluctuation at this location in Phang Nga Bay is about 10-12 feet.
7. The side of this rock tower reveals the typical gray color of the limestone that comprises the Ratburi Group.However, iron oxides from tropical soils often stain it orange.
8. The manner in which vegetation cover can vary based on slope is clearly shown in this image of a small tower island.The unvegetated sides of karst towers experience case hardening, which makes them resistant to weathering.
9. Towers rising from a mangrove forest at Phang Nga.Before the submersion of this region, these towers would have been surrounded by an alluvial plain.The multiple roots at the base of the mangroves help trap sediment and organic matter.The decay of the organic matter produces acids that dissolve the base of the towers.
10a. Stalactites form on the outside of the towers when calcium carbonate precipitates from the water in which it was dissolved.A cave is located at the base of this tower and a Buddhist shrine has been placed inside it.For scale, note the people standing at the foot of the cliff.
10b. Stalactites are visible along the sides of this sea arch that has developed in a small tower in the Andaman Sea.
11a. Although caves are not as developed in the tropics as they are in the mid-latitudes, they can develop some beautiful speleothems, such as this orange-colored flow stone.The orange is due to the iron oxides that form in the tropical soils above the cave.This cave is located near the city of Phang Nga.
11b. Stalagmites and columns in the cave near Phang Nga.
12. The small waterfall at Sa Bok Koronee National Park.As the small stream flows out of a cave at the base of the tower in the background, calcium carbonate precipitates to form the flowstone that creates the pool and "water fall".
13. This is the entrance to the cave near Phang Nga.Note the relatively small size of the cave.
14. This map of the Krabi region of Thailand shows the Island of Ko Phi Phi Lay.The small islands in this area are surely some of the most beautiful in the world.Source:
15. Bird Cave, on the island of Ko Phi Phi Lay, is visited by tourists who come to see the bamboo "towers" used by local people to harvest the bird nests high inside the cave.The nests are used to make soup that sells for up to $100 a bowl in Japan.
16. These children are standing under an overhang at the base of the tower located adjacent to Nail Head Island.The fact that the overhang can be so large gives some sense of the strength of the limestone in which tower karst develops.
17. When an overhang becomes large enough, the overlying rock eventually breaks free and sometimes forms a perfectly flat shear wall such as this one.
18. This larger-than-life Buddha image near Tiger Cave Monastery was placed under an overhang created by the dissolution of the limestone at the base of tower karst.The surrounding area is a rain forest that has acidic soils.
19. The west coast of peninsular Thailand is slowing subsiding below sea level because of tectonic forces acting in this region.As the area subsides, towers that were once surrounded by mangrove forests become surrounded by the sea.
20. This is one of several beach resorts in Phuket.
21. Notice the small homes located at the base of this isolated tower in the Andaman Sea.It is difficult to imagine what it must be like to live in a place surrounded by vast expanses of sea punctuated by the shear vertical walls of rock towers rising from the water.
22. The village of Panyee in the Andaman Sea was built almost entirely upon stilts.
23. The schoolyard at Panyee.When the author visited a few years ago, it was made of wood.On his most recent visit in 2000, it had been paved with concrete.However, it is a concrete surface on a pier above the water.If a child kicks a soccer ball too far, he or she has to have a boat to retrieve it!
24. Southern Thailand has a large Moslem population, and this is true of Panyee as well.The mosque shown near the center of the photograph was built on solid ground, which is also used for burials.
25a. Tourism has substantially changed the way of life for the people of Panyee.Once a fishing village, it has become a tourist trap that peddles all sorts of trinkets, such as those shown here.
25b. Even the tower adjacent to Nail Head Island has become a tourist trap.Tourists can buy film, postcards and other items on this small patch of sand in the middle of the sea.Kids can even have their pictures taken holding a hawk--for a fee.
26.Sunrise at Phang Nga Bay.