tropical karst

tropical karst Karst landscapes in the tropics are among the most spectacular anywhere in the world. Although the formative processes, primarily dissolution of carbonate rocks, are essentially the same as in other climatic zones, the higher temperatures, humidities, and rainfall, particularly in the humid tropics, accelerate and accentuate karst development (see karst).

High temperatures favour chemical erosion, and high humidities and rainfall ensure that abundant water is available for the dissolution process. Moreover, the warm, humid conditions encourage high levels of biological activity, particularly in the soil, generating the production of carbon dioxide, which is critical to the dissolution process in the near-surface, or epikarstic zone. Intense tropical rainfall may also result in rapid surface erosion and gullying by short-lived streamflow. In some tropical areas, surface weathering crusts develop through dissolution and recrystallization, a process termed case-hardening. This renders the rock sur-face less porous and increases its resistance to erosive stresses. Elsewhere, springs and waterfalls are associated with the precipitation of sheets and dams of tufa or travertine.

Extensive areas of tropical karst occur in southern Mexico, Central America, the Caribbean, in South-East Asia, and in southern China, where (in the provinces of Yunnan, Guizhou, and Guangxi) there is over 500 000 km² of the most impressive karst in the world. There are also significant tropical karst areas in South America, Madagascar, the Middle East, New Guinea, and northern Australia.

The most diagnostic elements of tropical karst terrain are large, more-or-less enclosed and polygonal depressions, sometimes termed cockpits because of their resemblance to cockfighting arenas, and systems of dry valleys, commonly dismembered by subsequent depression development. Bordering these negative relief features are sinuous ridges and interconnected or isolated residual hills, generally known as cones or towers. Although these hills may be striking, sometimes exceeding 100 m in height and with sheer cliffs, it is the valleys and depressions that are the active foci of the landscape, often channelling surface water into underground drainage systems. The enclosed depressions are complex forms of dolines or sinkholes, which act as centripetal drainage basins (see sinkholes). Although dissolution is the dominant process in their development, collapse of depression bases over subterranean voids may occur. For example, cenotes of the Yucatán Peninsula of Mexico are collapsed depressions, in the base of which groundwater flow is exposed.

Valleys in tropical karst are also highly variable. Steep-sided gorges develop where large rivers, rising in adjacent terrain, traverse karst belts, but many smaller rivers disappear underground at the end of blind valleys. Springs near the periphery of a karst area may supply rivers draining out of the karst via pocket valleys and some rivers may enlarge valleys and deposit alluvium within the karst to produce the large, flat-floored features known as poljes.

The classification of tropical karst is highly complex, with a tortuous terminology derived from several languages. The Chinese recognize a distinction between fenglin, or peak forest karst, in which residual hills are solitary and isolated; and fengcong, or peak cluster karst, in which numerous residual hills share a common bedrock base. Quantitative indices of tropical karst terrain have been developed, but are not employed widely.

In combination with the large landforms, a wide variety of smaller-scale features produced by dissolution are found. These features are generally called karren (see karren). Many of them are small and of eccentric morphology, but others are larger and mushroom- or spire-shaped. Among the most impressive of the latter are the pinnacle karsts, or stone forests of southern China, Borneo, and Madagascar, where the steep, fluted limestone spires may exceed 50 m in height.

Integral to tropical karst are some of the world's largest and most spectacular cave systems, the formation and evolution of which is favoured by the large volumes of water available underground (see caves). The caves take on a wide variety of forms, but they include some of the largest active river passages and several of the world's largest cave chambers. The largest of the latter, the Sarawak Chamber in Good Luck Cave, Sarawak, Malaysia, has horizontal dimensions of over 600 × 400 m, with a roof span of 300 m, a floor area of nearly 163 000 m², and a volume of some 12 million m³. Other massive cave chambers occur in Belize, Central America, and in Oman. Many tropical karst caves are extensively decorated with recrystallized deposits of calcite and other minerals.

Although karst in the humid tropics attracts the greatest attention, karst in arid and semi-arid tropical regions is also significant, in part because it illustrates the importance of climatic and environmental change in karst development. Tropical karst is developed on rocks ranging in age from the Palaeozoic (more than 250 million years old) to the Cenozoic (less than 65 million years old), but it is clear that it has often developed under changing environmental conditions, both at the surface and underground, with considerable inheritance of characteristics from karst terrain developed under earlier conditions.

Studies of tropical karst have influenced the understanding of karst in general. For example, in the early twentieth century, drawing on tropical experience, Alfred Grund developed models of groundwater circulation and landscape evolution in karst regions. The Chinese traveller Xu Xiake (1586–1641), who journeyed extensively throughout the tropical karst of southern China, was one of the first to appreciate the singular character of karst landscape.

Karst landscapes in the tropics are significant in terms of regional water supplies, as archaeological and cultural reserves, and as reservoirs for plants and animals. They are also increasingly important in the context of tourism. At the same time, they are inherently fragile, subject to severe water contamination, soil erosion, and environmental degradation. Increasing numbers of tropical karst landscapes are being designated as protected areas, but pressures upon them from agriculture, industry, and other sources are also increasing, and their conservation is of paramount importance. Notable protected areas of tropical karst include the Gunong Mulu National Park in Sarawak and the Lunan Stone Forest in Yunnan Province, China.

Michael Day