Karst in other lands: tropical regions

The best-developed karst regions are found in humid tropical environments (e.g. Jamaica and southern China). The reason for this is not fully known and has been the subject of considerable debate. The two arguments are:

  • Karst processes are more intense in the tropics because there is greater rainfall and greater carbon dioxide production in soils at higher temperatures.
  • But water in cool climates can hold greater quantities of carbon dioxide and is therefore more effective in dissolving limestone.

The reason is not just a simple matter of temperature. Temperature seems to be important only when it encourages the formation of deep soil and thick vegetation. Plant roots and decay by bacterial activity results in high levels of carbon dioxide in the soil and this increases solution of limestones.

So Nullarbor Plain, Australia, an arid karst region with sparse vegetation and thin soil, has a slow rate of surface and underground karst formation.

In Jamaica and south China, where soil is thick and the climate supports a dense vegetation cover, surface and underground karst formation is rapid.

The highest solution of limestone takes place in a region where

  • vegetation is lush,
  • moisture is high,
  • bacterial activity in the soil is high,
  • carbon dioxide production rate is high
  • water can move easily from soil to limestone

These conditions are present in both Jamaica and southern China and results in the overdeepening of cockpits and tower karst.

In this section we will consider Tower Karst of China, the Cockpits of Jamaica and the arid karst of Australia to see how they differ from the English karst.

China

Guangxi Province of southern China has the most spectacular karst topography found anywhere. The reason for this appears to be due to

  • the great thickness of carbonate rocks - massive crystalline limestone and dolomites of Precambrian to Triassic age reach thickness of 3 to 10 km.
  • the huge area of karst (between 1 and 2 million km3).
  • Gentle earth movements and slow uplift of limestone plateaus of horizontal or only gently dipping limestone.

Carbonate rocks of the central Guangxi Province date to the Devonian to Triassic times and are 3000 m thick in total. One of the features of the karst landscape of the central Guangxi Province is its lack of a surface drainage network and although there are several rivers, they do not develop a dendritic pattern of tributaries. Virtually all the streams sink through swallow holes and fissures into cave systems. Drainage is so rapid that, although there is up to 2m of rainfall in the province (up to 80% falling between May and August), drought conditions often occur.

We concentrate here on the unique tower karst of China, but other karst features occur. An example is the Sanshi Polje, which is surrounded by cockpit-like fengcong karst. The rivers in this polje drain through a swallow hole at its eastern end, flow east for about 10 km through the Banmen cave system, before joining the Hongshui River.

Tower Karst

Tower karst comprises numerous steep-sided residual hills, between 100 and 300m high. They are generally in groups of up to 10 towers per square kilometre. Tower karst forms only in a hot, humid climate and in China they are called fengcong, fenglin and Kufeng.

  • Fengcong are groups of peaks, connected together at their base and with depressions between them. Their formation is controlled by solution at limestone joints by water infiltrating down through a thick vadose zone.
  • Fenglin are also common karst features in China, for example near Guilin. They comprise groups of limestone pillars or small residual hills, that are not joined at the base, each pillar or hill being separated by valleys. The horizontal flow of ground-water is important in the formation of fenglin.
  • Kufeng are yet smaller hills that are separated from each other by flat plains.

It may be that Fengcong topography develops into fenglin topography and finally ending up as kufeng, although geologists are uncertain about this, especially because all three kinds of karst features appear to be of the same age.

Jamaica

The karst region of Jamaica is a dissected limestone plateau covered in tropical forest. The oldest limestones are impure and do not show good karst features. However, the younger, purer limestones, are up to 1500 m thick, and have well developed karst topography. Chalk is widespread and although folded, there are few fractures so that water moves through the pores of the rock rather than along solution channels. The limestone over central Jamaica, however, is recrystallised into a dense rock, the pores have been closed and the water follows fractures formed during earth movements. These fractures control the formation of the karst features.

Cockpits

Beneath the Cockpit Plateau, water drains mainly through joints widened by solution into small passages that coalesce to produce large conduits. Occasionally, storms cause flooding in the cave systems, the water backs up and may spill out onto the land surface. Amongst the solution features in Jamaica, are Cockpits and Poljes with mogotes.

There are several names for cockpits (one is 'cone karst', although this last name is not a useful one as the hills are rarely conical, outside China). They are often found in humid tropical areas such as Java, Puerto Rico and Jamaica. They are generally small, rounded hills (30–120 m in height), with a star-shaped depression between. There are usually 15 to 30 of these hills in a square kilometre.

Although it is not certain how cockpits developed, there are several theories. One is that cockpits formed in a similar way to dolines. The high rainfall and the torrential storms in tropical areas produced too much water to sink away quickly and it flowed over the surface of the limestone. This resulted in the erosion of gulleys along lines of weakness- usually the joints. In time, solution hollows developed and the streams sank into the underground drainage system at the lowest point of the depression. Gradually small solution depressions coalesced to form large, deep, irregular cockpits, surrounded by residual hills. Erosion and solution of the cockpit floors deepened them further. It may be that continued erosion and solution at the base of the conical hills will, in time, turn them into steep sided towers (karst towers or mogotes).

Mogotes are equivalent to the 'hums' in the temperate region. They are groups of steep-sided residual hills or towers that protrude up through a plain covered by younger sediments of sands and clays. It has been suggested that these sediments were originally the impurities within the limestone, and that they were left behind after the calcite had gone into the solution. This may in part be correct, but cockpits usually develop in areas of pure limestone, where impurities such as sand and clay form only a very small proportion (less than 5%) of the limestone. There is a question whether there was sufficient impurities in the limestone to carpet the plains.

Australia

Murrawijinie cave system.View from inside one

The Nullarbor karst region of southern Australia, is one of the largest karst regions in the world (about 200 000 km2), but it is also one of the most featureless. Nullarbor is an arid karst region where annual rainfall averages 250 mm near the coast, but only 150 mm inland. Any rainfall quickly evaporates or disappears underground through joints. The aridity is the reason for the absence of surface streams, the rarity of caves and the absence of towers, uvalas and poljes. Thus, a combination of the simple geological structure of the Nullarbor region, the resistance of the limestone to erosion and the arid climate, results in an essentially featureless surface.

The Nullarbor limestone is between 15 to 60 m thick and forms the best karst features in the region. These comprise a series of low ridges (1.5 to 6 m high) with flat-floored basins (up to 1 km wide and several kilometres long) between. These have been called 'ridge and corridor' features. They were probably formed by differential solution of the limestone surface and probably controlled by jointing, although how and when this occurred is unknown. In some areas there are dry valleys and dolines, though there is little rainfall today and no surface streams. It may be that the greatest solution took place during wetter periods in Pleistocene times.