Processes and landforms in tropical environments

Weathering is the breakdown or decay of rock in situ — that is, in place, where it is — at or near the Earth's surface.

Erosion is different because it involves the removal and transport of the loosened material away from the site by an agent such as water, wind or ice.

The key distinction is movement: weathering breaks the rock down but leaves it in place, whereas erosion carries the debris away.

Rainwater absorbs carbon dioxide from the air and the soil, forming a weak carbonic acid.

This weak acid reacts chemically with the calcium carbonate that limestone is made of.

The reaction produces calcium bicarbonate, which, unlike the original limestone, is soluble in water.

The soluble bicarbonate is then carried away in solution, so the limestone is gradually dissolved and removed, widening joints and bedding planes.

A bornhardt is a large, steep-sided dome of granite, such as Sugarloaf Mountain in Rio de Janeiro, and its formation has two main stages.

In the first stage, slightly acidic water penetrates the joints in the granite and hydrolysis rots the feldspar to clay deep below the surface, producing a thick regolith. Because the granite is not jointed evenly, the weathering cuts deepest where joints are closely spaced and only shallowly over the massive, widely-jointed rock, leaving an irregular weathering front and a resistant core.

In the second stage, the climate, uplift or erosion strips away the soft regolith — exhumation — and the massive, less-weathered core is left standing abruptly above the surrounding plain as an inselberg.

Pressure release (dilatation) then unloads the exposed granite, creating curved sheet joints roughly parallel to the surface; the rock peels off in layers (exfoliation), giving the bornhardt its smooth, rounded, dome-like shape.

The warm, wet tropical climate is important throughout because it makes the deep chemical weathering rapid, while the rock structure (the massive, widely-jointed core) determines that this particular landform survives as a dome.

Cockpit karst is a landscape of enclosed, star-shaped depressions (cockpits) separated by cone-shaped residual hills, well developed in places such as the Cockpit Country of Jamaica.

It forms by carbonation: rainwater absorbs CO₂ to make weak carbonic acid, which dissolves the calcium carbonate of the limestone into soluble bicarbonate that is removed in solution.

In the humid tropics the high rainfall, warmth and biological CO₂ make this dissolution rapid, and the acidic water sinks into the vertical joints and bedding planes where solution is concentrated.

Solution is focused in the hollows, which are deepened and enlarged into enclosed depressions (the cockpits), while the more resistant rock between them is left upstanding as rounded, cone-shaped hills.

Over time this produces the distinctive 'egg-box' landscape of deep cockpits separated by cone karst hills, with the pattern of joints controlling the spacing and shape of the depressions and hills.

Granite landforms in tropical environments — bornhardts (including dwala and ruware), kopjes and tors — are upstanding masses of resistant granite produced by deep chemical weathering followed by exhumation. This essay assesses the relative importance of weathering, climate and rock structure in forming them, and argues that weathering is the necessary process that creates the landforms, rock structure determines which specific landform appears, and climate controls how rapid and deep the process is.

Weathering has a strong claim to be the most important control because it is the process that actually creates the landform. Hydrolysis attacks the feldspar in the granite, converting it to clay and weakening the rock so that a thick regolith forms deep below the surface; when this regolith is later stripped away by exhumation, the resistant core is left standing as a bornhardt, kopje or tor. Pressure release (dilatation) then unloads the exposed granite and creates the curved sheet joints that give the bornhardt its rounded, dome-like shape. Without deep chemical weathering there would be no resistant core to expose and no landform at all, so weathering is the fundamental process.

However, weathering alone does not explain why the SAME rock produces three different landforms; rock structure is decisive here. Granite is crossed by joints, and the spacing of these joints controls how deeply it weathers. Where joints are widely spaced the rock is massive and resists weathering, surviving as a large bornhardt or inselberg such as Sugarloaf Mountain in Rio de Janeiro. Where jointing is moderate the rock breaks down further into a smaller kopje of rounded boulders, and where the rock is block-jointed it weathers into a small tor of stacked, rounded blocks. The structure therefore creates the irregular weathering front and decides which landform results, making it the key control of FORM.

Climate is also important, but more as an intensifier than as a determinant of the form. The warm, wet tropical climate speeds up the chemical reactions, keeps the rock permanently wet, and supplies CO₂ and organic acids from the dense vegetation, so weathering is unusually rapid and deep and the resulting landforms are large and well developed. Yet tors also form in cool, temperate climates such as Dartmoor in England, which shows that the tropical climate is not essential for the landform to exist — it controls the rate and scale rather than uniquely determining the type.

The three controls are best seen as interacting rather than acting alone. Joints (structure) let water in, which drives hydrolysis (weathering), whose rate depends on the warm-wet climate; and as weathering removes rock, unloading creates new joints. A bornhardt requires a massive jointed core (structure), deep hydrolysis and exhumation (weathering) and, for its great size, a warm-wet climate. No single control could produce the landform by itself.

Weighing these points, weathering is the most important in the sense that it is the indispensable process — nothing forms without it — and pressure release shapes the result. Rock structure is the most important control of WHICH landform appears, because the same processes acting on differently-jointed granite give bornhardts, kopjes or tors. Climate is the least independent control, important for the rate and scale but not for the existence or basic form, as shown by cool-climate tors.

Overall, the fairest judgement is that the three controls are not equally important in the same way: weathering is necessary for the landforms to exist, rock structure determines their specific form, and climate determines how rapidly and on what scale they develop. Weathering and structure are therefore the more fundamental controls, with climate acting as a powerful but secondary intensifier in the tropics.

Tropical karst — cone, cockpit and tower karst — is a landscape produced by carbonation of limestone in the humid tropics, ranging from rounded cone hills and enclosed cockpits to the isolated vertical towers of Guilin in China. This essay assesses whether rock structure is more important than climate in its development, and argues that both are essential but that they control different things: climate makes the dissolution rapid and deep enough to produce dramatic karst, while rock structure controls where dissolution concentrates and therefore the precise form of the hills and depressions.

There is strong support for the importance of rock structure. Limestone is crossed by vertical joints and horizontal bedding planes, and acidic water sinks into these lines of weakness and dissolves the rock fastest there. The joints are widened into deep clefts while the blocks between them are left standing, so the spacing and pattern of the joints control the spacing and shape of the residual hills. Thick, massive, well-jointed limestone gives the steepest towers, whereas more closely jointed rock gives the rounded cones and cockpits. Without suitable structure — pure, thick, well-jointed limestone — the spectacular towers of fenglin karst could not form, which is the basis of the claim.

However, climate is equally fundamental, because it controls the rate and depth of the dissolution that creates karst at all. Carbonation requires water and carbon dioxide, and in the humid tropics the high rainfall, high temperatures and abundant biological CO₂ from dense vegetation make the soil water strongly acidic, so the limestone dissolves rapidly and deeply. This is exactly why the most extreme karst landscapes on Earth are tropical: the same limestone in a cool, dry climate produces only gentle limestone pavements and shallow dolines, not towers. Climate therefore determines whether dramatic karst develops in the first place.

The two controls are best understood as acting together rather than competing. Structure provides the routes (joints and bedding) along which water moves, but it is the climate-driven acidity and abundance of water that make dissolution along those routes rapid enough to lower the land toward the water table and steepen the residual blocks into towers. Tower karst in particular needs a long period of intense, climate-driven solution AND a massive, well-jointed structure plus a near-surface water table to undercut the base — neither factor alone is sufficient.

It is also worth noting that time is a third control: tower karst represents a mature, long-evolved stage, so even with the right structure and climate the towers take a very long period of continued dissolution to form. This further shows that no single control, including structure, fully determines the landscape.

Weighing the argument, rock structure is decisive for the FORM of the karst — whether cones, cockpits or towers develop, and how they are spaced — because dissolution follows the joints. Climate is decisive for the EXISTENCE and intensity of karst — whether dramatic tropical karst develops at all, rather than a subdued cool-climate landscape. So neither is simply 'more important'; they govern different aspects of the landscape.

Overall, I only partly agree. Rock structure is the more important control of the precise form and pattern of the karst, but climate is at least as important because it determines whether the rapid, deep dissolution needed for tropical karst occurs at all. The most accurate judgement is that structure controls the shape while the warm, wet tropical climate controls the rate and intensity, so the distinctive tropical karst of Guilin or Cockpit Country requires both a favourable structure and a humid-tropical climate acting together over a long time.