Spheroidal weathering is a process of natural decomposition that transforms sharp, angular rock masses into smooth, rounded boulders. This phenomenon is a specific type of chemical weathering where the alteration of minerals begins along the faces and edges of fractured rock. Over time, the interaction of water with the rock’s internal structure causes differential decay and progressive inward rounding.
How Water Breaks Down Rock Corners
The initiation of spheroidal weathering requires a rock mass to be already broken by a network of pre-existing fractures, known as joints. These joints create a three-dimensional grid, which divides the bedrock into rough, cube-like blocks. Water, often containing dissolved carbon dioxide that makes it slightly acidic, penetrates deep into the rock by traveling along these fractures, carrying the chemical agents necessary to break down the rock-forming minerals.
Chemical reactions like hydrolysis and oxidation are the primary drivers of this decay. Hydrolysis occurs when water reacts with silicate minerals, such as the feldspar found in granite, transforming them into softer clay minerals like kaolinite. Oxidation often involves iron-bearing minerals reacting with oxygen dissolved in the water, which also weakens the rock structure. This chemical alteration begins simultaneously on all exposed surfaces of the angular rock blocks.
The geometry of the rock block dictates the speed of the weathering process. A corner is exposed to the chemical attack from three different sides, while an edge is exposed from two sides, and a flat face is exposed only from one side. Because the corners and edges have a significantly greater surface area exposed relative to their volume, they decompose at a much faster rate than the faces. This differential decay rate causes the initial angular block to progressively round inward, resulting in its characteristic spherical shape.
The “Onion Peel” Structure of Weathered Rocks
The visual signature of spheroidal weathering is a distinct layer-by-layer decay that is often described as an “onion peel” structure. As chemical reactions advance inward from the fractures, the outer layers of the rock block are converted into saprolite, a soft, highly altered material. Saprolite is composed of newly formed clay minerals and is easily eroded.
The conversion of primary minerals into clay minerals causes expansion, creating internal stresses within the rock mass. These stresses cause the altered outer shell, or rind, to crack and detach from the less-weathered material beneath it. These concentric shells peel away through physical spalling, leaving behind an unweathered, rounded core of the original rock.
This central, intact mass is called a corestone. Corestones are surrounded by the thick layer of soft, decomposed saprolite that forms the matrix of the weathered zone. When subsequent erosion removes this softer saprolite, the durable, rounded corestones are often left exposed on the landscape as freestanding boulders.
Why Certain Rocks Are Affected More Than Others
The effectiveness of spheroidal weathering depends highly on both the rock type and environmental conditions. Igneous rocks, such as granite, basalt, and dolerite, are particularly susceptible because they contain minerals like feldspar and pyroxene that readily undergo hydrolysis and oxidation. These chemical reactions are accelerated by the presence of moisture.
The availability of water is a major factor, which is why this type of weathering is most effective in humid or sub-humid climates. Warm temperatures also speed up the chemical reactions, making the process more pronounced in tropical and temperate regions. Conversely, the process is less common in arid or very cold environments where moisture is scarce or primarily locked in ice.
The density and pattern of the initial joints also influence the outcome. Closely spaced fractures allow water to penetrate the rock more thoroughly, creating smaller, more numerous angular blocks. This results in the formation of smaller, more tightly packed corestones. If the joints are widely spaced, the resulting corestones will be much larger.