Spheroidal weathering is a term used to describe the spalling away of concentric surphicial shells of the rounded surface of a boulder or rock mass.
Such “onion-skin” weathering its believed to result primarily from the mechanical effects of chemical weathering. When feldspars decompose the clay product has a greater volume than the parent feldspar.
The increase in volume disrupts the interlocking texture of mineral grains in the rock and causes breakage and separation of the layer of partially weathered rock near the surface. Comers of roughly rectangular blocks broken loose along joints are decomposed along three surfaces simultaneously and are progressively round by spalling until the rock mass assumes the spheriodal form.
Spheroidal weathering operates most effectively on relatively small rock masses such as those of boulder size. The kind of spheroidal weathering involving a larger-scale breaking off of concentric plates from bare rock surfaces is referred to as exfoliation.
ADVERTISEMENTS:
Rock that have experienced exfoliation may resemble those having sheet structure, but while sheet structure is caused by release of pressure when overlying rock masses are removed by erosion, all form of spheroidal weathering result from physical and chemical weathering.