Exfoliation domes are one of the more spectacular results of rock relaxation, which is the result of the removal of overburden by erosion.
When rock is buried inside the Earth’s crust, it is subject to enormous compressive stresses, which prevents joints and fissures from opening and slightly reduces the volume of the rock. Horizontal compression results largely from tectonic forces and changes only very slowly, but vertical compression results simply from the weight of the overlying rocks. This overburden may be removed relatively quickly by erosion, or even the activities of man, such as the excavation of large quarries.
When the overburden is removed, the compressive forces are reduced and the rock relaxes toward the unconfined side. What happens next depends on the properties of the rock as well as on the topography.
Soft materials such as clays simply expand toward the unconfined side whereas more competent rocks will also develop unloading joints (aka relaxation joints, dilatation joints or stress relief fractures).
In a homogeneous rock the stresses accumulate until they reach about half the rock strength, after which the outer, more rapidly expanding layer breaks loose.
Thus a series of unloading joints form subparallel to the unconfined surface.
The spacing of the joints increases with depth in the rock, but below about 25 m the confining pressures are too great to allow the development of joints.
In this way the rock body becomes covered with several layers of shells, like an onion. The outermost joints become particularly susceptible to further widening by weathering – by water pressure, freeze/thaw cycles or the action of vegetation – causing the outer sheets to break free and fall away. This process is called sheeting and the loose slabs thus produced are called exfoliation sheets.
In jointed rocks however, the expansion will be accommodated in part by pre-existing joints and the development of new unloading joints will also be masked by the pattern of bedding planes or pre-existing joints. Therefore sheeting will not be very obvious in these rocks.
The expansion which creates the unloading joints takes place perpendicular to the exposed surface, and therefore the shape of the sheets broadly follows the local topography. But as the stresses along the rock body are smoothed across surface irregularities, the unloading joints will become more gently curved over time. Thus exfoliation will itself further modify the local topography by creating more rounded profiles.
Typical exfoliation domes are therefore best developed in homogeneous, course-grained igneous rocks – such as granitic plutons – where sheeting is not masked by bedding planes or pre-existing joints and which often provide a suitable topography which is then further modified into their distinct domal shape.
But the process of exfoliation is by no means restricted to the creation of exfoliation domes. It occurs as well in valleys or along vertical canyon walls or cliffs. Unloading joints also develop on flat surfaces, but this merely creates a fractured and easily weathered surface without modifying the topography.
Interestingly, exfoliation also occurs on a much smaller scale – and due to an entirely different process, the chemical weathering of feldspars into clays. This process causes the spheroidal weathering of individual boulders, as the clays take up more volume than the feldspars and thus cause the outer layers of the boulder to peel away.
Paul De Schutter