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.