Geology Reference
In-Depth Information
Some interpret the joints and associated sheet structure as a primary feature of the rock which
has closely determined the gross morphology of the land surface. According to this view the joints
were first developed in the bedrock and the shape of the land surface is a response to this internal
structure. As Merrill (1897, p. 245) put it:
“… with many geologists these joints, in themselves, would be accepted as due to
atmospheric action. In the writer's opinion they are, however, the result of torsional stress
and once existing are lines of weakness which become more and more pronounced as
weathering progresses.”
According to Merrill (1897, p. 245), the boss or dome-like form of the bornhardts is “
incidental
and consequent
” on internal structure. The earliest proponent of this general endogenetic interpre-
tation was de la Beche (see Adams, 1954) but several others, besides Merrill (1897), subscribed to
the theory during the Nineteenth Century. Nowadays, however, such endogenetic theories of sheet
structure development find little support, for, since Gilbert (1904) published his seminal paper on
sheet fractures, the interpretation of sheet fractures as a response to the form of the land surface
(Chapman, 1956) has been the most widely, indeed almost universally, accepted, as epitomised in
the general usage of the terms offloading joint and pressure release joint; but wide acceptance does
not necessarily imply validity.
These are the two major competing interpretations of sheet fractures and associated sheet struc-
ture, but over the years many explanations and mechanisms have been proposed. Though most fail
as general explanations, some may have local validity. All fall into one of two major categories -
exogenetic or endogenetic.
2.3.1
Exogenetic explanations
Insolation was long ago suggested as a possible cause of sheet fractures. As rocks are poor con-
ductors of heat it has been argued that solar radiation heats the outer exposed zones of rock which
expand and become detached from the main mass, forming more-or-less thick slabs or sheets. But
because the effect of the Sun's radiation penetrates only a few centimetres at most into the rock,
whereas sheet jointing extends to considerable depths, this view can be discounted.
- Chemical weathering has also been cited. The gradual infiltration and penetration of meteoric
waters into rocks near the land surface has frequently been called upon in explanation of the
flaking and spalling of rock masses. Where the chemical alteration of rocks results in increased
volume and, hence, pressure, this appears feasible. However, not all chemical alteration leads
to volume increase and, hence, to expansive pressure and rupture. Also, if the weathering were
held to precede and to give rise to the fracturing, it is relevant to ask why chemical attack is
concentrated upon, and restricted to, a few gently arcuate planes.
Furthermore, many of the massive slabs and wedges of rock involved in sheet structure
display no sign of chemical alteration. Some do (
Fig. 2.5)
,
but such alteration of minerals can
more readily be explained as weathering associated with moisture seeping along pre-existing
fractures, rather than weathering having caused the development of the partings.
- The suggestion that sheet fractures are an expression of offloading or pressure release is widely
accepted. And all rock fractures are an expression of erosional offloading in the sense that at
depth other stresses are subordinate to the pressures exerted by the superincumbent load. It is only
through the release of vertical pressure that the other stresses are manifested as obvious frac-
tures. But, a basically different interpretation of sheeting joints, which attributes them solely
and wholly to pressure release without the previous application of stress, has long found favour.
The gist of the pressure release, or erosional offloading, hypothesis is that rocks which cool and
solidify deep in the Earth's crust (for example granites, whether of metasomatic or igneous origin)
do so under conditions of high lithostatic pressure, i.e. loading by overlying and adjacent rock.
That there are widespread granite outcrops is itself proof of deep erosion, for though some plutons
appear to have been emplaced at shallow depths (as for instance in eastern Papua New Guinea,
