Geology Reference
In-Depth Information
In this survey and analysis of the landforms developed in granitic terrains, the part played by
interactions between country rock and water, and hence the critical importance of rock structure
in facilitating - or inhibiting - water infiltration, has been emphasised. The role of water as a trans-
porting medium has also been highlighted. Water takes not only chemicals derived from the atmo-
sphere and the regolith but also biota, especially bacteria, into the rock mass. It translocates and
evacuates the products of alteration resulting from reactions between the rock-forming minerals
and water charged with chemicals and biota. In suitable environments and circumstances it carries
solids in the subsurface, in some areas sufficiently to cause surface subsidence as a result of volume
loss and compaction.
Water is supremely important in shaping the Earth's surface. Its effects are, however, more clearly
demonstrated in granitic terrains than in most. Dry granite is stable but in contact with water rapidly
decays. When fresh, granite, being crystalline, is of low porosity and permeability, but it is also char-
acteristically pervious so that water infiltration is concentrated along partings. Thus, fracture density
and geometry substantially determine topographic patterns at a range of scales from inselbergs and
plains, bornhardts and boulders, basins and gutters. Fracture geometry, in turn, reflects not only the
composition and texture of the granite but also the strength and sense of crustal stresses, and the
environment (pressure, temperature, and hence rheology) of the affected granite at the time of stress.
The part played by water in shaping the land surface underlines not only the importance of
avenues of entry and weathering such as fractures, but also the general point that dry sites may sur-
vive whereas moist ones tend to decay. And this in turn points up the importance of temporary
water retention, in regoliths, for example, in landform and landscape development. Uplands which
shed water tend to remain upstanding whereas lowlands are repeatedly weathered, eroded and
lowered. Reinforcement or positive feedback mechanisms are as important in granitic terrains as
they are elsewhere, and at various scales.
Most of the changes effected by the contact of water with granite take place beneath the land
surface, at the base of, or within, the regolith. At the weathering front groundwaters exploit varia-
tions (weaknesses) in the bedrock with which they are in contact. Some of those variations have
their origins in magmatic, thermal and dislocational events that took place long ago, even in geo-
logical terms. Thus, in a very real sense the origin of many landforms can be traced back tens or
hundreds of millions of years, or even a billion or more years. Many landforms are really multi-stage
in that sense, as well as varying in their development after exposure. The two stages of development
frequently cited, the preparation of forms at the weathering front, and their subsequent exposure,
are, however, critical.
The changes resulting from the interaction of granite and water do not take place instantaneously,
but in time, and the longer the contact, the greater the degree, say, of the rounding of a fracture-defined
block and its conversion to a corestone/boulder. That is why, on the one hand, many inselberg land-
scapes are associated with long exposed shield areas, and why on the other, and in general terms, the
shallower the depth below the surface, the more advanced are weathering and landform preparation.
Atmospheric climate obviously affects the physics and chemistry of groundwaters, both
directly (e.g. temperature) and indirectly (e.g. organic acids, biota), and it thus probably affects the
rate at which granite landforms develop. And there are some forms which appear to be zonal, that
is they are found mainly in specific climatic regions. Nubbins (humid tropics) and tafoni (arid or
semiarid areas) are two that come to mind. Granite areas affected by frost and ice are superficially
shattered, and angular peaks and screes for example are prominent, but so are similar features on
other massive bedrock exposures, and many features typical of granite in temperate or tropical regions
are also found in cold areas.
On the other hand, many granite landforms, major and minor, are azonal. This results in part
from the control exerted by structure
sensu lato
, but is also due to many of them being of etch origin.
Shallow groundwaters are ubiquitous, so that differential weathering at the base of the regolith, at
the weathering front, is also widespread. Though the rate and precise character of weathering
processes, and in some degree erosion, varies with atmospheric and regolithic climate, the end
results are similar. Also, and for the same reason, many landforms characteristic of granite are also
found well-developed on rocks that are genetically different from, but physically similar to, granite.
