Geology Reference
In-Depth Information
other rock types, the rock immediately above the weathering front (in the corestones that become
boulders - see Chapter 5 - adjacent to the kernels of fresh rock) is laminated, flaked or spalled.
Such changes have been attributed to moisture penetrating along crystal cleavages and microfis-
sures, and causing hydration or hydrolysis
(Fig. 3.2)
.
Though the changes effected are acknowl-
edged to be minute, they have been assumed to be sufficient to produce volume increase and
physical disruption. Flaking (or micro-sheeting) due to the hydration of biotite, for example, causes
it to expand. Where the crystal is confined or buttressed (Folk and Patton, 1982), this expansion is
converted to arching which finds expression in flaking, scaling or lamination, and eventually, given
the overall expansion caused by hydration and related reactions in grus the desintegrated and
altered rock.
In addition, however, once laminae have been formed, water can readily penetrate into the rock, and
not only continue the physical breakdown of the rock - an example of a positive feedback or rein-
forcement mechanism - but also effect various chemical changes. The flakes are fragmented and
the mica and feldspar are gradually altered to clay, the character of which depends on conditions
within the regolith, but which is commonly kaolinite. Eventually, the quartz must be also dis-
solved. Salts released by weathering are illuviated and, translocated by descending meteoric
waters and shallow groundwaters, tend to accumulate at the base of the regolith, just above the
impermeable fresh rock. In particular iron oxides and amorphous silica derived from the alteration
of micas, feldspars and quartz (Siever, 1962) are concentrated at the weathering front. In the
Ya rwondutta Quarry, Western Australia
(
Fig. 3.6)
iron and silica (which have replaced plant roots)
accumulated at the weathering front are 2-3 times as abundant as in the fresh rock.
In effect, water rots granite, changing it from a cohesive tough rock to a weak, puggy, gritty clay.
Well might MacCulloch (1814, p. 72) refer to the alteration of granite as a “gangrenous process”.
3.5
CONTROLS OF WEATHERING
Several factors influence the type and rate of rock weathering. All else being equal, the suscepti-
bility of minerals to alteration is the same as the order in which they crystallise out from an igneous
melt. The higher temperature minerals are in greater disequilibrium with the Earth's surface envi-
ronment than those that crystallise out at lower temperatures. Thus, the composition of the rock
(Bowen, 1918) strongly influences its rate of alteration, with rocks rich in such minerals as olivine,
augite and hornblende more susceptible than those composed of quartz and potash feldspars, for
example: all else being equal, basalt, norite and gabbro are more readily weathered than dacite and
granite (Hutton, Lindsay and Twidale, 1977). Granite is compositionally a resistant rock; yet dif-
ferent granites vary in their weathering characteristics because of variations in composition. For
instance, biotite is a weak link, and biotite granites in French Guyana for example, are preferen-
tially weathered. The granites of the Karkonosze Mountains of southern Poland are resistant
because they lack biotite and other ferromagnesian minerals. Even within massifs compositional
variations find morphological expression, the vertical western face of the Pão de Açucar, in Rio de
Janeiro, southeastern Brazil, for example, being partly due to the exploitation of a biotite-rich
zone. In parts of Corsica, bluffs and other steep slopes eroded in rocks rich in ferromagnesian min-
erals stand in contrast with the more gentle inclines of granodiorite and monzonite terrains. On
Haytor, on eastern Dartmoor, southwest England, the Giant Granite appears to be more resistant
than the Blue, with the result that a shallow alcove is associated with the latter (
Fig. 3.7);
though
it could reflect more concentrated attack by soil moisture at a time when the regolith stood higher
on the flanks of the residual.
In a more general sense, granodiorite is not only by far the most common of the granitic rocks,
but consisting, as it does, of over 40% feldspar, predominantly plagioclase, which is readily sus-
ceptible to reactions with water and resultant alteration to clays, it is, of all those in the granitic
domain, the rock most vulnerable to weathering by reason of its composition.
But composition is in many places overridden by fracture density. Fractures are avenues of
water penetration and thus of weathering. Variations in fracture density and the attitude of partings
