Geology Reference
In-Depth Information
saprock
, which represents the first stages of weathering.
Above the saprock lies saprolite, which is more weath-
ered than saprock but still retains most of the structures
found in the parent bedrock.
Saprolite
lies where it was
formed, undisturbed by mass movements or other erosive
agents. Deep weathering profiles, saprock, and sapro-
lite are common in the tropics. No satisfactory name
exists for the material lying above the saprolite, where
weathering is advanced and the parent rock fabric is not
distinguishable, although the terms '
mobile zone
', '
zone
of lost fabric
', '
residuum
', and '
pedolith
' are all used
(see Taylor and Eggleton 2001, 160).
Weathering can produce distinct mantles. The intense
frost weathering of exposed bedrock, for instance, pro-
duces
blockfields
, which are also called felsenmeer, block
meer, and stone fields. Blockfields are large expanses of
coarse and angular rock rubble occurring within polar
deserts and semi-deserts. Steeper fields, up to 35
◦
,are
called blockstreams. An example is the 'stone runs' of the
Falkland Islands.
Talus
(
scree
)
slopes
and
talus cones
are the result of weathering processes on steep rock faces
aided by some mass wasting.
silica. It occurs in humid and arid tropical environ-
ments, and notably in central Australia and parts of
northern and southern Africa and parts of Europe, some-
times in the same weathering profiles as ferricretes.
In more arid regions, it is sometimes associated with
calcrete.
Calcrete
is composed of around 80 per cent
calcium carbonate. It is mostly confined to areas where
the current mean annual rainfall lies in the range 200
to 600 mm and covers a large portion of the world's
semi-arid environments, perhaps underlying 13 per cent
of the global land-surface area.
Gypcrete
is a crust of
gypsum (hydrated calcium sulphate). It occurs largely
in very arid regions with a mean annual precipitation
below 250 mm. It forms by gypsum crystals growing
in clastic sediments, either by enclosing or displacing
the clastic particles.
Magnecrete
is a rare duricrust made
of magnesite (magnesium carbonate).
Manganocrete
is
a
duricrust
with
a
cement
of
manganese-oxide
minerals.
Hardpans
and
plinthite
also occur. They are hard
layers but, unlike duricrusts, are not enriched in a specific
element.
Duricrusts are commonly harder than the materials
in which they occur and more resistant to erosion. In
consequence, they act as a shell of armour, protecting
land surfaces from denudational agents. Duricrusts that
develop in low-lying areas where surface and subsurface
flows of water converge may retard valley down-cutting
to such an extent that the surrounding higher regions
wear down faster than the valley floor, eventually lead-
ing to
inverted relief
(Box 6.1). Where duricrusts have
been broken up by prolonged erosion, fragments may
persist on the surface, carrying on their protective role.
The
gibber plains
of central Australia are an example of
such long-lasting remnants of duricrusts and consist of
silcrete boulders strewn about the land surface.
Duricrusts and hardpans
Under some circumstances, soluble materials precip-
itate within or on the weathered mantle to form
duricrusts, hardpans, and plinthite.
Duricrusts
are
important in landform development as they act like a
band of resistant rock and may cap hills. They occur
as hard nodules or crusts, or simply as hard layers. The
chief types are ferricrete (rich in iron), calcrete (rich
in calcium carbonate), silcrete (rich in silica), alcrete
(rich in aluminium), gypcrete (rich in gypsum), mag-
necrete (rich in magnesite), and manganocrete (rich in
manganese).
Ferricrete
and
alcrete
are associated with deep weath-
ering profiles. They occur in humid to subhumid tropical
environments, with alcretes favouring drier parts of such
regions.
Laterite
is a term used to describe weathering
deposits rich in iron and aluminium.
Bauxite
refers to
weathering deposits rich enough in aluminium to make
economic extraction worthwhile.
Silcrete
, or siliceous
duricrust, commonly consists of more than 95 per cent
Weathering landforms
Bare rock is exposed in many landscapes. It results
from the differential weathering of bedrock and the
removal of weathered debris by slope processes. Two
groups of weathering landforms are (1) large-scale cliffs
and pillars and (2) smaller-scale rock-basins, tafoni, and
honeycombs.
