Geology Reference
In-Depth Information
no signs of deep weathering, and stripping and scarp
retreat seem unlikely as formative mechanisms. A third
possibility is mantle planation (Ollier 1978). In this
environment, weathering attacks any rock protruding
above the ground surface, levelling them off to create
a plane surface littered with a mantle of debris. Succes-
sive bevelling episodes of mantle planation would reduce
the level of the plains, leaving pockets of more durable
rock as high-standing residuals with their boundaries
corresponding with geological boundaries. Interestingly,
therefore, three different suites of processes may pro-
duce the same suite of landforms, a case of convergent
landform evolution.
Tors
, which are outcrops of rock that stand out on all
sides from the surrounding slopes, are probably formed
in a similar way to bornhardts. They are common on crys-
talline rocks, but are known to occur on other resistant
rock types, including quartzites and some sandstones.
Some geomorphologists claim that deep weathering is a
prerequisite for tor formation. They envisage a period
of intense chemical weathering acting along joints and
followed by a period when environmental conditions
are conducive to the stripping of the weathered mate-
rial by erosion. Other geomorphologists believe that tors
can develop without deep weathering under conditions
where weathering and stripping operate at the same time
on rocks of differing resistance.
fade with time. Flat sedimentary beds and folded sed-
imentary rocks produce distinctive suites of structural
landforms. Flat beds tend to form plateaux, mesas, and
buttes. Folded beds produce a range of landforms includ-
ing anticlinal hills, cuestas, and hogbacks. They also have
a strong influence on the courses of some rivers and pat-
terns of drainage. Faults and joints are foci for weathering
and produce large-scale landforms. Dip-slip faults may
produce fault scarps, grabens, horsts, and tilted blocks.
Strike-slip faults are sometimes connected with shutter
ridges, sag ponds, and offset drainage. Joints have a strong
influence on many landforms, including those formed
on granite. Characteristic forms include bornhardts
and tors.
ESSAY QUESTIONS
1 Explain the landforms associated
with folding.
2 Explain the structural landforms
associated with rifting.
3 To what extent do landforms result
from 'tectonic predesign'?
SUMMARY
FURTHER READING
Plutonic and hypabyssal forces intrude molten rock
(magma) into the deep and near-surface layers of the
Earth respectively, while volcanic forces extrude it on
to the Earth's surface. Volcanic and plutonic landforms
arise from the injection of magma into rocks and the
effusion and ejection of magma above the ground.
Intrusions include batholiths and lopoliths, dykes and
sills, laccoliths and phacoliths, all of which may express
themselves in topographic features (hills, basins, domes,
and so on). Extrusions and ejections produce volca-
noes of various types, which are tectonic landforms.
Impacts by asteroids, meteoroids, and comets pock the
Earth's surface with craters and impact structures that
Ollier, C. D. (1981)
Tectonics and Landforms
. Harlow,
Essex: Longman.
Old, given the pace of developments in the subject, but
still a good read for the novice geomorphologist.
Scheidegger,
A.
E.
(2004)
Morphotectonics
.
Berlin:
Springer.
Looks at the relationships between landform and tecton-
ics. Unusual and interesting.
Sparks,
B.
W.
(1971)
Rocks and Relief.
London:
Longman.
Very old by almost any criterion, but worth a look.
