Page 125 - Fundamentals of Geomorphology

108 STRUCTURE

It should be mentioned that a minority of geologists Continental rifting occurs at sites where the con-
have always spoken out against plumes. However, since tinental crust is stretched and faulted. The rift valley
about the turn of the millennium the number of voices runningnorthtosouthalongmuchofEastAfricaisprob-
has swollen, and the validity of the plume model has ably the most famous example (p. 98), and its formation
emerged as a key debate in Earth science (see Foulger et is linked with domal uplift. Volcanic activity is often
al. 2005; Huggett 2006, 21–5). associated with continental rifting. It is also associated
with hot-spots.
LANDFORMS RELATED TO Passive-margin landforms
TECTONIC PLATES
Figure 4.7 shows the basic geomorphic features of passive
Tectonic processes primarily determine large-scale land- or Atlantic-type margins with mountains (see Battiau-
forms, though water, wind, and ice partly shape their Queney 1991; Ollier 2004b). It seems likely that these
detailed surface form. Geomorphologists classify large- features start as an old plain (palaeoplain) of a conti-
scale landforms in many ways. One scheme rests on nental interior that breaks along a rift valley (Ollier and
crustal types: continental shields, continental platforms, Pain 1997).The palaeoplain at the new continental edge,
rift systems, and orogenic belts. It is convenient to discuss which is created by the rifting, experiences downwarp-
these large units under three headings – plate interiors, ing. Sea-floor spreading then favours the growth of a new
passive plate margins, and active plate margins. ocean in which post-rift sediments accumulate as a wedge
on the submerged palaeoplain to form a seawards-sloping
basal unconformity. This is the breakup unconformity
Plate-interior landforms
owing to its association with the fragmenting of a super-
Cratons are the broad, central parts of continents. They continent (Ollier 2004). Inland the palaeoplain survives
are somewhat stable continental shield areas with a base- as plateaux. Some plateaux may be depositional but
ment of Precambrian rocks that are largely unaffected most are erosion surfaces formed of uplifted palaeo-
by orogenic forces but are subject to epeirogeny. The plains. In areas where the sedimentary strata form folds,
main large-scale landforms associated with these areas the uplands are bevelled cuestas and accordant, level
are basins, plateaux (upwarps and swells), rift valleys, strike ridges. The plateaux may extend over large areas
and intracontinental volcanoes. Equally important land- or they may have suffered dissection and survive as frag-
forms lie along passive continental margins, that is, ments on the hardest rocks.They often retain the ancient
margins of continents created when formerly single land- drainage lines. Marginal swells are widespread asym-
masses split in two, as happened to Africa and South metrical bulges along continental edges that fall directly
America when the supercontinent Pangaea broke apart. into the sea with steeper (2 ) slopes towards the coast.
◦
Intra-cratonic basins may be 1,000 km or more They develop after the formation of plateaux and major
across. Some, such as the Lake Eyre basin of Australia valleys. Great escarpments are highly distinctive land-
and the Chad and Kalahari basins of Africa, are forms of many passive margins. They are extraordinary
enclosed and internally drained. Others, such as the topographic features formed in a variety of rocks (folded
region drained by the Congo river systems, are breached sedimentary rocks, granites, basalts, and metamorphic
by one or more major rivers. rocks) and separate the high plateaux from coastal plains.
Some continents, and particularly Africa, possess The great escarpment in southern Africa in places stands
extensive plateaux sitting well above the average height more than 1,000 m high. Great escarpments often sepa-
of continental platforms. The Ahaggar Plateau and rate soft relief on inland plateaux from highly dissected
Tibesti Plateau in North Africa are examples. These relief beyond the escarpment foot. Not all passive mar-
plateaux appear to have been uplifted without rifting gins bear great escarpments, but many do (Figure 4.8).
occurring but with some volcanic activity. A great escarpment has even been identified in Norway,