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