Page 117 - Fundamentals of Geomorphology

100 STRUCTURE

The ascent of internal energy originating in the Earth’s features, which resulted fromTertiary etching, are closely
core impels a complicated set of geological processes. adjusted to underlying rock types and structures (Hall
Deep-seated lithospheric, and ultimately baryspheric, 1991). Such passive influences of geological structures
processes and structures influence the shape and dynam- upon landforms are called structural geomorphology.
ics of the toposphere. The primary surface features of
the globe are in very large measure the product of PLATE TECTONICS AND VOLCANISM
geological processes. This primary tectonic influence is
manifest in the structure of mountain chains, volcanoes, The outer shell of the solid Earth – the lithosphere –
island arcs, and other large-scale structures exposed at is not a single, unbroken shell of rock; it is a set of
the Earth’s surface, as well as in smaller features such as snugly tailored plates (Figure 4.2). At present there are
fault scarps. seven large plates, all with an area over 100 million km .
2
Endogenic landforms may be tectonic or structural They are the African, North American, South American,
in origin (Twidale 1971, 1). Tectonic landforms are Antarctic, Australian–Indian, Eurasian, and Pacific
productions of the Earth’s interior processes without plates. Two dozen or so smaller plates have areas in
the intervention of the forces of denudation. They 2
include volcanic cones and craters, fault scarps, and the range 1–10 million km . They include the Nazca,
Cocos, Philippine, Caribbean, Arabian, Somali, Juan de
mountain ranges. The influence of tectonic processes Fuca, Caroline, Bismarck, and Scotia plates, and a host of
on landforms, particularly at continental and large microplates or platelets. In places, as along the western
regional scales, is the subject matter of morphotecton- edge of the American continents, continental margins
ics. Tectonic geomorphology investigates the effects of coincide with plate boundaries and are active margins.
activetectonicprocesses–faulting,tilting,folding,uplift, Where continental margins lie inside plates, they are pas-
and subsidence – upon landforms. A recent and prolific
development in geomorphology is the idea of ‘tectonic sive margins. The break-up of Pangaea created many
passive margins, including the east coast of South Amer-
predesign’. Several landscape features, patently of exo- ica and the west coast of Africa. Passive margins are
genic origin, have tectonic or endogenic features stamped sometimes designated rifted margins where plate motion
on them (or, literally speaking, stamped under them).
Tectonic predesign arises from the tendency of erosion has been divergent, and sheared margins where plate
motion has been transformed, that is, where adjacent
and other exogenic processes to follow stress patterns crustalblockshavemovedinoppositedirections.Thedis-
in the lithosphere (Hantke and Scheidegger 1999). The tinction between active and passive margins is crucial to
resulting landscape features are not fashioned directly interpreting some large-scale features of the toposphere.
by the stress fields. Rather, the exogenic processes act Earth’s tectonic plates are continuously created at mid-
preferentially in conformity with the lithospheric stress ocean ridges and destroyed at subduction sites, and are
(see p. 138). The conformity is either with the direc- ever on the move. Their motions explain virtually all
tion of a shear or, where there is a free surface, in the tectonic forces that affect the lithosphere and thus the
direction of a principal stress. Earth’s surface. Indeed, plate tectonics provides a good
Few landforms are purely tectonic in origin: exoge-
nous forces – weathering, gravity, running water, glaciers, explanation for the primary topographic features of the
waves, or wind – act on tectonic landforms, picking Earth: the division between continents and oceans, the
disposition of mountain ranges, and the placement of
out less resistant rocks or lines of weakness, to pro- sedimentary basins at plate boundaries.
duce structural landforms. An example is a volcanic
plug, which is created when one part of a volcano is Plate tectonic processes
weathered and eroded more than another. A breached
anticline is another example. Most textbooks on geomor- The plate tectonic model currently explains changes
phology abound with examples of structural landforms. in the Earth’s crust. This model is thought satisfacto-
Even in the Scottish Highlands, many present landscape rily to explain geological structures, the distribution and