Page 101 - Sedimentology and Stratigraphy

88 Continents: Sources of Sediment

Fig. 6.1 The pathway of processes

involved in the formation of a succession of

clastic sedimentary rocks, part of the
rock cycle.

between lithospheric behaviour, climate, weathering high ground that provide the source of clastic sedi-
and erosion are then considered in terms of the Earth mentary material.
Systems that are the sources of sedimentary material. However, not all vertical movements of the crust can
be related to the horizontal movement of plates. The
mantle has an uneven temperature distribution within
6.2 MOUNTAIN-BUILDING PROCESSES it, and there are some areas of the crust that are under-
lain byrelatively hot mantle, and other places where the
Plate tectonic theory provides a framework of under- mantle below is cooler. The hot regions are known as
standing the processes that lead to the formation of ‘plumes’, upwelling masses of buoyant mantle that in
mountains, as well as providing an explanation for how some instances can be on a large scale – ‘superplumes’
all the main morphological features of the crust have that probably originate from the core–mantle bound-
formed throughout most of Earth history (Kearey & ary. Above the hot buoyant mass of a superplume the
Vine 1996; Fowler 2005). Plate movements and asso- continental crust is uplifted on a vast scale to generate
ciated igneous activity create the topographic contours high plateau areas, such as seen in southern Africa
of the surface of the Earth that are then modified by today. Plateaux like these are distant from any plate
erosion and deposition. Areas of high ground on the boundary, but are important areas of erosion and
surface of the globe today can be related to plate generation of detritus for supply to sedimentary basins.
boundaries (Fig. 6.2). For example, the Himalayas is
an orogenic belt, a mountain chain formed as a
result of the collision of the continental plates of 6.3 GLOBAL CLIMATE
India and Asia, and the Andes have a core of igneous
rocks related to the subduction of oceanic crust of the The climate belts around the world are principally
east Pacific beneath South America. High ground also controlled by latitude (Fig. 6.3). The amount of energy
occurs on the flanks of major rifts, such as the East from the Sun per unit area is less in polar regions than
African Rift Valley, where the crust is pulling apart. in the equatorial zones so there is a temperature gra-
Interpretation of the stratigraphic record indicates dient from each pole to the Equator. These temperature
that the same mountain-building processes have variations determine the atmospheric pressure belts:
occurred in the past: the Highlands of Scotland and high pressure regions occur at the poles where cold air
the Appalachians of northeast USA are the relics of sinks and low pressure at the Equator where the air is
plate collisions resulting from the closure of past heated up, expands and rises. These differences in
oceans. Similarly, past subduction zones and related pressure give rise to winds, which move air masses
magmatic belts can be recognised in the Western between areas of high pressure in the subtropical and
Cordillera of western North America. Plate tectonic polar zones to regions of low pressure in between them.
processes are therefore the principal mechanisms for The Coriolis force imparted by the rotation of the
generating uplift of the crust and creating areas of globe influences these air movements to produce a