Page 102 - Sedimentology and Stratigraphy

Weathering Processes 89

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Fig. 6.2 The boundaries of the present-day principal tectonic plates.

basic pattern of winds around the Earth. The Coriolis low precipitation. In the middle lies the wet equato-
force is a consequence of the movement of any body rial zone of high rainfall and high temperatures.
travelling towards or away from the poles over the These climate zones are not uniform in width
surface of a rotating sphere, such that any moving around the world and have different local climatic
object – an air mass, water in the ocean, or an air- characteristics that are determined by the extent of
plane – will be deflected to the right in the northern continental land masses and the elevation of the land.
hemisphere and the left in the southern hemisphere. As both the positions and height of continents vary
The combination of temperature distribution and through geological time due to plate movements,
wind belts gives rise to four main climate zones. palaeoclimate belts can be related to the modern
Polar regions lie mainly north and south of the belts in only a relatively simplistic way unless com-
Arctic and Antarctic circles. They are regions of plex climate modelling is carried out.
high pressure and low temperatures with conditions
above freezing only part of the year, if at all. Between
about 608 and 308 either side of the Equator lie the 6.4 WEATHERING PROCESSES
temperate, moist mid-latitude climate belts which
have strongly seasonal climates and moderate levels Rock that is close to the land surface is subject to phys-
of precipitation. The dry subtropical belts are vari- ical and chemical modification by a number of different
able in width depending on the configuration of land weathering processes (Fig. 6.4). These processes
masses in the latitudes of the tropics of Cancer and generally start with water percolating down into
Capricorn. Over large continental areas these dry areas joints formed by stress release as the rock comes
are regions of high pressure, high temperatures and close to the surface, and are most intense at the