Page 109 - Sedimentology and Stratigraphy

96 Continents: Sources of Sediment

Relief tends to be greatest in areas that are under- ing results in fine-grained detritus and partial solution
going uplift due to tectonic activity and thermal dom- of the bedrock. High rainfall gives rise to high dis-
ing due to hot-spots in the mantle (Kearey & Vine charge in streams, although the dense permanent
1996; Fowler 2005). Rejuvenation of the landscape vegetation in these settings reduces soil erosion by
by uplift occurs mainly around plate boundaries, par- surface water, even on quite steep slopes.
ticularly convergent margins such as orogenic belts.
In tectonically stable areas the relief is subdued due to Arid subtropical regions
weathering and erosion resulting in a low, gentle
topography. The cratonic centres of continental plates The limited availability of water in arid regions means
are typically regions of low relief and hence rates of that chemical weathering processes are subdued.
denudation are low. The bedrock is frequently barren of soil or vegetation
cover, so when rainfall does occur it has little residence
time on the land surface, and hence little time for
6.6.2 Climate controls on denudation chemical alteration to take place. Mechanical break-
processes down can be significant, especially in desert regions
where cold nights and warm days promote freeze–
Chemical weathering processes are affected by factors thaw action, using whatever water is available. Exfo-
that control the rate and the pathway of the reactions. liation also occurs as a result of temperature changes.
First, water is essential to all chemical weathering However, the absence of soil and vegetation means
processes and hence these reactions are suppressed that infrequent but violent rainstorms can be very
where water is scarce (e.g. in deserts). Temperature effective at removing surface detritus: flash-floods
is also important, because most chemical reactions carry higher amounts of detritus than equivalent
are more vigorous at higher temperatures; hot cli- volumes of water occurring steadily over a longer
mates therefore favour chemical weathering. Finally, time. Fine-grained debris is removed from the regolith
water chemistry affects the reactions: the presence of by wind ablation, which is significant in barren
acids enhances hydrolysis and dissolved oxidising desert areas.
agents facilitate oxidation reactions (Einsele 2000).
The rates and efficiency of the reactions vary with
Polar and cold mountain regions
different bedrock types.
Rates of erosion are climatically controlled because Chemical weathering is less significant in cold, dry
the availability of water is important to the removal of regions where chemical reactions are slower. In
regolith by sheetwash and the extent to which rivers these areas physical weathering processes are more
and streams erode soil and bedrock. Temperature is effective, although these too rely on the presence of
also significant: the presence of ice is important in water. The products of weathering in cold mountains
mountains because wet-based, rapidly moving gla- are typically debris of the bedrock, broken up but with
ciers are more efficient at moving detritus than rivers. little or no change in the mineral composition. A
Denudation rates are therefore related to climatic granite breaks down into gravel clasts, plus grains of
regime, and general patterns can be recognised in quartz, feldspar and other rock-forming minerals.
each of the main global climate belts. Most of the products of physical weathering are
hence coarse material with little clay generated or
solution of the rock. Mountain glaciers are very
Wet tropical regions
powerful agents of erosion as they move downslope
In hot, wet, tropical areas, chemical weathering is over rock, but in polar regions the ice is permanently
enhanced because of the higher temperatures and frozen to bedrock and erosion due to glacial action
abundance of water. Bedrock in these areas is typi- is minimal (Chapter 7). Periglacial regions (areas
cally deeply weathered and highly altered at the sur- that border glaciers) have a seasonal cover of snow
face: seemingly resistant lithologies such as granite that melts in the summer months. However, the
are reduced to quartz grains and clay as the feldspars ground may remain frozen at depths of a few metres
and other silicate minerals are altered by surface all year round (permafrost – 7.4.4) and water
weathering processes. In general, chemical weather- accumulating near the surface may eventually