Chapter 5 — Geomorphic Processes

The endogenic and exogenic forces causing physical stresses and chemical actions on earth materials and bringing about changes in the configuration of the earth's surface are known as geomorphic processes. Diastrophism and volcanism are endogenic geomorphic processes; weathering, mass wasting, erosion, and deposition are exogenic geomorphic processes.

A process is a force applied on earth materials affecting them. An agent is a mobile medium — such as running water, glaciers, wind, waves, and groundwater — that removes, transports, and deposits earth materials. Exogenic processes and agents are one and the same unless stated separately.

The endogenic forces continuously elevate or build up parts of the earth's surface while exogenic forces work to wear them down. Because these two opposing sets of forces are always active, relief variations persist on the earth's surface.

Denudation is the general term covering all exogenic geomorphic processes — weathering, mass wasting/movements, erosion, and transportation — that strip or uncover the earth's surface. Denudational processes are driven by kinetic energy from gradients created by tectonic factors and the sun.

The three major groups are: (i) chemical weathering — solution, carbonation, hydration, oxidation, and reduction; (ii) physical or mechanical weathering — caused by gravitational forces, expansion from temperature changes or crystal growth, and water pressure from wetting and drying cycles; (iii) biological weathering — physical and chemical changes caused by organisms such as burrowing animals, plant roots, and microbial action.

Exfoliation is the flaking off of more or less curved sheets or shells from over rocks or bedrock, resulting in smooth and rounded surfaces. It is a result, not a process, and can occur due to expansion and contraction induced by temperature changes or due to unloading.

Mass movements transfer rock debris down slopes under the direct influence of gravity. Unlike erosion, no geomorphic agent — running water, glaciers, wind — carries the material; instead, debris may carry air, water, or ice with it. Mass movements range from slow (creep) to rapid (landslides, rock falls) and are aided but not pre-conditioned by weathering.

Landslides involve relatively rapid and perceptible movements of dry materials. Types include: slump (slipping with backward rotation), debris slide (rapid rolling or sliding without backward rotation), debris fall (nearly free fall from a vertical face), rockslide (sliding of rock masses along bedding, joint, or fault surfaces), and rock fall (free falling of rock blocks from steep slopes).

The five basic factors that control soil formation (pedogenesis) are: (i) parent material — the weathered rock debris or transported deposits; (ii) topography — slope gradient and drainage; (iii) climate — moisture and temperature; (iv) biological activity — vegetation, micro-organisms, and burrowing animals; (v) time — the duration over which soil-forming processes operate to develop a mature profile.

The two key climatic elements are moisture and temperature. Excess moisture causes eluviation (downward transport of soil components) and illuviation (deposition lower down). In dry climates, capillary action brings salts to the surface, forming hardpans. Higher temperatures deepen soil profiles, while freezing conditions produce largely mechanically broken materials with slow bacterial activity and peat accumulation.

Weathering breaks rocks into fragments that enable soil formation and support biomes and biodiversity through vegetation. It also enriches concentrations of valuable ores of iron, manganese, aluminium, and copper to economically exploitable levels — a process called enrichment. Without weathering, erosion and mass wasting cannot be significant.

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