Geomorphic Processes

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 5.

Why is the surface of the earth never flat?

Imagine the earth's surface as a playground with two teams pulling in opposite directions. One team pushes land up. The other team keeps shaving it down. Neither wins, and that unending tug of war is why we have mountains, plateaus, plains and valleys instead of one smooth ball. This chapter introduces both teams, then focuses on the shaving team: weathering, mass movements, erosion and deposition, and the soil that all this breakage finally produces.

The wearing down of high ground is called degradation and the filling up of basins is aggradation. The wearing down of relief differences through erosion is gradation. Because endogenic forces keep raising parts of the surface, exogenic forces never manage to level it completely.

Key terms that the whole chapter depends on

Why gravity gets its own mention. Gravity pulls every loose bit of matter down a slope and also puts stress on rocks. It indirectly drives waves, tides and winds too. Without gravity and slopes nothing would flow, slide or settle, so there would be no erosion, transport or deposition at all. Every movement on the surface runs from a higher level to a lower one, or from high pressure to low pressure.

The two lifting processes: orogeny and epeirogeny

Diastrophism has four parts: orogeny, epeirogeny, earthquakes and plate tectonics. Orogeny and epeirogeny are the two that students mix up.

Volcanism, the other endogenic process, covers the movement of magma towards or onto the surface and the formation of intrusive and extrusive volcanic forms.

How exogenic processes work: energy, climate and rock

Pause and predict: two identical granite rocks lie exposed, one in a hot, wet tropical forest and one in a cold, dry region. Where will the layer of weathered material on top be deeper?

Check your answer: in the tropical forest. Heat and water speed up chemical reactions, so rock rots to a greater depth. The weathering mantle gets deeper as precipitation and temperature rise, which is why the deepest weathered layers are found in tropical regions.

Temperature and precipitation are the two climatic elements that control exogenic processes. Vegetation, which itself depends on rainfall and temperature, then influences them indirectly. Even inside one climatic region the effects vary with altitude, with the direction a slope faces, and with how much sunlight (insolation) it gets. North and south facing slopes get different sunlight from east and west facing slopes. Wind speed, rainfall intensity, daily temperature range and how often water freezes and thaws also change the outcome.

Climate is not the only control. The same climate acts differently on different rocks, because rocks differ in their structure: folds, faults, the tilt of beds, joints, hardness of minerals, chemical susceptibility and permeability. A rock can resist one process and yield easily to another, so different rocks wear at different speeds and leave different landscapes. The effects are small and slow, but repeated contraction and expansion tires rock out, much like bending a wire back and forth until it snaps.

The three denudation processes each have their own driving force:

Weathering: breaking rock where it lies

Weathering is the mechanical breaking up and chemical decay of rocks by the action of weather and climate. Very little or no motion of material takes place, so it is an in-situ (on-site) process. If the broken pieces travelled, the process would have changed from weathering into something else.

There are three groups of weathering processes: chemical, physical (mechanical) and biological. One rarely works alone, but one often dominates.

Most physical weathering comes from thermal expansion and pressure release. Chemical weathering needs water and air (oxygen and carbon dioxide) together with heat to speed the reactions along. Decaying plants and animals add even more carbon dioxide underground.

The five chemical processes do different jobs:

Exfoliation: a result, not a process. Curved sheets or shells peel off bare rock and leave smooth, rounded surfaces. The causes are unloading, temperature expansion and contraction, and salt weathering. Exfoliation domes come from unloading, and tors come from thermal expansion. If a question calls exfoliation a "process", it has probably set a trap.

Why weathering matters. It breaks rock into small pieces and prepares the way for regolith, soil, erosion and mass movements. It also decides how deep the weathering mantle is, and forests depend on that depth, so biomes and biodiversity trace back to weathering. It enriches ores too: when weathering leaches away unwanted material, the valuable metals left behind, such as iron, manganese, aluminium and copper, become concentrated enough to mine profitably.

Two views. For the exam: weathering, mass movement and erosion are three separate denudation processes, and weathering is only an aid to the other two. Another reading: many geomorphology texts treat mass wasting and erosion as parts of one larger set of "removal" processes, because both shift material from higher to lower ground. The textbook separates them by asking whether a moving agent (water, ice, wind) does the carrying. Use that test for the exam.

Mass movements: when gravity does it alone

Mass movement is the transfer of rock debris down a slope under the direct influence of gravity. Air, water and ice do not carry the debris. If anything, the debris carries some air, water or ice along with it. Movements range from slow to rapid and from shallow to deep, and include creep, flow, slide and fall. Heave, flow and slide are the three forms of movement, where heave means soil being pushed up by frost growth and other causes.

Because no geomorphic agent is involved, mass movements do not count as erosion, even though material really does shift. Gravity acts on bedrock as well as on weathered material, so weathering is not a pre-requisite for mass movement, but it helps a lot. Mass movements are most active on weathered slopes.

What makes a slope ready to fail. Weak unconsolidated material, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs, heavy rain and bare slopes all favour mass movement. Then something triggers it:

Landslides. Landslides are the relatively rapid, easily noticed mass movements, and the material involved is relatively dry. Five types are named:

Pause and predict: a block of rock drops straight off a cliff face and bounces at the bottom. Is this a rockslide or a rock fall?

Check your answer: a rock fall. A fall is free movement away from the slope and comes from the superficial layers. A rockslide stays on a bedding, joint or fault surface and involves material to a substantial depth.

Why the Himalayas slip more than the Western Ghats. Himalayan landslides and debris avalanches are frequent because the mountains are tectonically active, mostly made of sedimentary rocks and unconsolidated or semi consolidated deposits, and have very steep slopes. The Nilgiris and Western Ghats are more stable and made of hard rock, yet they still see landslides and many rock falls. The reasons are almost vertical cliffs and escarpments, strong mechanical weathering from temperature changes and ranges, and heavy rain over short periods.

Beyond the textbook. For the exam: mass movements are triggered by the nine causes in the table above. In reality: the human triggers in that table, such as road cuts, quarrying, building on slopes and clearing vegetation, are within our control, while earthquakes and heavy rain are not. In hill regions it is usually the combination, a cut slope followed by a heavy downpour, that produces the disaster. This is why hill road design and slope protection are part of disaster management (see Natural Hazards and Disasters).

Erosion and deposition

Erosion is the picking up (acquisition) and transport of rock debris by an agent. Debris carried by the agent also scrapes the surface, which is called abrasion, and this adds to erosion. Erosion lowers relief, so it is mainly responsible for the continuous changes in the earth's surface. Weathering, mass wasting and erosion are all degradational processes. Weathering aids erosion, but it is not a pre-condition for it.

The five erosional agents are running water, groundwater, glaciers, wind and waves.

Deposition is the sequel to erosion. On gentler slopes the agents lose velocity and energy, so the load starts to settle. Coarser material drops first and finer material later. Deposition fills depressions. It is not the work of a separate agent: the same five agents that erode also deposit, and they are then called aggradational or depositional agents.

Soil formation: the final product

Soil is more than crushed rock. It is a living, changing medium of chemical, physical and biological activity, and it responds to the seasons: biological activity slows or stops when soil is too cold or too dry, and organic matter builds up when leaves fall or grasses die.

The sequence. Soil formation (pedogenesis) depends first on weathering, because the weathering mantle is the basic raw material. Bacteria and simple plants such as mosses and lichens colonise the weathered material first, and small organisms take shelter in it. Their dead remains accumulate as humus. Grasses and ferns follow, then bushes and trees, whose seeds arrive with birds and wind. Roots push down and burrowing animals bring particles up, until the mass becomes porous and sponge-like, able to hold water and allow air to pass. The result is mature soil, a complex mixture of mineral and organic products.

Process versus factors. Pedogenesis is the process, and the five factors are the conditions that control it. The factors work together and influence each other's effect. Parent material, topography and time are passive control factors. Climate and biological activity are active factors.

Climate in more detail. The terms below describe what rainfall and temperature do to the soil.

Temperature works in two ways. Chemical activity rises in heat, falls in cold (carbonation is the exception) and stops in freezing conditions. That is why tropical soils have deeper profiles, while tundra soils contain mostly mechanically broken material.

Biological activity in warm and cold regions. Humus builds up in cold climates because bacterial growth is slow. Undecomposed matter forms layers of peat in subarctic and tundra regions. In humid tropical and equatorial climates bacteria work intensely, dead vegetation is oxidised quickly, and the soil keeps very little humus. Rhizobium bacteria live in root nodules of leguminous plants and fix nitrogen for the host plant. Earthworms change the soil's texture and chemistry as it passes through them.

Beyond the textbook. For the exam: soil forms from weathered material plus organisms over time, with five controlling factors. In reality: soil forms extremely slowly, often a centimetre over hundreds of years or more, while it can be lost in a few monsoon seasons when vegetation is stripped from a slope. Once the topsoil is gone the weathering mantle below, not the soil, is exposed. That is why soil conservation and vegetation cover matter more than any one-time fix.

Exam traps

60-second revision

Where this chapter connects

• Previous: Interior of the Earth explains the internal heat and plate movements that power endogenic forces.
• Next: Landforms and Their Evolution shows what each agent of erosion and deposition builds.
• Related: Natural Hazards and Disasters treats landslides as a hazard, and Structure and Physiography uses these ideas for Indian relief.