Endogenic Forces & Weathering Processes

5.1 Why Is the Earth's Surface Uneven?

By now you have learnt how the Earth was born, how its crust evolved, how the rigid plates have been moving for hundreds of millions of years, and how earthquakes, volcanoes, rocks and minerals make the crust what it is. The next natural question is much simpler: why is the surface of the Earth so uneven? Why are there mountains and valleys, plains and plateaus, deserts and deltas? Why does the surface refuse to settle down to a single, smooth level?

The Earth's crust is dynamic. It moves both vertically and horizontally, and it has done so — though more vigorously in the past — throughout geological time. The differences in the internal forces that originally built up the crust are responsible for the variations on its outer surface. At the same time, the surface is being continuously attacked from the outside by forces driven mainly by sunlight (and to a smaller extent by gravity). The internal forces are still at work, though with changed intensities. So the Earth's surface is forever the meeting ground of two opposing groups of forces — one acting from inside, the other from outside.

The forces from within the Earth are called endogenic forces; the forces from outside are called exogenic forces. Their actions are opposite. Exogenic forces wear the surface down (degradation) and fill up depressions (aggradation). The wearing down of relief variations through erosion is given a single name — gradation^?. Endogenic forces, in contrast, continuously elevate or build up portions of the surface, so the exogenic forces never quite manage to level out the relief. As long as the two groups keep pulling against each other, the surface variations will remain. In short: endogenic forces are mainly land-building forces; exogenic forces are mainly land-wearing forces.

The surface we live on is sensitive. We depend on it intensively for food, water, shelter and minerals. Almost every organism contributes to keeping the Earth's environment in balance, but humans have caused extensive damage through the over-use of resources. Understanding the processes that have shaped — and are still shaping — the surface, and the materials of which it is made, is essential if we want to use the Earth without diminishing its potential for the future.

5.2 Endogenic Processes — Building the Crust from Within

The energy that drives endogenic processes is generated inside the Earth. The principal sources are radioactivity, rotational and tidal friction and the primordial heat retained from the Earth's origin. This energy creates geothermal gradients and a steady heat-flow outward, which together induce diastrophism and volcanism in the lithosphere. Because the geothermal gradient, the heat-flow, the crustal thickness and the crustal strength all vary from place to place, the action of endogenic forces is not uniform — and the original tectonically-controlled crustal surface ended up uneven.

Diastrophism — Four Kinds of Crustal Movement

Diastrophism^? is a single umbrella term that covers all processes that move, elevate or build up portions of the Earth's crust. Geographers recognise four kinds:

In orogeny (oros = mountain) the crust is severely deformed into folds; in epeirogeny (epeiros = continent) the deformation is simple — a slow up-warp or down-warp of large blocks. Orogeny is a mountain-building process; epeirogeny is a continent-building process. Through orogeny, epeirogeny, earthquakes and plate tectonics, the crust gets faulted and fractured. All four cause changes in pressure, volume and temperature (PVT changes), and these PVT changes are exactly what induce the metamorphism of rocks.

Volcanism — Magma on the Move

Volcanism includes the movement of molten rock (magma) onto or toward the Earth's surface and the formation of all kinds of intrusive and extrusive volcanic forms. Many aspects of volcanism — types of eruption, dykes, sills, laccoliths, volcanic landforms — were dealt with under volcanoes in Unit II and under igneous rocks in the previous chapter, so they are not repeated here. The point to remember is that, like diastrophism, volcanism is an endogenic process driven by the heat from inside the Earth.

5.3 Exogenic Processes — Energy from the Sun

The exogenic processes draw their energy from the atmosphere — and behind the atmosphere lies the Sun. The Sun's heat creates the temperature differences and pressure differences that drive winds, the water cycle, ocean currents and ice movements. To this is added the slope gradients created by tectonic factors: without slopes there would be no flow, and without flow there would be no erosion. Gravitational force acts on every Earth material that lies on a sloping surface and tends to pull it downslope.

The force applied per unit area is called stress. Stress is produced in a solid by pushing or pulling, and it leads to deformation. Forces acting along the faces of Earth materials are shear stresses (separating forces) — these break rocks and produce angular displacement or slippage. On top of gravitational stress, Earth materials also undergo molecular stresses caused by temperature changes, crystallisation and melting. Chemical processes, in turn, loosen the bonds between grains, dissolve soluble minerals and weaken cementing materials. The basic reason that leads to weathering, mass movement and erosion is therefore the development of stresses inside Earth materials.

Because there are different climatic regions on Earth — created by latitudinal variations, seasonal changes, and the way land and water are distributed — the exogenic processes vary from region to region. Vegetation density, type and distribution depend on temperature and rainfall, and they too influence exogenic processes indirectly. Within a single climatic region, local effects are produced by altitudinal differences, by aspect (north- vs south-facing slopes, east- vs west-facing slopes), by wind speeds and directions, by intensity of precipitation, by the precipitation–evaporation balance, and by the depth of frost penetration.

Climate apart, the type and structure of rocks control how vigorously a process can act. Folds, faults, orientation and inclination of beds, presence or absence of joints and bedding planes, the hardness or softness of constituent minerals, their chemical susceptibility, and the permeability of the rock all matter. A single rock may be highly resistant to one process but offer little resistance to another. Under different climates, the same rock may behave differently — which is why the same rock can produce different topographies in different parts of the world.

5.4 Weathering — Action of Weather and Climate

Weathering^? is the action of elements of weather and climate over Earth materials. A number of processes act either alone or together to reduce rocks to a fragmental state.

Weathering is conditioned by many complex geological, climatic, topographic and vegetative factors, of which climate is the most important. Not only do weathering processes differ from one climate to another, but the depth of the weathering mantle (the layer of weathered rock above the fresh bedrock, also called regolith^?) also varies sharply. Geographers recognise three major groups of weathering processes:

Chemical Weathering Processes

Chemical weathering is a group of processes — solution, carbonation, hydration, oxidation and reduction — that act on rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions with oxygen, surface water, soil water and various acids. Water, air (oxygen and carbon dioxide) and heat must all be present to speed up chemical reactions. The carbon dioxide content of soil air is greatly increased by the decomposition of plants and animals, so soil-water is far more chemically aggressive than rain-water alone. The chemical reactions on minerals are very similar to the reactions you study in a chemistry laboratory.

Physical / Mechanical Weathering Processes

Physical or mechanical weathering processes depend on some applied force. The applied force can be: (i) gravitational forces such as overburden pressure, load and shearing stress; (ii) expansion forces due to temperature changes, crystal growth or animal activity; (iii) water pressures controlled by wetting and drying cycles. Many of these forces act both at the surface and within the rock, leading to fracture. Most physical weathering is caused by thermal expansion and pressure release. Each individual stress is small and slow, but rocks suffer continued fatigue from endless contraction–expansion cycles, and eventually fail.

Unloading and Expansion

When the deep rocks that originally formed under high pressure are exposed to the surface by erosion of overlying rocks, the load on top is removed. This pressure release allows the deep rocks to expand. The expansion produces fractures parallel to the rock surface, and slabs of rock peel off — a process closely linked to the formation of exfoliation domes on granite hills.

Temperature Changes and Expansion

Different minerals in a rock expand and contract by different amounts when heated and cooled. In hot deserts, a rock can experience a daily temperature swing of 40 °C or more, so its surface heats up and expands while the cooler interior holds back. The repeated stresses crack the rock surface, sometimes producing granular disintegration (loose grain-by-grain breakdown) or thin curved sheets.

Freezing, Thawing and Frost Wedging

In cold and high-altitude regions, water enters cracks during the day and freezes at night. Frost wedging^? occurs because water expands by about 9–10 per cent when it turns into ice, exerting tremendous pressure inside the crack. Repeated freeze–thaw cycles widen the crack and eventually split the rock. This is why rock faces in the Himalayas and the Alps are often littered with sharp angular debris.

Salt Weathering

Salts dissolved in water enter rock pores and cracks. When evaporation removes the water, the salt crystallises and grows. The crystallisation pressure can be considerable — and in arid coastal areas, repeated cycles of salt growth pit and pulverise even hard rock. Hydration of certain salts (gypsum, anhydrite, magnesium sulphate, calcium chloride) also adds an extra wedging force.

Biological Activity and Weathering

Biological weathering is the contribution to (or removal of) minerals and ions from the weathering environment by living organisms, plus the physical changes caused by their growth or movement. Burrowing and wedging by earthworms, termites and rodents expose new rock surfaces to chemical attack and help moisture and air to penetrate. Humans too contribute by disturbing vegetation, ploughing and cultivating the soil, all of which mix and create new contacts between air, water and minerals. Decaying plant and animal matter releases humic, carbonic and other acids, which sharpen chemical decay and increase the solubility of certain elements. Plant roots exert tremendous mechanical pressure on rocks and prise them apart along bedding planes and joints.

5.5 Significance of Weathering

Weathering is responsible for breaking down rocks into smaller fragments and so preparing the way for the formation of regolith and soils, and for erosion and mass movements. Biomes and bio-diversity are basically the result of forests, and forests depend on the depth of the weathering mantle. Erosion cannot be significant if rocks are not weathered first. Weathering aids mass wasting and erosion; the reduction of relief and the gradual changes in landforms are a consequence of erosion. So the chain runs: weathering → mass wasting → erosion → landform change.

Weathering of rocks and deposits also helps in the enrichment and concentration of certain valuable ores — iron, manganese, aluminium, copper — that are crucial for the national economy. When rocks undergo weathering, some materials are removed by chemical or physical leaching by ground-water; the remaining (valuable) material gets concentrated. Without such weathering, the ore concentration in many places would not be sufficient to make extraction economically viable. This concentration mechanism is called enrichment.

5.6 Special Effects of Weathering

Exfoliation

Exfoliation^? is a result of weathering, not a process in itself. It refers to the flaking off of more or less curved sheets or shells from over rocks or bedrock, leaving smooth, rounded surfaces (NCERT Figure 5.3). Exfoliation can be caused by expansion and contraction due to temperature changes, by unloading, or by salt weathering. The two best-known forms are exfoliation domes (large rounded hills like the Half Dome of Yosemite, or the granitic domes of Karnataka), produced by unloading on once-deeply-buried granite, and tors (smaller, blocky residual hills), produced by thermal expansion in tropical and sub-tropical climates.

Soil Formation — A First Look

Weathering also begins the long process by which solid rock turns into soil. The weathered material — the regolith — is the basic input. First, bacteria and lower plants like mosses and lichens colonise the weathered surface; minor organisms shelter inside it; the dead remains add humus; minor grasses and ferns grow; bushes and trees follow; plant roots penetrate; burrowing animals turn up particles. Slowly the mass becomes porous and sponge-like, with the capacity to hold water and pass air, and finally a mature soil — a complex mixture of mineral and organic matter — is born. (The five soil-forming factors and the full process are dealt with in Part 2 of this chapter.)