Chapter 9: Weathering
A boulder sits beside a hill road, high enough that the nights freeze right through winter. No lorry strikes it. No worker touches it. Yet when the road crew comes past in spring, the boulder is lying in two clean pieces, and a scatter of grit has collected at its base. Nothing pushed it over. It came apart where it stood. What happened is easy to miss, because it happened a few hundred times, very slowly. Each afternoon, meltwater seeped into a hairline crack in the rock. Each night the temperature fell below freezing and that water turned to ice. Ice takes up more room than the water it came from, so it pressed outward against the walls of the crack, widening it by a fraction. By morning it melted and drained deeper in. Freeze, thaw, freeze, thaw — one winter’s worth of tiny shoves, and the crack ran right through. A thousand kilometres away, in a hot desert with almost no water at all, boulders split too. Same result, different cause. Rock is being taken apart everywhere, all the time, by the weather itself.
Diagram
Fig. 9.A1 Caption: The boulder split along a crack that widened a little on each freezing night. Brief: Roadside boulder broken into two pieces with grit at its base; label the original hairline crack, the widened split line and the fallen fragments; inset arrow showing the crack line running top to bottom.
9.1 Weathering
The forces that work inside the earth are described as ‘Constructive Forces’, because they build things: they raise mountains, lift plateaus and make new landforms on the surface. But forces also operate on the surface itself, and these work in the opposite direction. They are known as exogenic forces — from exo, the Greek word for outside — and they act on the land from above rather than from below.
These external forces reach the surface through several different processes: weathering, mass wasting, erosion, transportation, deposition and the formation of soils. All of them draw their energy from the sun. Climate, which is the sun’s energy expressed as heat, cold, rain and wind in one place, is therefore the driving agent behind every one of them.
Flowchart
Fig. 9.A2 Caption: The sun powers a chain of surface processes, and weathering is the first link in it. Nodes: energy from the sun → climate acts as the driving agent → exogenic forces work on the surface → weathering breaks the rock where it stands → mass wasting → erosion → transportation → deposition → formation of soils
What Weathering Means
Weathering is the process of breaking down of rocks, but not their removal. It is described as the disintegration or decomposition of a rock in size, by natural agents, at or near the surface of the earth. Every part of that definition is doing work. Disintegration means the rock is broken into smaller pieces; decomposition means the rock is chemically altered. Both happen only at or near the surface, and neither involves carrying the broken material anywhere.
The name itself explains the cause. Rock is broken down like this only where it is exposed to the weather, and it is the change in weather conditions — temperature, moisture, precipitation — that disintegrates or breaks the rock down. Where those conditions do not exist, the process stops.
The moon makes the point exactly. Weathering cannot take place on the moon’s surface, because the weather conditions that drive it do not exist there. This is why the footprints left on the moon by astronauts will last for millions of years, while a footprint in soil on earth is gone in a season. Weathering is thus the basic process and an essential phase of denudation, the wider wearing down and stripping away of the land surface, which the next chapter deals with in full.
Figure 9.1 shows what the difference looks like over long periods. The same mountain range is drawn twice: once with its slopes still sharp, and once after weathering has worked on the exposed rock.
Diagram
Fig. 9.1(a) Caption: A mountain range before weathering. Brief: Photograph of a steep, sharp-crested mountain range with clean unbroken rock faces; label the intact summit and the smooth slope.
Diagram
Fig. 9.1(b) Caption: A mountain range after weathering. Brief: Photograph of the same kind of range with a broken, ragged crest, split blocks and loose debris on the slope; label the shattered rock face and the loose material at the base.
9.1.1 Chief Characteristics Of Weathering
Weathering has a set of features that hold wherever it happens, and they are worth reading as a description of how the process behaves. (a) Weathering involves the disintegration or decay of solid rocks, so what was once one continuous mass ends up as separate, smaller pieces. (b) It depends on climatic elements and on the characteristics of the rocks themselves. A rock’s chemical composition, its hardness, its texture and its permeability — how far it allows water to penetrate — together determine the weathering pattern that develops on it. (c) Weathering affects the surface of the earth, because that is where rock is exposed to the conditions that break it. (d) Weathering causes the formation of soil, since broken and altered rock is the raw material from which soil is built. (e) Weathering involves not one but several processes acting together to cause the disintegration of rocks. (f) Weathering changes hard massive rocks into finer materials, which is why an outcrop that once had to be quarried can end as grit. (g) It prepares rock materials for transportation by the agents of gradation — the agents that level the land by wearing down what is high and filling in what is low.
Read together, points (f) and (g) explain why weathering matters so much. On its own it moves nothing; but by turning solid rock into loose, fine material it hands the running water, ice and wind something they are able to carry.
9.2 Types Of Weathering
Weathering, as already stated, is a static process: the rock is attacked where it lies, and nothing is carried away. The disintegration of rocks may occur through physical forces, through chemical forces, or through the roots of plants and the action of animals and insects. Weathering is therefore of three types — physical, chemical and biological.
The three are separated by what happens to the rock’s chemistry. Physical weathering breaks the rock but leaves its chemical make-up untouched, chemical weathering alters the minerals themselves, and biological weathering is living things doing one or both of these.
Flowchart
Fig. 9.A3 Caption: The three types of weathering, and the named processes that belong to each. Nodes: weathering → physical (mechanical) weathering / chemical weathering / biological (organic) weathering; physical weathering → block disintegration / granular disintegration / frost action / exfoliation; chemical weathering → oxidation / carbonation / hydration / solution; biological weathering → humans / plants / animals and insects
9.2.1 Physical Weathering
Physical weathering, also known as mechanical weathering, involves rock disintegration without any change in the chemical constituents of the rock. A granite boulder reduced to a heap of grit is still granite; only its size has changed.
The main factors responsible for physical weathering are set out below. (a) Temperature changes, which make the rock expand when it is heated and contract when it cools. (b) Moisture, which enters cracks and joints and works on the rock from inside them. (c) Frost action, where water inside the rock freezes and pushes the walls of a crack apart. (d) Wind, which drives loose material against exposed rock faces.
Weathering due to changes in temperature is more rapid in hot deserts. This is because temperature changes are sharpest in a desert, where the ground heats fiercely under a cloudless sky by day and loses that heat just as fast after sunset.
Block Disintegration
The sudden rise and fall of temperature causes the expansion and contraction of rocks, and the repeated action causes their breakdown. At night the temperature suddenly falls and during the day it rises suddenly, so the rock is pulled in one direction and then the other, again and again. This leads to tension and splitting of rocks, known as block disintegration, in which the rock comes apart along its joints into large angular blocks.
Granular Disintegration And Frost Action
Rocks are made up of different minerals, and those minerals do not all respond to heating and cooling in the same way. Weathering may therefore reduce rocks to pieces and fragments rather than to blocks, and this type of disintegration is called granular disintegration, in which the rock crumbles grain by grain.
This disintegration may also take place in cold places and at high altitudes, where a second mechanism takes over. The cracks and joints of rocks are filled with water during the day, and at night the water freezes, leading to the rupture of rocks on account of the repeated actions of melting and freezing. This is called frost action, and Fig. 9.2 shows how it works.
Diagram
Fig. 9.2 Caption: Mechanics of Weathering (Physical Weathering). Brief: Two-stage cross-section of a jointed rock mass; upper stage labelled water collects in cracks with downward arrows into the crack, lower stage labelled water freezes and expands in the cracks with outward arrows widening the crack.
Exfoliation
Sudden temperature changes may also cause fissures in the rock, into which water penetrates, and this in turn stimulates chemical weathering. Most rocks are not good conductors of heat, so a change in temperature reaches only a thin outer skin and leaves the interior at its old temperature. The sudden contraction and expansion of that skin peels off the outer layer, which is known as exfoliation.
Fig. 9.3 sets the three forms of mechanical weathering side by side, so that the shape of the debris each one leaves can be compared.
Diagram
Fig. 9.3 Caption: Types of Mechanical Weathering. Brief: Three labelled panels — (a) block disintegration, a jointed rock breaking into large rectangular blocks; (b) granular disintegration, a rounded rock crumbling into loose grains scattered at its base; (c) exfoliation, a curved outer shell peeling away from a rounded rock core.
9.2.2 Chemical Weathering
In chemical weathering, some or all of the minerals in the rocks suffer decay or alteration by such agents as water, oxygen, carbon or various organic acids. It leads to the decomposition of rocks and minerals, as chemical reactions transform them into different chemical combinations. The rock does not merely break here; it becomes a different substance.
The chemical weathering of rocks is accomplished in a number of ways, and each one is given a name to distinguish the different processes involved. Four main processes are recognised. (a) Oxidation means the reaction of minerals in rocks to atmospheric oxygen, which rainwater also carries down into the rock. On coming into contact with iron-compounds in the rock, the iron starts rusting or crumbling, and the colour of the rock changes to red, yellow or brown. (b) Carbonation occurs when mineral constituents of rocks such as carbon dioxide come into contact with water and produce an acidic effect on rocks. This action dissolves most calcareous rocks, such as gypsum, marble and limestone. (c) Hydration is the process in which expansion of minerals occurs on coming into contact with rainwater, so that the minerals become heavy and start disintegrating. Rocks like feldspar are converted by it into kaolin, a soft white clay. (d) Solution is the process in which minerals in the rocks dissolve in water, and all rocks are subjected to solution to some extent. Some rock minerals, such as salts, are more soluble in water than others, which is why rainwater causes the chemical disintegration of rock-salt and gypsum.
Two Processes Easily Confused
Carbonation should not be mistaken for solution, although both end with material being carried away in water. In the case of carbonation, rainwater converts calcium carbonate into calcium bicarbonate, which is soluble and is taken away by the ground water. A chemical change happens first, and only then does the new compound dissolve; in solution the mineral simply dissolves as it is.
Hydration sits on a similar edge. Its swelling and crumbling can be described as physical or mechanical, but its effect in hydration is a chemical process, because the mineral itself has taken water into its structure and become a new one.
The rate at which solution takes place is affected not only by the mineral composition of the rock but also by its structure. Sedimentary rocks, for example, have pore-spaces between the grains in which air and water can lodge, and the water thus attacks the rock from within rather than only at its outer face.
Table 9.1 sets the mechanical and the chemical routes against each other point by point.
9.2.3 Biological Weathering
Biological weathering is also known as organic weathering. Its main agents are animals, insects, plants and man. In all these cases, however, both physical disintegration and chemical decomposition are involved, or one of the two. This is because all biological matter is made up of organic compounds, which react with the minerals present in rocks.
The living agents work in the following ways. (a) Humans are the most active agent of both physical and chemical weathering of rocks. Mining, excavations and the construction of roads and buildings cause physical disintegration of rocks, while agriculture and the dumping of chemicals and wastes underground promote both physical and chemical weathering. (b) Plants weather rock through their roots. The roots of large trees reach deep into rocks and cause physical disintegration due to the pressure they exert as they thicken. (c) Animals and insects such as rabbits, worms and moles bring large quantities of fine material to the surface. Burrowing animals help to loosen the surface materials around the rocks, facilitating their physical disintegration, and on death the decaying animals also provide many chemicals and acids for rock disintegration.
Plants, though, work in both directions at once. Most vegetation prevents the disintegration of rocks, because it binds the surface layer and does not allow the rock beneath to be exposed to the elements of weathering. A tree root splits the rock it grows into, while the cover of grass and leaves above shields everything around it.
Diagram
Fig. 9.4 Caption: Biological Weathering due to roots. Brief: Photograph of a thick tree root spreading across and into a fractured rock outcrop; label the root, the widened joint it has entered and the loosened rock slabs.
9.3 Weathering In Different Climatic Regions
From the processes of weathering discussed above, it is clear that climate is the most important factor of all. The rock itself matters, but the same rock will be attacked in quite different ways depending on where in the world it is exposed.
Two broad patterns run through everything that follows. Physical weathering is more rapid in desert climates, where temperature swings are extreme and water is scarce, while chemical weathering occurs in moist and cold climates, where water is available to carry the reactions.
The dominant process in each of the world’s climatic regions can be set out region by region. (a) In equatorial climates, near the equator, humidity and temperature are constantly high, and weathering — especially chemical weathering — is more active in these regions. (b) In tropical climates there is a marked dry season and a marked wet season, and the differences in heating lead to a consistently high rate of evaporation. During the wet season, precipitation of oxides of iron and aluminium takes place from rocks, and this promotes the formation of laterite soils. (c) In dry climates or deserts, mechanical weathering is most dominant, because of the high diurnal range of temperature — the gap between the day’s highest and the night’s lowest reading. (d) In mid-latitudes or temperate climates, lying between the tropics and the polar circles, the most powerful agent of weathering is frost action. In limestone areas, ‘Solution’ or chemical weathering operates on a large scale. (e) In polar climates, physical weathering due to frost action is more prevalent, while chemical and biological weathering in these areas is negligible.
Flowchart
Fig. 9.A4 Caption: The same rock is broken down differently depending on the climate it is exposed to. Nodes: climate of the region → equatorial: constantly hot and humid → chemical weathering dominant; tropical: wet and dry seasons → chemical weathering, oxides of iron and aluminium precipitated → laterite soils; dry or desert: high diurnal range → mechanical weathering dominant; temperate: winter freezing → frost action, and solution on limestone; polar: permanently cold → frost action dominant
A Recent Change In The Polar Picture
The last of these has been questioned. Recent studies have shown that the melting water of polar regions contains more carbonic acid — the weak acid that forms when carbon dioxide dissolves in water. This is because carbon dioxide is more soluble at low temperatures than at high temperatures, so cold meltwater holds more of it than warm water can.
This suggests that chemical weathering is more active in polar regions than was once thought. The general rule that cold means physical weathering still holds, but the picture underneath the ice is not as chemically dead as the older account assumed.
9.4 Summary
- The forces working inside the earth are called ‘Constructive Forces’ because they build landforms, while exogenic forces work on the surface from outside and take them apart.
- Exogenic forces act through weathering, mass wasting, erosion, transportation, deposition and the formation of soils. All of them draw their energy from the sun, and climate is the driving agent.
- Weathering is the breaking down of rocks but not their removal — the disintegration or decomposition of a rock in size by natural agents at or near the surface of the earth.
- Weathering happens only where rock is exposed to changing weather conditions such as temperature, moisture and precipitation. It cannot take place on the moon, which is why astronauts’ footprints there will last for millions of years.
- The chief characteristics of weathering describe how the process behaves.
- It involves the disintegration or decay of solid rocks.
- It depends on climatic elements and on the rock’s composition, hardness, texture and permeability.
- It affects the surface of the earth and causes the formation of soil.
- It uses several processes together, not one.
- It changes hard massive rocks into finer materials and prepares them for transportation by the agents of gradation.
- Weathering is of three types — physical, chemical and biological.
- Physical or mechanical weathering breaks rock without any change in its chemical constituents, and its main factors are temperature changes, moisture, frost action and wind.
- Block disintegration is the splitting of rock into blocks by the tension set up as sudden heating and cooling expand and contract it.
- Granular disintegration crumbles rock grain by grain, because its different minerals do not respond alike.
- Frost action ruptures rock when water in its cracks and joints freezes and melts repeatedly.
- Exfoliation peels off the outer layer, because rock conducts heat poorly and only its skin expands and contracts.
- Chemical weathering is the decay or alteration of minerals by water, oxygen, carbon and organic acids, and it works through four processes.
- Oxidation rusts the iron-compounds in rock and turns its colour red, yellow or brown.
- Carbonation makes rainwater acidic enough to dissolve calcareous rocks such as gypsum, marble and limestone, converting calcium carbonate into soluble calcium bicarbonate.
- Hydration swells minerals that take in rainwater, converting rocks like feldspar into kaolin.
- Solution dissolves soluble minerals directly, and works faster where a rock’s structure gives water pore-spaces to lodge in.
- Biological or organic weathering is carried out by humans, plants, animals and insects, and involves physical disintegration, chemical decomposition, or both.
- Humans are the most active agent, through mining, excavation, construction, agriculture and the dumping of chemicals and wastes.
- Tree roots split rock by pressure, but most vegetation protects rock by binding the surface layer above it.
- Burrowing animals loosen surface material, and decaying animals supply chemicals and acids.
- Climate decides which type of weathering dominates in a region: chemical weathering in the constantly hot and humid equatorial belt, chemical weathering and laterite soil formation in the tropics, mechanical weathering in deserts with their high diurnal range, frost action and limestone solution in temperate mid-latitudes, and frost action in polar climates.
- Recent studies show that polar meltwater carries more carbonic acid, because carbon dioxide dissolves more readily in cold water, suggesting that chemical weathering is more active in polar regions than was assumed.
Flowchart
Fig. 9.A5 Caption: Exposed rock is broken and altered where it stands, and the loose material it yields is what everything else can then carry away. Nodes: solid rock exposed at the surface → changing weather conditions of temperature, moisture and precipitation act on it → physical weathering: broken smaller, chemistry unchanged / chemical weathering: minerals decayed and new ones formed / biological weathering: living things doing both → rock becomes finer, softer and greater in surface area → more surface exposed → back to weather conditions acting on it; and finer material → formation of soil, and material prepared for transportation by the agents of gradation
What This Chapter Cannot Tell You
The chapter names the processes but gives no rates, so it cannot say how long any of this takes — whether a boulder splits in one winter or in a thousand. It also treats the three types separately for teaching, while on a real rock face they act together and their effects cannot be told apart by eye.
The climate-by-climate account works at the scale of a whole region, and a region is a coarse unit. Inside a desert, a shaded north-facing rock and a sunlit one weather differently, and the regional label hides that difference completely.
9.5 Check Your Concepts
Set out, in order, the steps by which frost action ruptures a rock at a high altitude, beginning with water entering the cracks and joints. Then state what would happen to the process if the night temperature never fell below freezing.
Row 4 of Table 9.1 states that mechanical weathering increases the surface area of rocks. Using this, explain how physical weathering makes chemical weathering easier, and name one process from each type that could act on the same rock face.
The chapter links the formation of laterite soils to tropical climates rather than to equatorial or desert climates. Using what the chapter says about the tropical wet and dry seasons, explain why.
The chapter states that chemical weathering is negligible in polar climates, and then reports that polar meltwater carries more carbonic acid. Explain how both statements can stand together, and state one thing the evidence about meltwater still does not tell us.
Distinguish between carbonation and solution.
Define weathering and describe the chief characteristics of weathering.
Give a geographical reason for each of the following:
- Physical weathering is also called mechanical weathering.
- Physical weathering is more rapid in desert climates.
- Chemical weathering occurs in moist and cold climates.
Draw a well labelled diagram to show the process of physical weathering.
Describe chemical weathering, mentioning the processes involved in it.
Give a geographical reason for each of the following:
- Biological weathering is also known as organic weathering.
- Vegetation prevents disintegration of rocks.
- Chemical and biological weathering is negligible in polar climates.