Weathering, erosion & soil

Where rock meets air, water and life, it breaks down. The pieces are carried downhill and out to sea, the residue becomes soil, and along the way the chemistry quietly draws carbon out of the sky.

By the end of this module, you will be able to
  • Tell physical weathering from chemical weathering, and explain why each speeds up the other
  • Name the six things that control how fast rock weathers, and see why grinding rock matters so much
  • Read the weathering equation that turns CO2 into dissolved bicarbonate
  • Explain how water, wind, ice and gravity move sediment, and why grains get smaller, rounder and better sorted with distance
  • Follow sediment from mountain to deep sea, using the Ganges–Brahmaputra as the example
  • Read a soil profile and list the five factors that make one soil different from another

Three words get muddled, so let's pin them down. Weathering is rock breaking down where it sits. Erosion is the removal of that broken material, by water, wind, ice or gravity. Deposition is where it finally settles out again, as new sediment. Weathering makes the pieces; erosion moves them; deposition stores them. Together they are the surface half of the rock cycle.

Two ways to break a rock

Physical weathering

Breaking rock into smaller pieces without changing its chemistry. Examples include frost wedging, roots prying cracks apart, and grains rubbing together in rivers. Smaller pieces have more surface area, which speeds up chemical attack.

Chemical weathering

Changing the minerals themselves. Rainwater is weakly acidic because it dissolves CO2 to make carbonic acid. Soil water is more acidic still, thanks to roots and microbes. This acid slowly dissolves silicate minerals, turning feldspar into clay and freeing calcium and magnesium.

The two work as a team. Physical weathering opens cracks and exposes fresh surfaces for chemistry to attack. Chemical weathering weakens grain boundaries, so the rock crumbles more easily.

What controls how fast rock weathers

Chemical weathering is a reaction between water and mineral surfaces, so anything that changes how much surface is exposed, how reactive it is, or how quickly fresh water reaches it changes the rate. Six controls matter most.

Mineralogy

What the rock is made of

Minerals that crystallised hot and early, like olivine and calcium-rich feldspar, are the least stable at the surface and dissolve fastest. Quartz barely dissolves at all. See Minerals & lithology.

Surface area

How finely broken it is

Reactions only happen at surfaces. The same mass of rock as fine powder exposes vastly more surface than a boulder, so it reacts far faster (worked example below).

Temperature

How warm it is

Chemical reactions speed up with heat. As a rough rule of thumb, silicate weathering rates roughly double for every 10 °C of warming, which is why the tropics weather fastest.

Water

How wet, and how flushed

No water, no reaction. Water also has to keep moving: if it sits still, dissolved products build up and the reaction slows towards a standstill. Rain that flushes through soil keeps it going.

Biology

Roots, fungi and microbes

Roots and microbes breathe out CO2, so soil air often holds 10–100 times more CO2 than the atmosphere. Roots and mycorrhizal fungi (fungi that partner with roots) also release organic acids and physically grip grains to extract nutrients.

pH

How acidic the water is

pH measures acidity: lower is more acidic. Most silicate minerals dissolve faster in more acidic water, because hydrogen ions attack the bonds holding the mineral together.

halve edges halve again 6 cm² 1 cube, 1 cm across 12 cm² 8 cubes, 0.5 cm across 24 cm² 64 cubes, 0.25 cm across
Original outside facesNew faces made by cutting
The same amount of rock, cut smaller and pulled apart so you can see inside. Every cut creates two new faces (purple), so each time the edges are halved the total surface area doubles. The volume never changes.
Worked example

Take a 1 cm cube of basalt. It has 6 faces of 1 cm², so 6 cm² of surface.

Now grind it into cubes 1 µm (one micrometre, a thousandth of a millimetre) across. Each edge is now 10,000 times shorter, so you get 10,000 × 10,000 × 10,000 = 1012 (a million million) tiny cubes. Each has a surface of 6 × (0.0001 cm)² = 6 × 10−8 cm². Multiply: 1012 × 6 × 10−8 = 60,000 cm², about 6 m², roughly the floor area of a small bathroom.

That is 10,000 times more surface from the same rock. The rule is simple: surface area goes up in proportion to how many times smaller you make each grain. Real grains are not cubes and real powders have a mix of sizes, but the principle holds.

Try it · grind the rock finer

Pick a grain size and see what happens to one gram of basalt cut into perfect cubes of that size. Basalt weighs about 2.9 g per cm³.

gravel sand silt clay 10 mm 1 mm 100 µm 10 µm 1 µm finer → (a human hair is ~50–100 µm thick) 100 µm
Grainsin one gram
Surface areaper gram (smooth cubes)
Time to dissolverelative, same conditions

Geometry only. Surface area per gram is 6 ÷ (density × size), so it rises tenfold for every tenfold cut in size. If every square centimetre of surface dissolves at the same speed, a grain's surface retreats at a steady rate, so the time to dissolve it completely is proportional to its size. Real grains are rough and irregular and come in a mix of sizes, so measured surface areas are usually higher than this smooth-cube estimate, and real dissolution slows as the easy surfaces go and the soil water fills with products. How a real powder dissolves over months and years is the subject of The chemistry of enhanced weathering; the energy cost of grinding finer is covered in From quarry to field.

The carbon-removal reaction

Here is chemical weathering of a simple calcium silicate, written as an equation:

CaSiO3 + 2CO2 + 3H2O → Ca²⁺ + 2HCO3⁻ + H4SiO4

In words, one unit of calcium silicate plus two molecules of CO2 plus water gives dissolved calcium, two bicarbonate ions and dissolved silica. The CO2 is now locked up as bicarbonate in water. Rivers carry it to the ocean, where it can stay for more than 10,000 years. Magnesium silicates react the same way.

Some of the calcium may instead come out of solution as solid carbonate mineral (calcium carbonate, the stuff of limestone), in soils, rivers or the sea. That still stores carbon, but only half as much: building calcium carbonate from one calcium ion and two bicarbonate ions puts one of the two CO2 molecules back into the air.

Ca²⁺ + 2HCO3⁻ → CaCO3 + CO2 + H2O
CO₂ H⁺ Ca²⁺ HCO₃⁻ Ca²⁺ + HCO₃⁻ CO₂ River Ocean HCO₃⁻ 1 2 3 4 5 6 7
  1. Rain picks up CO₂. Rainwater dissolves a little CO₂ from the air and becomes weak carbonic acid.
  2. Soil adds more. Roots and microbes breathe out CO₂, so soil water is more acidic still.
  3. The acid attacks a grain. Hydrogen ions (H⁺) break the bonds of a silicate grain, here a speck of rock powder, and free its calcium and magnesium.
  4. Carbon now travels as bicarbonate. Each Ca²⁺ in the draining water is balanced by two HCO₃⁻ ions.
  5. Rivers carry it to the sea.
  6. The ocean stores it for more than 10,000 years. A small share, roughly 10–20%, returns to the air as seawater chemistry adjusts.
  7. A possible detour. If solid carbonate forms in the soil or a river, it keeps one CO₂ per calcium and releases the other.
The carbon-removal pathway, from sky to sea (not to scale). Enhanced rock weathering adds step 3 on purpose, by spreading crushed rock (grey specks) on fields. Measuring removal means following the products along this path: see Measuring removal (MRV).

Chemists call the dissolved bicarbonate (and related ions) alkalinity: the water's capacity to neutralise acid. Positively charged ions like Ca²⁺ and Mg²⁺ are called cations. For every cation the rock releases, the water has to carry matching negative charge, and in natural waters that is mostly bicarbonate. So counting cations is a way of counting captured carbon.

The same reaction runs Earth's slow natural thermostat (first met in Deep time). Volcanoes keep adding CO2. If it builds up, the planet warms and gets wetter, weathering speeds up and draws more CO2 down, and the climate cools again. If the planet cools, weathering slows and volcanic CO2 builds back up. This kind of self-correcting loop is called a negative feedback. It works over hundreds of thousands of years, far too slowly to cancel today's emissions, but over geological time it has kept Earth habitable. You can run the thermostat yourself in A brief history of Earth.

More CO₂ in the air traps more heat Planet warms warmer and wetter Weathering speeds up more CO₂ captured CO₂ falls, planet cools weathering slows again A balancing loop acts over hundreds of thousands of years volcanoes add CO₂
Earth's weathering thermostat. Each step pushes the next, and going round the loop undoes the original change, so the climate is nudged back towards balance. Temperature drives it twice over: warmth speeds up the reaction itself, and a warmer world is a wetter one, flushing more water through soils.
Try it · climate and weathering speed

Move the temperature and rainfall, or pick a place, to see roughly how fast silicate rock weathers compared with a cool, moderately wet reference (15 °C, 1,000 mm of rain a year). Stronger colour means faster.

0102030 °C 1,0002,0003,0004,000 Mean annual temperature → Rainfall, mm a year → slowerfaster

Illustrative only. The model uses two rough rules: the rate doubles for every 10 °C of warming (real minerals range from about 1.5 to 3 times), and more rain helps, with diminishing returns, because it flushes away the products. The places use approximate annual averages. It ignores evaporation, irrigation (which matters a lot in Punjab), soil depth, how much fresh mineral is left, pH and biology, any of which can change real field rates many times over.

Why this matters at Alt Earth

Enhanced rock weathering applies everything in this section. We choose a fast-weathering mafic rock (see Minerals & lithology), crush it to a fine powder to increase surface area, and spread it on soils where warm temperatures, rain, roots and microbes drive the reaction above. The Enhanced rock weathering part of the course covers each step in depth.

Proving the carbon was removed means measuring the reaction's products: the cations and alkalinity that appear in soil water, drainage and rivers, and the rock that has disappeared from the soil. Soil pH matters in both directions. Basalt neutralises acid, so it can raise the pH of acidic farm soils, which can benefit crops. But a higher pH also slows further dissolution, and if a strong acid (for example from nitrogen fertiliser) rather than carbonic acid dissolves the rock, the cations are released without capturing CO2.

Weathering matters in mineral exploration too. It can enrich metals near the surface, as in supergene copper, or hide a deposit beneath a thick weathered layer. Soil geochemistry surveys depend on knowing how weathering has moved metals around.

Erosion and transport

Once rock is broken, something has to move the pieces. Four agents do almost all the work.

Water

Rivers, rain and waves

By far the most important on Earth. Rivers roll boulders, bounce sand along the bed, carry silt and clay in suspension and carry dissolved ions invisibly. Monsoon floods do much of a year's work in a few weeks.

Wind

Dust and dunes

Air is thin, so wind can only lift sand and dust. It matters most in dry places like the Thar Desert, and it can carry fine dust across oceans.

Ice

Glaciers

Glaciers are slow but powerful. They grind bedrock into fine "rock flour" and carry everything from clay to house-sized boulders, dumping it unsorted when they melt.

Gravity

Landslides and creep

Mass wasting is gravity moving material downslope directly: rockfalls, landslides and slow soil creep. In steep, wet mountains like the Himalaya, landslides deliver much of the sediment that rivers then carry away.

How fast must water flow to move a grain? You might guess that the smaller the grain, the slower the water needs to be. That's true for keeping grains moving, but not for picking them up. In the 1930s the Swedish geographer Filip Hjulström plotted this, and the result is now called the Hjulström curve.

Try it · the Hjulström curve

Set the speed of the river and see what happens to five grain sizes on its bed. Both scales are logarithmic: each step along an axis is ten times the one before.

0.11101001,000 0.0010.010.11101001,000 claysiltsandgravel, cobbles, boulders Grain size, mm (log scale) → Water speed, cm/s (log scale) → Erosion Transport Deposition 30 cm/s Riverbed

    Schematic, with approximate values: the real speeds depend on water depth, grain shape and density, and the erosion line is really a broad band. Above the upper (purple) line, flowing water lifts grains off the bed (lifted). Between the lines, grains already moving keep moving, but grains resting on the bed stay put (carried means "kept moving if already in motion"). Below the lower (blue) line the water is too slow to carry them and they settle. The surprise is on the left: clay is so fine and sticky that its particles cling together, so it takes faster water to erode clay than sand, even though once lifted, clay stays suspended in almost still water.

    Journeys change the grains themselves. Each knock in a river chips off corners, so grains become rounder the further they travel. And because faster water carries bigger grains, a river naturally separates sizes as it slows: boulders drop out first, then gravel, then sand, then silt. A deposit of grains that are all about the same size is called well sorted. The mix of minerals changes too. Soft rock fragments and feldspar break up or weather to clay along the way, while tough, stable quartz survives (the stability ladder from Minerals & lithology). A sand that is almost pure quartz is called mature.

    Mountain stream Upland river Lowland river Beach or delta angular, mixed sizes corners wearing off rounded, similar sizes small, rounded, sorted almost all quartz a mix of minerals distance travelled from the source rock
    QuartzFeldsparRock fragments
    Three clues in every sandstone. Angular, poorly sorted grains mean a short, violent trip, such as a flash flood near a mountain front. Rounded, well-sorted grains mean a long journey or many years of reworking by waves or wind. And a sand that is almost pure quartz has been weathered and worn for a long time.

    Source to sink: where sediment ends up

    Geologists follow sediment from its source (where rock is eroded) to its sink (where it is finally buried). Each stop along the way is a depositional environment with its own typical sediment. Learn to recognise them and an old rock tells you what the landscape looked like when it formed.

    sea level turbidity currents Mountains Rivers Floodplain Delta Shelf Deep-sea fan
    Mountains:
    bedrock, boulders
    Rivers:
    gravel, sand
    Floodplain:
    silt, clay, sand
    Delta:
    sand, mud
    Shelf:
    fine mud
    Deep-sea fan:
    sand and mud in layers
    Not to scale. Grains get finer and better sorted downstream. Rivers drop their coarsest load where they leave the mountains and spread finer silt and clay across the floodplain in floods. At the coast the current dies and a delta builds out into the sea. Mud drifts across the shelf. Periodically, sediment piled at the shelf edge collapses into a turbidity current, a dense underwater avalanche of muddy water that races down the slope and spreads a layer of sand and mud across the deep-sea floor.

    India has the world's best example on its doorstep. The Himalaya are rising fast and eroding fast, helped by monsoon rain and landslides. The Ganges and Brahmaputra rivers carry the debris east across the plains of north India and Bangladesh, building the Ganges–Brahmaputra delta, the largest delta on Earth, home to the Sundarbans mangroves. From there, sediment funnels through a submarine canyon into the Bay of Bengal and builds the Bengal Fan, the largest submarine fan on Earth.

    Himalaya Tibet Ganges Brahmaputra India Deccan Kerala Delta Bengal Fan ~3,000 km Equator 1,000 km
    Simplified map: real coastlines and river courses, with the fan outline approximate. The Ganges drains the southern side of the Himalaya; the Brahmaputra drains Tibet, loops round the eastern end of the range and joins it in Bangladesh. At the coast, sediment is funnelled down a submarine canyon (short line at the delta front) and spread across the floor of the Bay of Bengal as the Bengal Fan (sand colour).
    ~1 billiontonnes of sediment a year carried by the Ganges and Brahmaputra, among the most of any river system
    ~3,000 kmlength of the Bengal Fan, stretching south from Bangladesh to beyond the equator
    up to ~16 kmthickness of sediment in the fan, mostly eroded from the Himalaya since India hit Asia
    The Bengal Fan is a mountain range, taken apart grain by grain and laid flat on the ocean floor.

    Reading a soil profile

    Soil is the top layer of the weathering story: broken-down rock mixed with organic matter, water, air and life. Dig a pit and you'll usually see distinct layers called horizons.

    1. O · Organic layertop few cmLeaf litter and decaying plant matter
    2. A · Topsoiltypically ~10–30 cm thickDark, rich in organic matter and life; where most roots liveEnhanced weathering: rock powder is spread on the surface and usually mixed into this layer, where roots, microbes and water are most active.
    3. E · Leached layer (eluviation)~10–30 cm; often absentPale: water has washed clay and iron downward
    4. B · Subsoiltens of cm to about 1 mWhere clay, iron and minerals washed down from above accumulateSome released calcium and magnesium can be held on clay surfaces here, delaying their journey.
    5. C · Weathered parent rockup to several metresBroken, partly altered bedrockDissolved cations and bicarbonate drain on towards groundwater, field drains and rivers.
    6. R · Bedrockfrom under 1 m to many metres downSolid, unweathered rock
    Rock powder (enhanced weathering)Dissolved Ca²⁺, Mg²⁺ and bicarbonate draining down
    A typical soil profile. Thicknesses are rough and vary hugely from place to place, and not every soil has every horizon. For enhanced weathering, the profile is the reaction vessel: the rock goes in at the top, and the products have to be tracked on the way down.

    That downward journey is why measuring removal samples soil, soil water and drainage at different depths: some cations are held on clay or taken up by crops on the way, and only what reaches groundwater and rivers as bicarbonate is on its way to long-term storage. In warm, wet tropical climates, intense weathering can strip a soil down to deep red, iron- and aluminium-rich layers called laterite; you will meet one below.

    Why soils differ: five factors

    Why is the soil on the Deccan Traps black and sticky while the soil a few hundred kilometres away in Kerala is red? In the 1940s the soil scientist Hans Jenny argued that every soil is the product of five factors, remembered by the acronym CLORPT.

    FactorWhat it doesExample
    ClimateSets temperature and rainfall, and so the speed of chemical weathering and how much is leached awayHot, wet climates leach soils deeply and can leave red laterites
    OrganismsPlants, fungi, microbes and burrowing animals add organic matter, acids and CO2, and mix the soilForest soils differ from grassland soils on the same rock
    ReliefSlope and position: steep slopes lose soil downhill, hollows collect it and stay wetThin soils on ridges, thick soils in valleys
    Parent materialThe rock or sediment the soil formed from sets its starting minerals and chemistryBasalt weathers to clay-rich, often fertile soils; granite gives sandier soils
    TimeSoils mature over thousands to millions of years, developing deeper, more distinct horizonsYoung river-deposited soils versus ancient plateau soils

    The Deccan's black soils, called regur or black cotton soil, formed on basalt in a climate with a long dry season, which keeps the clays that swell when wet and crack when dry. Where rainfall is heavier and the wet season longer, as along the Konkan and Kerala coasts, rocks (in places the Deccan basalt itself) weather much further, losing even more of their soluble elements and leaving red soils and laterite. Parent material and climate are both at work.

    Regur (black cotton soil), Deccan

    1. Black cracking claysurface to ~1 m or moreSticky clay that swells when wet and cracks open when dry. Topsoil falls into the cracks, so the soil churns itself and few separate layers form.
    2. Lime nodules (kankar)around 1–1.5 mLumps of calcium carbonate left where soil water evaporates in the dry season
    3. Weathered basalt (murrum)metre or twoCrumbly, partly altered rock
    4. Deccan basaltFresh lava-flow rock

    Laterite profile, Kerala

    1. Thin topsoiltop ~10–30 cm
    2. Laterite~1 to several metresRed-brown, rich in iron and aluminium oxides; hardens in air and is cut into building blocks
    3. Mottled zonemetresRed and pale patches where iron has moved and gathered
    4. Pallid zonemetresAlmost white clay, stripped of iron and nearly every soluble element
    5. Saproliteup to tens of metresRock rotted in place: keeps its original texture but crumbles in your hand
    6. BedrockOften ancient gneiss in Kerala; Deccan basalt along parts of the Konkan coast
    Two soils from the same subcontinent, drawn to different depth scales: a regur profile is typically a metre or two deep, while a laterite profile can reach tens of metres. Regur keeps much of its basalt's calcium and magnesium in clay and nodules. Laterite has lost almost everything soluble, leaving iron and aluminium behind. The name comes from the Latin later, a brick, after the blocks cut from it on India's south-west coast.