Geomorphic processes and forces
Geomorphic processes are natural processes that change the shape of the Earth's surface. The forces behind them are:
| Endogenic (inside) | Exogenic (outside) |
|---|---|
| Energy: the Earth's inner heat (radioactivity, rotation, tidal friction, heat left from formation) | Energy: the Sun's heat and gravity, working through air and water |
| Build up land: land-building forces | Wear down land: land-wearing forces |
| Diastrophism, volcanism, earthquakes | Weathering, mass movement, erosion, deposition |
| Make the surface uneven (relief) | Try to make the surface level (gradation) |
The two work at the same time. That is why the Earth's surface never becomes fully flat.
Gradation = wearing down of high land (degradation) + filling up of low land (aggradation).
Endogenic processes: diastrophism and volcanism
Diastrophism
All processes that move, lift or build up parts of the crust. They are slow.
- Orogeny (mountain building): severe folding of crust into long mountain belts.
- Epeirogeny (continent building): uplift or sinking of large parts of the crust without much folding.
- Earthquakes: sudden local movements.
- Plate tectonics: horizontal movement of plates.
Diastrophism causes folding (bending), faulting (breaking and moving), and it changes pressure and temperature, which can change rocks (metamorphism).
Volcanism
The movement of molten rock (magma) towards or onto the surface, and the landforms it creates, both inside the crust (intrusive: batholiths, sills, dykes) and on the surface (extrusive: volcanoes, lava plateaus).
Exogenic processes and weathering
Exogenic processes happen because of stress from the atmosphere and gravity. Where rock is under stress (heat, freezing, water), it breaks. Key exogenic processes: weathering, mass wasting, erosion, transport and deposition. Agents: running water, groundwater, glaciers, wind, waves and currents.
Weathering: breaking rock in place
Weathering is the breaking and decaying of rock where it is, with little or no movement. Climate decides which type is strongest.
| Type | How | Example |
|---|---|---|
| Chemical | Water and air change minerals: solution (dissolving), carbonation (rainwater with CO₂ dissolves limestone), hydration (minerals absorb water and swell), oxidation (rusting of iron minerals), reduction | Limestone caves; red rusty rocks |
| Physical (mechanical) | Pressure release (unloading), heating and cooling (expansion and contraction), freezing and thawing of water in cracks (frost wedging), salt crystals growing | Exfoliation: outer layers peel like onion skin in hot deserts |
| Biological | Roots grow into cracks, burrowing animals, humans digging; plant acids and decaying matter help chemical weathering | Tree roots splitting rocks |
Why weathering matters: it makes soil, helps erosion and mass movement, and concentrates useful ores (like bauxite of aluminium) that make mining worthwhile.
Try it at home: put a piece of chalk in vinegar (weak acid). Watch it fizz and dissolve: that is like chemical weathering of limestone.
Mass movements and landslides
Mass movement is the movement of rock, soil and debris down a slope under gravity. It needs no river, wind or ice to carry it. Weathering prepares the material; steep slopes, heavy rain, earthquakes, road cutting and removal of vegetation trigger it.
Slow movements
- Creep: very slow movement of soil; fence posts and trees lean downhill.
- Solifluction: slow flow of water-soaked soil, common in cold regions over frozen ground.
Rapid movements
- Earthflow: water-soaked clay or soil slides down slopes.
- Mudflow: very wet, thick mud racing along valleys, often after heavy rain or volcanic eruptions.
- Debris avalanche: very fast, in narrow, steep valleys of humid regions.
Landslides (fast, relatively dry)
- Slump: a block slips down and rotates backwards.
- Debris slide: debris rolls or slides down quickly.
- Rockslide: blocks slide along a sloping surface.
- Rockfall: rocks fall freely from a steep cliff.
In India the Himalayas and the Western Ghats face many landslides. Ways to reduce them: plant trees and grass, build terraces and retaining walls, avoid cutting steep slopes, and drain water safely.
Erosion and deposition
Erosion is the picking up and carrying away of rock pieces by moving agents: running water, wind, glaciers, waves and groundwater. Weathering helps erosion, but erosion is what really changes landforms. Deposition happens when the agent slows down and drops its load. Low places get filled.
Weathering vs erosion: weathering = break in place; erosion = pick up and carry away.
Soil formation
Soil is a thin, living layer of loose material on the surface, made of mineral bits, humus (decayed plant and animal matter), water and air.
Process
- Weathering breaks the rock into loose material.
- Bacteria, mosses and lichens settle; they add organic matter.
- Plants grow; roots and animals mix the material; decaying leaves add humus.
- Over hundreds to thousands of years, layers called horizons form: A (topsoil with humus), B (subsoil where minerals collect), C (broken parent rock), above the bedrock.
Soil-forming factors
- Parent material (active/passive): the rock the soil comes from decides texture and minerals.
- Climate (active): rain and temperature control weathering and leaching (washing down of minerals).
- Relief / topography (passive): steep slopes have thin soil; flat lowlands collect thick soil.
- Biological activity (active): plants, animals and microbes add humus and mix soil.
- Time (passive): the longer the time, the more mature the soil.
Climate and living things are called active factors; parent rock, relief and time are passive factors.
Key formulas and definitions
- Endogenic = inside, builds up (diastrophism + volcanism); Exogenic = outside, wears down
- Diastrophism = orogeny (mountain building) + epeirogeny (continent uplift/sinking) + earthquakes + plate tectonics
- Gradation = degradation (wearing down) + aggradation (filling up)
- Weathering types: chemical, physical, biological; Exfoliation = peeling of rock layers
- Mass movement = downslope movement under gravity: slow (creep, solifluction) / rapid (earthflow, mudflow, debris avalanche) / landslides (slump, debris slide, rockslide, rockfall)
- Soil horizons: A (topsoil) → B (subsoil) → C (weathered parent rock) → bedrock
- Soil factors: parent material, climate, relief, biological activity, time (CROPT)
Worked examples
1. A limestone hill develops caves after many years of rain. Which process?
Chemical weathering by carbonation and solution: rainwater with dissolved CO₂ forms a weak acid that dissolves limestone.
2. In a hot desert, rocks peel off in curved sheets. Why?
Exfoliation, a physical weathering: strong heating by day and cooling at night make the outer layers expand and contract more than the inside, so they peel off.
3. Fence posts on a hill slowly lean downhill over years. Name the process.
Soil creep, a very slow mass movement under gravity.
4. Classify as endogenic or exogenic: folding, landslide, volcanic eruption, river erosion.
Folding and volcanic eruption are endogenic; landslide and river erosion are exogenic.
5. Why is soil thin on steep slopes but thick in valleys?
On steep slopes, soil is washed and pulled down by gravity and running water before it can build up. It collects in valleys and plains.
Common mistakes
- Calling weathering and erosion the same. Weathering breaks rock in place; erosion carries it away.
- Thinking mass movement needs a river or wind. Gravity alone moves the material.
- Believing endogenic forces are only sudden. Most diastrophism is very slow; earthquakes and eruptions are the sudden ones.
- Thinking soil is just broken rock. Soil also needs humus, air, water and living things, and takes a long time to form.