2.2.1 geomorphic processes

What are geomorphic processes?

Natural mechanisms of weathering, erosion and deposition that result in the modification of sediment and landforms on Earth.

What is regolith?

By-products of weathering of the parent rock, including broken rocks and other minerals.

What are the 3 main rock types and how are they formed?

• Igneous → crystallisation + solidification of magma/lava

• Sedimentary → deposition → compaction + cementation → lithification

• Metamorphic → existing rocks altered by heat and/or pressure without melting

How do intrusive and extrusive igneous rocks differ?

• Intrusive: slow cooling underground → large crystals, coarse-grained

• Extrusive: rapid cooling at surface → small crystals, fine-grained

What are the 3 types of sedimentary rocks?

• Clastic/mechanical → deposited rock fragments

• Chemical → precipitation/crystallisation of dissolved minerals

• Organic → accumulated organic material

What is weathering and how does it differ from erosion?

Weathering = alteration of the physical/chemical nature of rocks in situ.
Erosion = removal and transport of material.

What is chemical weathering?

Breakdown of rocks through alteration of their chemical composition by agents such as water, oxygen and acids.

How does solution/dissolution occur and what controls it?

How does carbonation weather rocks such as limestone?Soluble minerals dissolve in water/acidic water → solution.

Rate ↑ with:

• ↑ surface area : volume ratio

• ↑ mineral solubility

• Suitable pH

How does carbonation weather rocks such as limestone?

CO₂ + H₂O → carbonic acid (H₂CO₃)
→ reacts with CaCO₃
→ soluble bicarbonates form
→ minerals removed in solution.

Note: carbonation can be greater at lower temperatures because CO₂ is more soluble in cold water.

How does hydrolysis weather and produce corestones?

Water reacts with feldspar → clay minerals (e.g. kaolinite) + dissolved materials → rock weakens.

In jointed granite:
water enters joints → joint intersections/corners weather fastest → edges become rounded → corestones form.

How do oxidation and reduction differ?

Oxidation: oxygen added → oxides/hydroxides form; expansion may stress and weaken rock.

Reduction: oxygen removed under anaerobic/waterlogged conditions → compounds become more soluble/mobile.

What is physical weathering?

Breakdown of rocks by physical/mechanical forces into smaller units without significant chemical alteration.

How does pressure-release weathering occur?

Erosion/uplift removes overlying material
→ pressure ↓
→ rock decompresses + expands
→ tensile stress
→ joints/fractures form.

May cause exfoliation, where outer rock layers peel off parallel to the surface.

How does freeze-thaw weathering occur and when is it most effective?

Water enters cracks → freezes → expands by ~9% → pressure widens cracks → repeated cycles break rock apart.

Most effective when:

• Temperature repeatedly crosses 0°C

• Moisture is available

• Existing cracks/weaknesses are present

How does salt weathering occur and where is it most effective?

Saline water enters pores → evaporation → salt crystals form/grow → pressure on rock → disintegration.

Most effective in hot arid environments due to high evaporation + low rainfall.

How does thermal weathering occur and what affects its effectiveness?

Repeated heating/cooling → expansion + contraction → stress → cracking.

Favoured by:

• Large diurnal temperature range

• Little cloud cover

• Sparse vegetation

• Dark/heterogeneous rocks

Evaluation: heating/cooling alone may cause little breakdown; moisture + chemical weathering can enhance it.

What are the main natural controls of weathering?

Climate, vegetation, topography, rock characteristics, altitude and aspect.

How does vegetation influence weathering?

• Decomposition → organic acids → chemical weathering ↑

• Vegetation → infiltration ↑ → water for chemical weathering ↑

• Roots widen cracks → root wedging

How do temperature and precipitation control weathering?

↑ temperature → chemical reactions ↑.
Van’t Hoff’s Law: reaction rate ↑ about 2.5× per 10°C rise.

↑ precipitation → more water/reactants → generally chemical weathering + weathering depth ↑.

What does Peltier’s Diagram show and what are its limitations?

Shows how temperature + precipitation affect weathering intensity.

• Warm + wet → strongest chemical weathering

• Physical weathering in the model considers ONLY freeze-thaw

Limitations:

• Does not account for other physical weathering processes

• Does not fully account for local factors such as water chemistry/pH and mineral-specific responses

What does Strakhov’s Model show about climate and regolith depth?

↑ temperature + ↑ precipitation → deeper regolith

• Humid tropics → deepest regolith

• Deserts → shallow regolith

Exception: deep regolith may develop where relief is gentle because weathered material is not efficiently removed.

How does slope steepness influence weathering?

Gentle slopes: regolith accumulates → protects underlying rock → further weathering may slow.

Steep slopes: mass movement removes regolith → exposes fresh rock → continued weathering.

What 3 rock characteristics control weathering?

• Strength/hardness

• Chemical composition

• Rock structure

How do rock strength and chemical composition affect weathering?

Hard/strong rocks → generally more resistant to physical weathering.

More reactive minerals → greater chemical weathering.

Basic minerals generally more reactive than acidic minerals.

Exception: feldspar is susceptible to hydrolysis.

How does rock structure affect weathering?

Joints + bedding planes are zones of weakness.

More joints
→ surface area ↑ + water/air penetration ↑
→ chemical + physical weathering ↑.

How do altitude and aspect influence weathering?

Altitude:

• Higher altitude → lower temperatures → freeze-thaw ↑ where temperatures repeatedly cross 0°C.

• Extremely high altitude → temperatures may stay below 0°C → freeze-thaw ↓.

Aspect: slope orientation controls exposure to sun, wind and precipitation.

In the Northern Hemisphere:

• North-facing slopes → less direct sunlight → cooler + moister → greater freeze-thaw potential.

• South-facing slopes → more direct sunlight → warmer + drier → greater thermal weathering potential.

How can human activities increase weathering?

• Tourism: physical damage + erosion expose rock

  • CS: Uluru, Australia → tourists climbing and urinating on sandstone contributed to weathering.

• Construction: mining, quarrying and cutting slopes exposes rock; explosives create fractures

• Agriculture: irrigation/tilling alter infiltration

• Deforestation: solar exposure + temperature fluctuations ↑ → thermal weathering ↑

• Pollution: acid rain → carbonation/hydrolysis ↑

How does the Taj Mahal demonstrate human-enhanced weathering?

Acid rain → marble discolouration + pitting + weakening.

Acid rain can reach pH 4.2, compared with normal rain at ~pH 5.6.

How does silicate weathering regulate atmospheric CO₂?

Atmospheric CO₂ dissolves in water → carbonic acid
→ reacts with silicate rocks
→ bicarbonates transported to oceans
→ incorporated into sediments/limestone
→ CO₂ stored long-term.

Why can natural silicate weathering not offset current anthropogenic CO₂ emissions?

Silicate weathering/carbon burial is too slow.

Fossil-fuel CO₂ emissions occur hundreds–thousands of times faster than natural carbon burial.

Enhanced weathering: crush + spread silicate rocks → surface area ↑ → weathering ↑ → atmospheric CO₂ removal ↑. 2.2.1 geomorphic processes