Geography (9696) | Rocks and weathering

🌍 Core Physical Geography: Rocks and Weathering Study Notes

Welcome to the fascinating world of Rocks and Weathering! This chapter is foundational to understanding how the Earth's surface has been shaped over millions of years and how the landscapes we see every day are constantly changing.

Don't worry if the processes sound complicated—we'll break them down into simple steps. By the end, you'll understand everything from continental drift to why your local statue looks worn down!

Quick Review: Defining Key Terms

• Weathering: The breakdown of rocks in situ (in their original place) by physical, chemical, or biological processes. It does not involve movement.
• Erosion: The removal and transport of weathered material by agents like wind, water, or ice.
• Mass Movement: The bulk transfer of material downslope under the direct influence of gravity.

1. Plate Tectonics: The Earth's Moving Foundation (Syllabus 3.1)

Before rocks can weather, they need to be formed, uplifted, and exposed. This all starts with Plate Tectonics—the theory that the Earth's outer layer (the lithosphere) is broken into large slabs called tectonic plates that float and move slowly on the semi-molten mantle (the asthenosphere).

Nature of Tectonic Plates and Global Patterns

• Plates move due to convection currents within the mantle, driven by heat from the Earth's core.
• Plates can be oceanic (denser, thinner, made of basalt) or continental (less dense, thicker, made of granite).

Types of Plate Boundaries

The movement of plates means they interact at their boundaries. There are three main types, each associated with specific processes and landforms.

1. Divergent (Constructive) Boundary:
   

   Plates move apart. New crust is created as magma rises to fill the gap.
   Example: Mid-Atlantic Ridge.

   Processes and Landforms: Sea floor spreading, volcanism, and the formation of ocean ridges (underwater mountain ranges).

2. Convergent (Destructive) Boundary:
   

   Plates move towards each other, resulting in crust being destroyed.

   • Oceanic-Continental: The denser oceanic plate is forced under the continental plate (subduction). Forms deep ocean trenches and volcanic island arcs (on the continental plate).
   • Continental-Continental: Neither plate subducts easily; they collide and buckle, creating massive mountain ranges. Forms fold mountain building (e.g., the Himalayas).
3. Conservative Boundary:
   

   Plates slide past each other, neither creating nor destroying crust. Example: San Andreas Fault, USA.

   Process and Landforms: Intense friction builds up stress, released as powerful earthquakes. No significant volcanism.

Key Takeaway: Plate tectonics is the engine of the Earth, responsible for uplift, mountain building, and creating the rocks that are later subjected to weathering.

2. Weathering: Breaking Down the Rocks (Syllabus 3.2)

Weathering is the vital first step in shaping landscapes. It transforms solid rock into loose material (regolith) that can then be eroded or moved downslope.

2.1 Physical (Mechanical) Weathering Processes

These processes break rocks into smaller pieces without changing their chemical composition. Think of it like smashing a glass bottle—it's still glass, just smaller fragments.

1. Freeze-Thaw (Frost Shattering):
   

   Water seeps into cracks (fissures) in rocks. When the temperature drops below freezing (0°C), the water turns to ice and expands by about 9%. This expansion exerts immense pressure on the rock walls, widening the crack. Over many cycles, pieces break off.

   Where it happens: Mountainous or high-latitude regions where temperatures frequently fluctuate around freezing point (diurnal range).

2. Heating/Cooling (Insolation Weathering):
   

   In deserts, extreme temperature changes between day and night cause the outer layers of the rock to heat up and expand, and then cool down and contract. Since rock is a poor conductor of heat, the outer layers move more than the inner layers, causing stress. This leads to exfoliation (peeling off) or granular disintegration.

3. Salt Crystal Growth:
   

   Water containing dissolved salts evaporates on rock surfaces, leaving salt crystals behind, often in pores or cracks. As these crystals grow over time, they exert pressure, forcing the rock apart (similar to freeze-thaw).

   Where it happens: Coastal areas or hot, arid environments where high evaporation rates occur.

4. Pressure Release (Dilatation):
   

   When overlying material (like ice or other rocks) is removed by erosion, the rock underneath is released from the immense pressure it was under. The rock expands slightly, causing joints and cracks parallel to the surface (sheeting).

   Did you know? This is why large granite intrusions often weather into curved, dome-shaped hills (known as tors or exfoliation domes).

5. Vegetation Root Action:
   

   Tree roots grow into small fissures to find moisture. As the roots expand, they wedge the rock apart, widening the cracks.

2.2 Chemical Weathering Processes

These processes involve a chemical reaction that changes the composition of the rock, weakening it and making it unstable.

1. Carbonation:
   

   This is the key process affecting limestone and chalk (rocks rich in calcium carbonate).

   Rainwater absorbs carbon dioxide from the atmosphere, forming weak carbonic acid:
   \(H_2O \text{ (Water)} + CO_2 \text{ (Carbon Dioxide)} \to H_2CO_3 \text{ (Carbonic Acid)}\)
   The carbonic acid reacts with calcium carbonate (limestone), dissolving it to form soluble calcium bicarbonate, which is then carried away in solution.

   Analogy: Like dissolving a sugar cube in water. The sugar (limestone) disappears completely.

2. Hydrolysis:
   

   The reaction of rock minerals with the hydrogen ions (\(H^+\)) or hydroxide ions (\(OH^-\)) in water. This process is highly effective on rocks containing silicates, especially granite (which contains feldspar and quartz).

   The water attacks the mineral structure, changing it into a softer, clay-like substance (kaolin). This weakens the rock structure significantly.

3. Hydration:
   

   The physical absorption of water molecules onto the mineral structure. When minerals absorb water, they expand, causing internal stress within the rock, potentially leading to disintegration.

   Example: Some iron oxides can expand by up to 60% when hydrated.

2.3 Factors Affecting Weathering Type and Rate

The speed and type of weathering depend on several general and specific factors.

General Factors:

• Climate: This is the most crucial factor (see specific factors below).
• Rock Type (Lithology): Is the rock soluble (like limestone)? Is it permeable (allows water through)? Is it made of minerals susceptible to hydrolysis (like granite)?
• Rock Structure: The presence of weaknesses like joints (cracks) and fissures increases the rate of weathering by providing entry points for water, air, and roots.
• Vegetation: Can accelerate physical weathering (root action) and chemical weathering (decaying organic matter forms humid acids).
• Relief (Slope): Steeper slopes allow weathered debris to be removed faster, exposing fresh rock to attack, thus speeding up the rate of weathering.

Specific Factors: Temperature and Rainfall (Peltier Diagram)

The Peltier Diagram shows the dominant type and rate of weathering based on the combination of temperature and rainfall:

• High Temperature + High Rainfall: Maximum chemical weathering (e.g., equatorial rainforests). Hot, wet conditions speed up all chemical reactions.
• Low Temperature + Low Rainfall: Minimal weathering overall, but physical weathering (e.g., frost action) may occur if moisture is present and cycles around 0°C.
• High Temperature + Low Rainfall: Maximum physical weathering (e.g., deserts). Heating/cooling and salt crystal growth dominate. Chemical weathering is slow due to lack of water.

Memory Aid: Chemical weathering loves Heat and Water (like making soup). Physical weathering dominates where it's Dry (Salt growth, thermal expansion) or where Temperature Fluctuates around Freezing (Freeze-thaw).

3. Slope Processes: Gravity Takes Over (Syllabus 3.3)

Once rock has been weathered, gravity pulls the resulting debris downslope. The collection of processes that move material down a slope is critical to landscape evolution.

Slope Processes and Conditions

The key condition governing slope processes is the balance between the shear strength of the material (how well it resists movement) and the shear stress (the force of gravity pulling it down). When stress exceeds strength, mass movement occurs.

Mass Movement: Types and Characteristics

Mass movement is classified based on the speed and the moisture content of the moving material.

1. Creep (Heaves):
   

   The slowest form of mass movement, often measured in millimetres per year. Material moves downhill due to repeated expansion and contraction (e.g., freeze-thaw or wetting/drying).

   Effects on slopes: Tilted fence posts, curved tree trunks (called pistol butts), and small ripples in the soil surface.

2. Flows (Earthflows, Mudflows, Solifluction):
   

   Material moves downhill like a viscous fluid, saturated with water.

   • Earthflows: Slower than mudflows, usually occur in fine-grained soil on gentle slopes.
   • Mudflows: Rapid movement of extremely saturated fine material (mud and debris). Often follow existing channels, like rivers, and are common after heavy rainfall or volcanic eruptions (lahars).
   • Solifluction: Specific to periglacial areas (areas bordering ice sheets). Saturated active layer (topsoil) slowly flows over the underlying impermeable frozen layer (permafrost) in summer.
3. Slides (Slumps and Landslides):
   

   Material moves as a distinct block or mass along a shear plane (a line of weakness). They are typically rapid and disastrous events.

   • Rotational Slides (Slumps): Material moves along a curved slip plane, often creating a concave (bowl-shaped) scar at the top. Common in weak, impermeable clays.
   • Translational Slides (Landslides): Material moves along a straight slip plane (often a layer of rock or a bedding plane). Very destructive.
4. Falls:
   

   The fastest form. Material detaches and drops vertically or nearly vertically. This often happens on very steep slopes or cliffs due to physical weathering (like freeze-thaw) removing the support.

   Resultant Landform: A pile of debris, known as scree or talus, accumulates at the bottom of the cliff.

Water and Sediment Movement on Slopes

Water plays a role not only in mass movement (by saturation) but also in transporting sediment across the surface of the slope.

• Rainsplash: When raindrops hit bare soil, they detach and splash soil particles, transporting them a short distance. On flat land, this effect cancels out, but on slopes, the net movement is downhill.
• Surface Runoff: Water flowing over the land surface.
  • Sheetwash: A thin, uniform layer of water moving over the surface, often removing fine soil particles evenly (hence 'sheet').
  • Rills: If the sheetwash concentrates, it erodes tiny channels (rills). If these channels enlarge, they become gullies.

Key Takeaway: Mass movements range from slow creep (heaves) to rapid falls. Water is critical, either by lubricating the soil (flows/slides) or by surface erosion (sheetwash/rills).

4. The Human Impact on Slope Stability (Syllabus 3.4)

Human activities frequently disrupt the delicate balance between shear stress and shear strength, often leading to instability and catastrophic mass movements.

4.1 Impact on Slope Stability

Human actions can either decrease or increase the stability of slopes:

• Decreasing Stability (Making slopes dangerous):
  • Removing Vegetation: Deforestation removes the binding effect of roots, reducing soil strength. It also decreases interception and evapotranspiration, increasing the water content of the soil (saturation).
  • Loading the Slope: Building structures (houses, roads, retaining walls) near the top of a slope adds weight, increasing the downward shear stress.
  • Undercutting the Toe: Quarrying or building roads at the base (toe) of a slope removes the natural support, making the slope much more prone to sliding or falling.
  • Artificial Saturation: Leaking pipes, irrigation, or poorly managed drainage systems can introduce large amounts of water, drastically reducing the material's shear strength.
• Increasing Stability (Making slopes safer):
  • Drainage: Installing drainage channels or pipes to remove excess water reduces saturation and increases shear strength.
  • Afforestation: Planting deep-rooted vegetation helps bind the soil and removes water through transpiration.
  • Reducing Slope Angle: Grading the slope to make it less steep reduces shear stress.

4.2 Strategies to Modify Slopes to Reduce Mass Movements

These are direct engineering and biological methods used to manage unstable slopes.

1. Pinning (Rock Bolts or Soil Nailing):
   

   Long steel rods (bolts) are driven deep into the rock face or soil and anchored, often with cement. These act like massive stitches, holding layers of unstable rock/soil together and preventing blocks from falling or sliding.

2. Netting (Wire Mesh):
   

   Heavy-duty wire mesh or chain-link fencing is draped over steep rock faces. This prevents individual pieces of rock from falling onto infrastructure below (like roads or railways). While it doesn't prevent the movement, it controls the hazard.

3. Grading (or Benching):
   

   Involves reshaping the slope, often by cutting it into a series of steps or terraces (benches). This reduces the overall angle of the slope, lowering the shear stress, and provides flat areas for debris accumulation.

4. Afforestation:
   

   The large-scale planting of trees, particularly fast-growing varieties with extensive root systems. This stabilizes the soil biologically by interlocking the ground material and reducing soil moisture.

Case Study Requirement (Syllabus 3.4):

Remember, for the exam, you need a specific case study (e.g., a landslide event in an urbanized area like the Hong Kong landslides or the La Conchita mudslide, USA). Your case study must cover:
1. The impacts of human activity on slope stability (e.g., development, drainage, deforestation).
2. The effects on the stability of the slope (e.g., increased slides, flows).
3. An evaluation of the attempts made to reduce mass movement (e.g., how successful were the pinning or afforestation projects?).

Key Takeaway: Human development frequently leads to slope instability (especially by removing the toe or vegetation). Management strategies like pinning and netting are essential to mitigate mass movement risk, particularly in densely populated areas.