Regolith & Critical Zone

define Regolith

all material between bedrock and the atmosphere

• Includes: soil, saprolite, transported sediments, duricrusts.

• Regolith covers a very large portion of the Australian continent

vertical profile of ground: parts

Soil

Pedolith

Saprolith

Bedrock

define:

Soil

Pedolith

Saprolith

Bedrock

Soil	Organic matter + leached minerals
Pedolith	No original rock fabric; secondary structures (Fe, Al oxides, clays)
Saprolith	Weathered rock retaining original textures  Saprolite: >20% minerals altered Saprock: <20% altered Grus: granite-derived coarse debris
Bedrock	the hard, solid layer of rock that lies beneath loose surface materials like soil, sand, and grave

what is:

Pedoplasmation

Cementation fronts

Weathering front

Pedoplasmation	Transformation of saprolith → pedolith
Cementation fronts	Fe/Al accumulation zones
Weathering front	Boundary between fresh rock and saprolith

landscape factors impacting regolith formation

• Topography

• Bedrock type and structure

• Climate

• Vegetation and biota

• Groundwater flow

• Time. / palaeoladnscape

drainage and minerology impact on regolith formation

drainage impacts type of iron oxide formed

• Well‑drained → hematite

• Poorly drained → goethite

• Goethite is found where large pores, while hematite forms in micro-pores

 

Drainage depends on

• Porosity

• Permeability

• Rock fabric

• Slope position

what are Weathering Processes

Mechanical (Physical) Weathering

Frost action

Pressure release / unloading

Salt crystallisation

Biological mechanical weathering

Chemical Weathering

Dissolution

Hydrolysis

Carbonation

Oxidisation

Mechanical (Physical) Weathering

Breakdown without chemical change

Frost action	Freeze-thaw cycles = widens cracks Water expands 9% when frozen Produces talus cones
Pressure release / unloading	Granite batholiths expand when overburden removed Forms sheet joints and exfoliation domes
Salt crystallisation	Near coasts Salt crystalises and expands rock Salt grows = honeycomb weathering
Biological mechanical weathering	Roots widen cracks in rock Bioturbation mixes soil Animal burrows

Chemical Weathering

Breakdown by reactions with water, acids, oxygen

Dissolution	Congruent: mineral dissolves completely E.g. halite, quartz Incongruent: mineral alters → secondary mineral + solutes E.g. feldspar → kaolinite E.g. smectite after pyroxene
Hydrolysis	Hydrogen ions from water replace positive ions in minerals to make new minerals
Carbonation	CO₂ + H₂O → carbonic acid → dissolves carbonates Forms karst landscapes E.g. Iron + oxygen = hematite
Oxidisation	Fe²⁺ → Fe³⁺ → hematite/goethite Forms ferricrete in Australian regolith

Biological Chemical Weathering

• Organic acids from plants 

• Root ion exchange → clay formation 

  • Clay content of soil increased

Factors that impact weathering rates

Climate

Goldich Weathering Series

Acidity

Particle size

Porosity and Permeability

Topographic Effects

Geomorphology & Tectonics

Regolith Strength

impact of climate on weathering rates

• Humid/more water = more erosion

 

Chemical weathering depth

• Tropical: tens of metres 

• Temperate: moderate 

• Arid/arctic: centimetres to metres

impact of acidity on weathering rates

• Acidity increases rates of chemical weathering

impact of goldich weathering series on weathering rates

• Vulnerability to weathering is opposite of Bowen's reaction series

  • More felsic = more resistant to weathering

  • More mafic = less resistant

• Olivine → Pyroxene → Amphibole → Biotite → Plagioclase → K‑spar → Muscovite → Quartz

• Feldspar → kaolinite 

• Biotite → vermiculite, goethite 

• Amphibole → smectite 

• Quartz → remains mostly unchanged

impact of Particle size on weathering rates

• Smaller particles are more vulnerable to weathering → increases SA

  • More weathered particles experience more weathering

  • More SA = more weathering

impact of porosity and permeability on weathering rates

• Porosity = Controls how much rock surface water contacts

• Permeability = Controls fluid movement and thus reaction rates

Examples

• Granite vs volcanic rocks → different void ratios with depth

• Clay = porous but impermeable

• Fractured rock = permeable but not porous

impact of topography on weathering rates

• Slope positions

  • Hilltop: stable, thick regolith

  • Upper slope: erosional, thin regolith

  • Lower slope: depositional, thick regolith

  • Valley: thick composite regolith + fluvial erosion

• Slope shapes

  • Concave → landslips

  • Free-face → resistant caprock collapses

  • Convex → saprolite erosion exposes basement

impact of … on weathering rates

• cuestas

• volcanic plugs

• drainage evolution

• landscape inversion

Cuestas

• Resistant strata overlying weaker layers

• Dip slope + scarp slope

• Formed by differential erosion of tilted beds

 

Volcanic plugs, quartz veins

• Highly resistant → stand as ridges

 

Drainage evolution

• Australian drainage systems range from Proterozoic to Cainozoic.

 

Landscape inversion

• Durable materials (lava flows, ferricrete) become ridges after surrounding weaker rock erodes

strength of different parts of regolith

Ferricrete Formation Models

1. Vertical accumulation

   1. Remove 50 m saprolite → 0.5 m ferricrete

2. Lateral movement

   1. Move Fe from 100 m × 0.5 m saprolite → same ferricrete thickness

3. Conglomeratic/pisolitic ferricrete

   1. Reworked nodules cemented by Fe oxides

4. Vesicular ferricrete

   1. Fe-rich groundwater impregnates sands

5. Ferruginised palaeochannel sediments

   1. Megamottles, pisoliths formed by Fe mobilisation