Critical zone

What is the Critical zone?

Earth's permeable layer from the tops of trees to the bottom of actively cycling groundwater” .

It is where rock, soil, water, air and organisms interact.

Why is the critical zone considered ‘critical’?

It provides energy and nutrients to ecosystems, controls runoff and infiltration, mediates toxin release, and controls groundwater chemistry.

What makes the critical zone a scientific approach?

It links processes into one system, focuses on feedbacks, and aims for long‑term sustainability of critical functions.

What are the 6 major controls on the Critical zone?

• Lithology

• Climate

• Biology

• Topography

• Time

• Disturbance

How are CZ sites compared?

Along gradients due to the number of controls. Limit variables, making it easier to indentify and quantify feedbacks between processes

Which lithological properties influence CZ processes?

• Grain size

• texture

• porosity

• mineralogy

• chemical composition

• thickness

• age

What are the properties of the Critical zone?

• Thickness

• Porosity

• Mineralogy

• Chemical composition

• Topography

• Organism types and abundances

What controls weathering in volcaniclastics [in Puerto Rico]?

Via pyrite dissolution which generates acids and Mg minerals dissolve quickly

What are the CZ characteristics of volcaniclastics?

• Fastest weathering

• highest porosity

• highest infiltration

• deepest soils

Why does quartz diorite produce steep topography [in Puerto Rico]?

It is prone to landslides dues to its layered structure

What controls weathering in quartz diorite?

Biotite oxidation which causes spheroifal fracturing

What are the characteristics of metavolcanics?

Slowest erosion

lowest infiltration

cloud forest, bogs, and highest peaks.

What makes ultramafic rocks unique in the CZ

• Very high Mg and heavy metals

• low in major nutrients

• toxic to many plants and microorganisms

What are the ecological consequences of ultramafic soils?

• Low biodiversity

• Prescence of genetically adapted species (hyperaccumulation of metals)

• Endemic species

What is the signicance of silicate rocks?

Their mineral structues are chemically stable, making them slower to weather.

Which physical properties of rocks influence soil formation?

• Permeability

• porosity

• grain size

• root penetration

• carbon sequestration

• nutrient release and water ingress.

Which minerals to crysallise first and last in Bowens reaction series?

first: olivine (high temp)

last: quartz (low temp, more resistant to weathering)

How do intrusive and extrusive igneoius rocks differ texturally?

Intrusive rocks cool slowly → large crystals, interlocking textures, low pore space.

Extrusive rocks cool rapidly → fine crystals, volcanic glass, bubbles → higher pore space.

How does silica content influence eruption types?

Higher silica increases melt viscosity, leading to explosive eruptions

Lower silica produces effusive, slower-cooling lava flows

Why are volcanoes important in the Critical zone development?

• They act as lithological and disturbance agents; releasing elements, perturbing landscapes, interrupting biogeochemical processes and supplying new CZ materials

What are ignimbrites and how do they form?

Extremely hot pyroclastic flows that weld together into rock → they contain no minerals and are just fused fragments

What is the difference between consolidated and unconsolidated volcanic deposits?

Consolidated deposits are lithified (e.g., ignimbrites), while unconsolidated deposits include ash, tephra, and loose pyroclastics.

What are andisols and why are they important?

Soils formed from porous, low‑density volcanic clastic rocks with high nutrient potential. They cover

Why do volcanic rocks often produce fertile soils?

They contain abundant alkalis (Na and K) and nutrient‑rich minerals that weather into nutrient‑rich soils.

How does volcanic disturbance influence soil development?

Eruptions reset ecosystems, add fresh mineral material, and create heterogeneous substrates that the Critical Zone exploits.

How does the Critical zone differe from solid rock?

It is disaggregated - broken up into pieces, less dense, has more pore space, more water, different colours and minerals and contains plants

What is congruent dissolution?

Weathering where products are dissolved ions and leaves no/little solid residue.

What is incongruent dissolution?

Weathering where some solid produts remain (clays, hydroxides)

What factors influence the order in which minerals weather?

• Spatial relationships

• mineral abundance/assemblage

• water–rock interaction

• mineral surface area

• accumulation of weathering products.

What are the main types of primary minerals involved in chemical weathering?

• Carbonates

• phosphates

• sulfates

• aluminsilicates

What clay minerals form under weak vs intense weathering of aluminiosilicates?

Weak weathering forms silicic acid and soluble cation and 2:1 clay

Intense weathering forms more silicic acid, soluble cation and 1:1 clay

How do clays differ from primary aluminosilicates?

They have high OH and water:oxygen ratios and higher Al:Si ratios due to Si loss

What weathering products do silicates form?

Soluble ions, metal hydroxides, and amorphous/poorly crystalline materials.

What is volcanic glass and how does it weather?

An igneous rock that cooled too quickly to form crystals; it dissolves and precipitates as allophane (amorphous) or imogolite (paracrystalline)

What does the presence of allophane or imogolite indicate?

An andisol → a soil formed from volcanic ash

What is residual parent material?

Material formed in situ from weathered rock

How does climate affect residual plant material

Warm/humid → strongly leached, oxidised, red/yellow

Cool/dry → resembles original rock; parent material dominates soil properties

Where does organic parent material form?

Wet areas with poor drainage where decomposition is .limited by lack of O2 allowing organic matter to accumulate

What characterisies colluvial parent material?

Coarse, rocky angular, poorly sorted, well-drained but landslide prone

What is loess and why is it important?

Wind blown silt with fine sand and clay: forms fertile silty soils (e.g: China)

What are the characteristics of volcanic ash parent material?

forms andisols. ashe weather quick, is nutrient rich, light, porous abnd accumulates organic matter rapidly

What are the 4 main components fo soil?

Air, water, mineral particles and organic matter

What proportion of soil is typically pore space vs solid material?

50:50

What are soil colloids and why are they important?

Particles <1 μm with high surface area and electrical charge; they have a soil fractions which makes them “responsible for most chemical activity” in soils.

Which materials make up most soil colloids?

Clays and soil organic matter

What is the difference between primary and secondary materials?

Primary = form by cooling magnma

secondary = form form weathering

Why particle fraction contributes most to nutrient retention?

Clay - as it has high attraction to water and nutrients

What is soil texture?

The proportion of sand, silt and clay in a soil

What is a loam?

A soil with roughly equal proportions of sand, silt and clay

What is SOM and why is it important?

Soil organic matter is a major nutrient source and has high warer-holding capacity

Humus is the main SOM colloid

What gases dominate soil air compared to atmosphere?

Soil CO2 is 1–5% vs ~0.04% in the atmosphere; soil O₂ is lower (~5–15%).

What is field capacity?

The amount of water soil holds after gravitational water drains; it is the water available for plant growth.

What are the four main soil-forming processes?

1. Additions: SOM (plant debris), dust, salts/minerals

2. Losses: Leaching, erosion, SOM decay

3. Transformations: chemical weathering, dissolution, oxidation, precipitation; SOM decay

4. Translocations: movement of material within the soil

What distinguishes young from mature soils?

The degree of profile development (horizon formation), not necessarily its chronological age

What is the equation to work out the mass transfer in a CZ profile?

What do different mass transfer values mean?

Tau = 0, immobile

Tau = -1, element totally lost

Tau ˃ 0, element gained

What does a depletion profile look like?

When tau DECREASES upward (away from parent rock), elemnets are being lost overtime

What does an addition profile look like?

When tau INCREASES above the parent rock, elements are being gained overtime [creates paleosol]

What does a Biogenic profile look like?

When tau of a NUTRIENT element INCREASES at surface, it’s likely plants arer concentrating the element (biolifting)

What are the three main fractions of soil organic matter?

(1) Decomposing residues

(2) lLiving biota

(3) Resitant organic matter

What are the essential macronutrients for soil fertility?

N, P, K, Ca, Mg, S, Fe.

What are the essential micronutrients?

Fe, Mn, B, Cu, Zn, Mo, Se.

Why is SOM important for soil fertility?

It stores nutrient ions and water, buffers polutants and pH/mosirure fluctuations, affects colour and temp and is a major carbon reservoir

What are the three SOM resistrtance categories?

• RPM: Resistant plant material → hard to break down

• POM: physically → not accesible to organisms can’t get through

• COM: chemically → its chemical form makes it hard to break down

What is dynamic equilibrium in SOM?

Continuous flow between SOM categories over different timescales

What is the average turnover time of soil organic carbon?

~22 years

What are the turnover years of the five SOC fractions?

• DPM: 0.165 years

• BIO: 1.69 years

• RPM: 2.31 years

• POM: 49.5 years

• COM: 1980 years

What is the equilibrium composition of SOM?

COM 50%, POM 47%, RPM 2%, BIO 1%, DPM 0%.

How does SOM influence soil structure?

• It forms aggregates via reactions with clays and Fe/Al oxides, improving aeration, water retention and stability

Why do clay-rich soils have more SOM?

small pore spaces restrict aeration → slows decomposition allowing SOM to accumulate

Which nutrient pool is most bioavailable on agricultural timescales

Adsorbed ions

What is CEC and what factors controls it?

The total amount of positively charged ions a soil can hold → measures nutrient availability and soil fertility

Factors: colloid type, surface area, relative abundant and pH

How is CEC measured?

• Leach soil with NH₄‑acetate to remove adsorbed cations

• Displace NH₄⁺ with KCl

• Measure NH₄⁺ released

What is the importance of base saturation?

Soils with high BS are more fertile because they have higher pH, little/no Al³⁺, and more nutrient cations (K⁺, Ca²⁺, Mg²⁺).

What are the six hazard categories

Geophysical, hydrological, meteorological, climatological, biological, extraterrestrial.

What is risk

Hazard × vulnerability.

How can hazards interact

E.g., rainfall → floods → saturation → slope failure.

What controls mass movement susceptibility

Gradient, pore pressure, cohesion, particle packing, mineralogy.

Why are volcanoes prone to landslides

They are structurally weak piles of fragmented rock.

What are examples of hazard mitigation

Early warning systems, slope protection, afforestation, governance, hazard mapping.

How does vegetation affect the CZ

Influences infiltration, SOM, nutrient cycling, microbial activity, soil temperature/pH.

What is the rhizosphere

~2 mm zone around roots with intense biological activity.

What do mycorrhizae do

Increase nutrient uptake (especially P) in exchange for plant carbon.

How does biodiversity support soil processes

Increases SOM, nutrient accumulation, productivity, and ecosystem resilience.

How does topography affect soils

Controls erosion, deposition, soil thickness, moisture, and weathering.

What has the strongest effect on weathering: temperature or precipitation

Precipitation.