Block 7: Weathering, Soils, & Surface Waters Flashcards
(23 cards)
What is chemical weathering?
The chemical alteration of rocks, e.g. dissolution & secondary mineral formation
What is physical weathering & how does it assist chemical weathering?
The mechanical breakdown of rocks into smaller pieces
This creates more points of access for chemical weathering agents
What are primary & secondary minerals?
Primary minerals are inherited from igneous, sedimentary, or metamorphic rocks (e.g. feldspars, micas, quartz, olivine, plagioclase, amphiboles)
Secondary minerals are formed by chemical weathering (e.g. clays, oxides, hydroxides, carbonates)
How do primary & secondary minerals trend along the soil profile?
Primary minerals dissolve up the soil profile, whereas secondary minerals form
What are congruent & incongruent weathering?
In congruent weathering, minerals completely dissolve into constituent ions (i.e. evaporites & carbonates)
In incongruent weathering, minerals only partially dissolve & leave behind residual soil weathering products (i.e. sulphides & silicates)
How does weathering affect evaporites, carbonates, sulphides, & silicates?
Evaporites are highly soluble & the most easily weathered in the presence of a polar solvent
Sulphides weather through oxidation as bonds are formed from reduced Fe & S
Carbonates are highly soluble in acidic solutions through acid hydrolysis
Silicates have mineral-dependent solubilities & weathering rates
What is soil & how is it produced?
Soil is a mixture of minerals & organic matter consisting of secondary minerals made during silicate weathering
What are the 4 main soil horizons?
O Horizon: above ground material
A Horizon: organic-rich surface soils
B Horizon: mineral-rich subsoil
C Horizon/Saprolite: transition to bedrock, including primary minerals
Which 2 methods are used for interpreting weathering in soil profiles?
- Normalisation of data to the parent material to yield concentration change
- Normalisation of mobile to immobile content, correcting for volumetric strain
What is volumetric strain?
The change in mineral volume during weathering as water leaches out original mineral mass, creating higher porosity & lower bulk density in soils than in the protolith
What is the ion exchange capacity?
The amount of electric charge on the surface of secondary phases, which allows soil to retain ions important for pH regulation
What is the Chemical Index of Alteration?
An index using mol/kg oxide concentrations to quantify degrees of (mainly thermal) weathering experienced in soils
* Higher CIA indicates more weathering
Which common elements are immobile in surface waters?
Be, Al, Ti, Fe(III), Ti, sometimes Si
How does conservative Cl trace rain inputs & evapotranspiration?
As Cl typically doesn’t react or degrade, surface/soil water concentration directly reflects rainfall quantity & source
Cl also concentrates in remnant water according to degree of evapotranspiration
How can [Cl] track silicate weathering, input to grass & input from silicates, & clay uptake?
Silicate weathering: relationship with [Na]
Grass & silicate uptake: relationship with [K]
Clays: negative values
What is the difference between hydrolysis & hydration?
Hydrolysis: H+ replaces a cation
Hydration: H2O is incorporated into the molecule
Which silicates are the least & most susceptible to chemical weathering?
How does this relate to order of crystallisation from igneous melts?
Least to most: quartz, muscovite, biotite, amphibole, pyroxene, olivine
This is the inverse of Bowen’s reaction series
What general trend occurs in heavy metal mobility with pH?
As pH decreases heavy metals become more mobile
This is the same for Al
In what environments are soils generally acidic?
Environments in which high amounts of rain weathering of primary minerals & leaching of base cations, i.e. humid tropics or moist temperate forests
How does the plot of Na/(Na+Ca) vs TDS change across streams in different climate zones?
Tropics: High Na & low TDS due to rainfall dilution
Temperate: High Ca & medium TDS due to limestone weathering
Arid: High Na & high TDS due to evaporation; Ca becomes supersaturated & precipitates out of waters
How does hydrolysis affect acidity?
Solutions are buffered by consumption of H+ ions in hydrolysis, decreasing acidity
Which metals are amphoteric?
Arsenic, cadmium, iron, lead, & zinc can act as an acid or base & can therefore have higher solubilities at acidic & basic pH than at neutral ph
How does precipitation have the capacity to dissolve surface minerals?
Rainwater contains little TDS & CO2 incorporation creates carbonic acid
