#13 Weathering + Soils

Definition: Soil = layer of sediment (coarse - fine), organic matter (living + dead, plants + animals, microscopic - large), water, + air that covers most of Earth's surface. Soil is produced by weathering (chemical, physical, + biological breakdown of rock at or near Earth's surface), located on top of bedrock (geology) + capable of supporting plant growth (soil science). Soil must contain nutrients, e.g., P, N, + K. Weathering = important part of rock cycle, where rock reacts to produce sediment + dissolved ions, both can eventually convert to sedimentary rock.

Lots of excellent information at web sites of the National Resources Conservation Service (US Dept. of Agriculture) + NASA.

General Reaction for Weathering of Silicate Rock

Unaltered Silicate (Parent) Rock + O2 + H2O + H2CO3 -->
Soil Minerals [Clay Minerals (smectite, kaolinite, Al-hydroxides) + Fe-oxides] + Dissolved ions (Na+, K+, Ca2+, SiO2(aq), H2CO3-) + Quartz (unaffected)

I) Importance of studying weathering + soils

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

II) Weathering (physical, chemical, + biological changes in rock at Earth' s surface due to action of water, air, plants, + animals; results in formation of soil).

Why does weathering occur? Why do rocks react to form other minerals?

 

Does weathering occur on the Moon?

 

Types of weathering

A) Physical (mechanical) = breaking rock into smaller pieces

Frost wedging - rock contains fractures (joints), which are forced apart when water freezes (~9% expansion).

Action of plant roots + burrowing organisms

Exfoliation (pressure release) - when erosion of overlying rock exposes igneous pluton, pressure < + rock expands, causing sheet-like, concentric fractures that break away.

Growth of salt crystals in joints (similar to frost wedging)

Abrasion - rocks break as they are carried by rivers or wind.

B) Chemical = dissolution of rock, need liquid water, greatly enhanced by physical weathering. Why?

Acid usually > dissolution (rainwater is naturally acidic due to presence of carbonic acid, weak acid from atmospheric CO2, CO2 + H2O --> H2CO3; produce more H2CO3 in soils from CO2 generated from oxidation of organic matter, Corg + O2 --> CO2).

Total Dissolution (solution) - entire mineral dissolves, e.g., halite or calcite (limestone caves) products = dissolved ions in solution

CaCO3 + H2CO3 --> Ca2+ + 2HCO3-

Partial Dissolution (hydrolysis) - part of mineral dissolves, most silicate mineral weathering, e.g., feldspar partly dissolves, producing dissolved ions + clay minerals.

2NaAlSi3O8 (Albite) + 2H2CO3 + 9H2O -->
Al2Si2O5(OH)4 (Kaolinite) + 2Na+ + 2HCO3- + 4H4SiO4

Oxidation - react with O2 usually dissolved in H2O, e.g., Fe-metal rusts to Fe-oxide.

Fe2SiO4 (Fayalite) + 0.5O2 + 2H2O --> Fe2O3 (hematite) + H4SiO4

Environmentally important oxidation reaction = weathering of sulfide minerals, e.g.,

2FeS2 (pyrite) + 7.5O2 + 4H2O --> Fe2O3 (hematite) + 4H2SO4 (sulfuric acid)

Producing sulfuric acid during sulfide mineral weathering = acid mine drainage, occurs in coal mines + sulfide ore deposits exposed to atmosphere. Above reaction is unusual weathering reaction because it produces acidity; most weathering reactions of silicates + carbonates consume, i.e., neutralize, acidity.

III) Factors controlling soil formation (state in terms of developing thick, mature soil)

Climate - rainfall + temperature (most important)

Warm + wet (rainforest) favors?
Hot + dry (desert) favors?
Very cold (polar) favors?

In addition, kinds of soil minerals produced depends on climate:

smectite (rich in soluble elements) forms in ~dry climates,

kaolinite (rich in ~insoluble elements) forms in warm + wet climates

bauxite (contains only very insoluble elements) forms in hot + very wet climates

Topography - steep slopes?

Valley bottoms + flat slopes?

Organics - > plants + animals
interdependence on climate

 

Time - > time (1,000's to 100,000's of years)

Bedrock/Mineral Composition - Different minerals (+ rocks) weather at different rates. Goldich's weathering sequence = inverse of Bowen's reaction series.

Why?

Olivine + Ca-plagioclase feldspar weather much faster than K-spar, muscovite, + quartz (extremely stable).

Mafic igneous rock vs. silicic igneous rock?

In chemical terms, explain sequence of mineral "weatherability" in Table 4-5

IV) Soil Horizons - characteristic set of layers, collectively = soil profile.

Fig. 5.24a shows most common layers (horizons) for soil in temperate climate.

Nearest surface = accumulation of organic matter (O horizon). O (+ A) horizon of fertile soil is usually teeming with life including bacteria (2 trillion per kg of soil), molds, fungi (400 million/kg), algae (50 million/kg), + insects (thousands/kg) including ants, worms, + spiders. Source of CO2 + organic acids for chemical weathering.

Below that = gray to black, coarse-grained layer (A horizon), contains both mineral + organic matter (humus - decomposed organic matter). A horizon = zone of leaching because downward percolating water has chemically dissolved minerals + physically carried fine minerals away from this layer; most intensively weathered zone + where plant roots are abundant.

Below that can be E (eluviated or leached) horizon, which contains little to no organic matter. Leaching of soluble elements also occurs here.

Below that = B horizon, which is dark brown, fine-grained layer rich in iron oxides + clay minerals, where some dissolved mineral matter + fine-grained minerals from above are deposited (zone of accumulation or illuviation). In arid climates, B horizon can be rich in calcite. Brief + heavy rains dissolve calcite from upper part of soil + transport it downward, eventually precipitates as white layer = caliche.

Below that = layer of fragmented, partially weathered (usually oxidized) bedrock (C horizon).

Below that = unaltered bedrock (R horizon.)

Boundaries between horizons are usually gradual not abrupt + not all horizons may be present (need lots of time + other favorable conditions).

V) Soil Minerals vs. Depth - In humid climates, kaolinite dominates near surface, smectite dominates deeper in soil. Why?

Residual soil (horizons form over bedrock) vs. transported soil (form on sediment). Which type would tend to produce thicker soil?

VI) Soil Classification

A) General Zonal Classification (function of rainfall + temperature)

pedalfer - soil rich in clay minerals + Fe-oxides (i.e., rich in Al + Fe); characteristic of humid, temperate regions e.g., eastern USA.

pedocal (aridosol) - soil rich in calcite, characteristic of dry regions, e.g., desert SW USA, evaporation concentrates salt + calcite, little leaching.

laterite (oxisol) - highly leached soils characteristic of hot + humid tropical zones, only most insoluble phases remain (Al- + Fe-oxides), usually brick red color; economic source of Al (bauxite); unproductive after deforestation most nutrients are in plants not in soil, dries to brick-like texture.

B) Soil taxonomy - complex classification used by soil scientists, based on physical + chemical characteristics including horizons, nutrients, organics, color, + climate. Examples = mollisols (black, organic-rich prairie soils) + alfisols (forest soils); 12 soil orders; link to State Soils Photo Gallery.

VII) Selected Engineering Properties + Environmental Problems of Soils

A) Water content (wt. % water in soil) With > water content, soil behavior changes from solid (breaks into clumps) to plastic (moldable) to liquid (flows when slightly disturbed). Plastic limit (PL) = water content at solid/plastic transition; liquid limit (LL) = water content at plastic/liquid transition; plasticity index (PI) = difference between plastic + liquid limit (more useful).

What are expected PL, LL, + PI values for clay-rich soils?

Expected PL, LL, + PI values for sandy soils?

Low PI (<5%) = soil flows readily (liquefaction) + susceptible to landslides.

High PI (>35%) suggests possible swelling soils.

B) Soil Geohazard - Swelling soil (vertisol) - contain smectite, which when wet can absorb large amounts of water between sheets + swell to many times its volume (up to 10 - 15 x), exerting great upward pressure (up to 5 - 10 tons/ft2). As it dries, it shrinks back to original volume. Wet/dry cycles can be due to heavy rain, snow melt, lawn watering, etc. Shrink-swell action can damage walls, foundations, + roads. Average annual damage from swelling soils in USA is $2 - 6 billion, one of most costly natural hazards. Swelling soils are worst where there is bentonite - layer of volcanic ash that has altered by reaction with groundwater to smectite. Where do swelling soils occur? Montana, WY, Dakotas, CO (Denver), TX, + LA.
Mitigation = Avoid building on them; anchor structures deep within soil; improve drainage (prevent water buildup); keep trees away from foundations; treat with lime (Ca exchange); excavate; difficult to reduce damage for roads.

C) Compressibility - tendency of soil to < volume (settlement) when loaded; Highly compressible soils (organic + clay-rich soils) are problem for construction because compression (+ building settlement) happens slowly + unevenly so walls + foundations crack. As soil water is removed, soils become more compressible (e.g., Leaning Tower of Pisa).

D) Strength - Strength of soil determines its ability to support load before failing.

CASE HISTORY - Transcona grain elevator near Winnipeg, Manitoba built in 1913 by Canadian Pacific Railway. As elevator was filled, underlying soils failed + elevator rotated to 27° angle from vertical. Structure was relatively undamaged + was restored upright (7 m deeper). Soil = very weak + compressible (clay-rich).

E) Cohesion - degree to which soil sticks together, important to soil erodibility (+landslide development). Clay-rich soils vs. sandy soils?

Soil Erosion

A) Problem - Lose valuable soil resource (< fertility or complete loss) + create sediment pollution (damage to humans, plants, + animals from sediment deposition). Eroded soil deposited in rivers, lakes, reservoirs, causes > dredging, flooding (sediment fills river channel), treatment of surface drinking water supply (sediment removal), filling reservoir (small reservoirs fill in decades, large reservoirs in centuries), + negative impact on ecosystem (cloudy water = plants get no sunlight + fish cannot breathe).

CASE HISTORY - Lake Ballinger Dam, TX built in 1920 for drinking water supply, maximum water depth of 11 m; abandoned in 1952 because sediment filled it.

B) Factors controlling soil erosion (natural process, large potential human impact). Soil erosion rate = complex function of following interrelated parameters. Discuss which factor > soil erosion rate

soil properties - cohesion
rainfall (climate) + vegetation - desert vs. humid climate

 

wind -
slope angle -
land use (human impact) -

C) How to < soil erosion?

Avoid disturbing (building on) problem areas; need soil surveying (mapping)
Good construction practice - build sediment traps (ditches or ponds to capture soil on-site); replant trees; build immediately + provide soil cover (e.g., straw) while building
Good crop planting practices - terracing (creating level areas in hill sides), crop rotation, no till