SCSC 301 chapter 2

Weathering

biological, chemical, or physical process that destroys or decomposes rocks

• may also include subsequent construction of new minerals

Weathering usually leads to…

formation of regolith, and the formation of soil due to 5 soil forming factors

Physical Weathering

abrasion by water, ice, wind, temperature, and biota

chemical weathering

hydration/hydrolysis

dissolution/acid dissolution redox

complexation

Weathering results in….

• increasingly smaller particles

• increasingly stable minerals

  • release of soluble materials (calcium, magnesium, potassium, sodium, sulfate)

Igneous Rock

formed when molten lava cools. w

What rocks are more susceptible to weathering?

minerals with high Fe, and Mn are more easily weathered. also usually darker colored

Sedimentary Rocks

deposition and recrystallization of weathering products from other rocks

• compacted or cemented

• sand stone

  • limestone

metamorphic

igneous or sedimentary rock transformed by high heat and pressure

granite (igneous) → gneiss

Quartz (I) —> quartize)

Sandstone (S) —> slate

Limestone (S) —> marble

Rock Vs Mineral

Rocks are naturally occurring aggregate of minerals with no definite chemical composition.

Minerals are naturally occurring substance that has a characteristic composition, ordered atomic structure, and physical properties

Abundance of elements in earth’s crust

Acids

• Fe Al(not essential)

Bases

• Ca, MG, Na ( beneficial/toxic), K

What is the difference in aluminum content in rock compared to soil?

soil is much higher in Al and Fe than rock. also much lower in basic cations

silicates

quartz - dominant in sands

*feldspars

*amphiboles

*pyroxenes

*micas

* = form clays

Oxides

hematite, Fe2O3

Goethite FeOOH FeOH)3

Gibbsite AlOH3

• very insoluble

Carbonates

Calcite CaCO3

Dolomite CaMG CO3)2)

• somewhat soluble

Phosphates

apatite CA5 (OH,F,CL)(PO4)3)

• insoluble

sulfates

gypsum CASO$ 2H2O

• relatively soluble

Halides

Halite NACl

Sylvite KCl

• very soluble

Primary vs 2ndary minerals

primary minerals have not been altered chemically since their solidification from magma. most are silicates except for clay minerals

secondary minerals form near the surface. they are recrystallized from the breakdown of primary minerals. most other minerals including oxides, carbonates, and sulfates

6 types of chemical weathering

dissolution, acid dissolution, reduction (redox) , hydration, hydrolysis, complexation

Dissolution

Some minerals will dissolve in water

• gypsum

Acid Dissolution

weathering is accelerated by acid

• enhances hydration and dissolution reactions

• acid comes from water, root respiration, (CO2 to carbonic acid)

• organic acids from decomposition

  • mineral acids (decomposition)

reduction oxidation (REDOX)

minerals with Fe and Mn and S are susceptible to redox reactions

• chemical breakdown where the oxidation state of an element is changed by the transfer of electrons

  • FE 2 ← → Fe III + e-

redox and weathering

rust: iron with oxyfen and water with fe 2+ which futher reacts to form Fe 3+ which forms iron oxide which is rust (a soil material)

0 iron in a car or nail reacts similarly to the iron in a rock causing it to degrade or weather overtime in the presence of water and air

Hydration

intact water molecules can change the structure of minerals

• raisins to grapes

hydrolysis

water molecules are split into hydroxyl and hydrogen ions at the mineral surface and part is adsorbed into the mineral

hydro = water lysis = seperation

• the hydrogen ion often replaces the cation

Complexation

organic acids complex or chalte many metal cations and remove them from the mineral

parent material

determines….

type of clay

texture

affects…

water movement

soil acidity

plant nutrients

parent material origins

regolith or rock (can be transported from another location)

In place formation

residual

gravity formation and water formation

colluvial

formation from a river

alluvial

formation from oceans

marine

formation from lakes

lacustrine

formation from ice

glacial

formation from wind

eolian

formation from accumulated plant debris

organic

residual plant material

develops in place from the underlying rock

warm and humid climates

leached and oxidized soil

red and yellow colors

cooler and drier climates

• piedmont plateau and great plains soil

Marl

soil developed from residuum

• soft calcite rich material that is often deposited with clays in a marine environment

colluvial parent material

sediment that is moved downhill by gravity

• made of poorly sorted rock and sediment

• rock fragment (talus) slope

• cliff rock debits (detritus)

avalanches

• highly weathered deep soil

alluvial parent material

flowing water carries sediments that are deposited in three general classes

• flood plain

• alluvial fan

• delta

what is true about all streams

ALL streams carry sediment that will be deposited causing alluvial soils

Marine Parent Materials

Beach or shore deposits

often uplifted sediments

coastal plain soils

• fairly level

• horizontal layering

  • vary from sandy to clayey

lacustrine parent material

similarto marine parent materials, except water is from lakes rahter than oceans

• sediments are rich in clays and silts

• often layered appearance due to annual deposition of sediments

• usually uplifted lakebeds

Glacial Parent Materials

ice age (10,000 to 10,000,000 years ago)

• advanced into north america, europe, and asia

• gouged the earth

• accumulated soil, rock

• rounded hills and valleys

• left debris

glacial till

materials deopsited in place by ice

varies in size from boulders ato clay

may appear similar to colluvial material but more roundedg

glacial outwash

coarse grained alluvium mostly gravel and sand from flowing melt water

drumlin

elongated whale shaped hill formed by glaciers acting on other glacial till or moraine

moraine

any glacially accumulation of unconsolidated debris (soil and rock)

• end (terminal moraines) form where the glacier stopped

• grounded moraines formed under glaciers

Eolian parent materials

wind picks up material and deposits it elsewhere

• dune sand

• aerosolic dust

  • volcanic as h

Loess

silt, fine sand, and clay. glacial dust that is carried out

organic deposits

organic soils usually develop in wet, cool areas where plant growth exceeds the rate of residue decomposition

how does wetness affect organic matter?

excessive wetness inhibits organic matter decomposition

• microorganisms do not have enough oxygen for aerobic activities that facilitate organic matter decomposition

Climate (climosequence)

influences soil formation by

• acting on parent material, controlling the nature and intensity of weathering

• influences biota

what two main variables affect climate?

temperature and effective precipitation

precipitation

water is essential for chemical weathering

• water must penetrate the regolith to perform chemical weathering processes to translocate soluble weathering products out of the profile

effective precipitation

amount of rainfall that is actually penetrating into the regolith

factors influencing effective precipitation

season, topography, temperature and evaporation, permeability

how does weathering rates differ with climate?

weathering rates are higher in humid, warm areas than cold, dry areas

biota (biosequence)

soil organisms influence

• organic matter accumulation degradation

  • type and quantity of plants

  • cycling of nutrients

• biochemical weathering

  • organic acids contribute to complexation

• profile mixing

  • animals and humans

• vegetative cover reduces soil erosion

  • less precipitation, wind, and sun

topograhpy (toposequence)

elevation, slope, lay of the land

• topography affects where plants grow by affecting water movement

• topography and vegetation affect soil formation

Slope aspect

south facing slopes are generally warmer and lower in moisture than north facing

salt build up - arid and semi arid regions accumulate salt in low areas

parent material — often topography reflects distribution of parent material

• residual upper slopes, colluvium on lower slopes, and alluvium filling valley bottom

Chronosequence

age of soil is usually related to degree of weathering rather than specific time period

darkening of horizon 50-100 years

B horizon formation - centuries

accumulation of silicate clays (2nd minerals) - thousands of years

4 Broad Soil Forming Processes

Transformations

• mineral weatheringand degradation of organic residues

Translocations

• movement of materials from one horizon to another

Additions

• deposition of organic residues

Losses

• leaching to groundwater

• erosion

Soil Profile

made up of the master horizons

O,A,E,B,C