GEOL101: Weathering and Soils

What is weathering?

All processes that physically break down and/or chemically alter rocks

Is weathering always a uniform process

No

What are some ways physical weathering occurs?

Ice growth, salt crystal growth, sheeting

What is frost wedging/heaving?

Liquid water gets into rock, freezes, then pushes rock apart

What is salt wedging?

Salty water penetrates into rock, salt crystals push rock apart

What is sheeting

Reduction of weight reduces pressure —> results in fractures near surface

What are the results (products) of physical weathering?

• Rock fragments

• Sediment (gravel, sand, silt)

• Increased surface area for chemical weathering

• Regolith (loose surface material)

What are the results (products) of chemical weathering?

• Clay minerals

• Dissolved ions (e.g., calcium, potassium, bicarbonate)

• Altered rock composition

• Soil formation components

What is the most important agent for chemical weathering?

Water drives dissolution, hydrolysis, and transports ions.

What are the factors that control (affect) soil formation?

• parent material

• climate

• topography

• organisms

• time

Ice growth

• Frost wedging

• Liquid water gets into cracks and later freezes

• Ice growth pushes rock apart

• Frost heaving

  • Subsurface ice lens grows as subsurface water freezes

  • Ice growth pushes rocks up and fractures overlying rock

Salt crystal growth

• Salty water penetrates crevices and pore spaces in rock

• Salt crystal growth pushes rock apart

Sheeting

• Pressure reduction results in fractures near surface

  • Large mass of igneous rock previously below surface exposed by erosion

• Reduction of weight reduces pressure

  • “Unloading”

• Pressure reduction results in fractures near surface

  • Sheets eventually break off from surface

Thermal stressing

• Repeated rock heating and cooling → fractures

  • Diurnal cycles

  • Seasonal cycles

Biological activity

• Organisms break apart rocks (and other materials)

  • Growth of plant roots

  • Plucking by attached organisms

  • Human construction activities

Spheroidal weathering

creates rounded shapes from initially angular rocks

Dissolution

• Chemical breakup of a material by solvent to produce homogenous solution

• Dissolution of rocks by water → interesting landscapes

  • Karst terrane

Hydrolysis

Water reacts w/ mineral + produces diff mineral

• important for decomposing silicates

Hydration

• Water is added to a mineral

  • Changes the structure of the original mineral

  • EX:

    • Anhydrite to Gypsum

    • Clay mineral hydration

Oxidation

• Oxygen (or other oxidizing agent) reacts w/ rocks and minerals

  • Changes chemical composition of original mineral

Rate of weathering depending on

• Material composition

• Material texture

• Environment (climate)

• Degree of seasonal variability

Differential weathering

• ifferential weathering:

  • Uneven weathering of rocks and rock formations due to

    • Local variations in environmental conditions, and/or in rock properties

  • Can produce interesting landforms

Regolith

Layer of rock and mineral fragments produced by weathering

Soil is a combo of:

• Unconsolidated mineral gains

• Unconsolidated organic matter

• Water

• Air and other gases

Developed soil structure

• Layers = horizons

  • O (organic) horizon: mostly organic matter

  • A horizon: organic and mineral matter

  • E (eluviation) horizon: little organic matter

  • B horizon: zone of accumulation

  • C horizon: partially altered parent material

• Vertical slices thru soil horizons = soil profiles

Soil erosion

• Natural process

• Human activities can enhance

  • Over-cultivation

  • Clearcutting forests

• Human activities can enhance

  • No-till farming

  • Contour flowing

  • terracing

Facts that affect erosion rates:

• Climate

• Topography

• Geologic material

• Extent and type of vegetation

Erosion can occur as a result of:

• Flowing liquid water

• Glacier advancements

• Wind

• Gravity

Physical weathering (also known as mechanical weathering) is

the breaking of a rock into smaller pieces without changing the chemical composition or
mineral structures of the rock

The dissolution (dissolving) of a rock is an example of

chemical weathering

Chemical weathering operates

on the exposed surfaces of a rock

When liquid water freezes it

expands

Soils typically have

most of their organic matter in their upper layers

Sedimentary rocks are built from the products of

physical + chemical weathering

In the context of sediments and sedimentary rocks, the terms “well sorted” and “poorly sorted” refer to the

sediment size range in the sample

Sediments that have angular shapes

were likely transported only a short distance and/or for a short period of time

Limestone is an example of a

chemical or biochemical (also called non-clastic) sedimentary rock

Most of the limestone deposits we see today were formed in

marine environments