ES1002 Week 6: Weathering and Soils

What defines the critical zone?

It's the Earth's surface layer where rock is broken down, less dense than bedrock, has pore spaces, supports more water and plant life, and has a different color due to altered minerals.

What are the main types of weathering?

Physical weathering (breaks rocks into fragments) and chemical weathering (alters chemical composition).

What are key causes of physical weathering?

Temperature changes, frost wedging, abrasion by water, wind, and ice, and biological activity like plant roots.

What is frost wedging?

Water enters cracks, freezes, expands, and splits the rock apart.

How does abrasion cause weathering?

Particles carried by water, wind, or ice rub against rocks, wearing them down and rounding them.

Why does chemical weathering occur?

Most minerals are unstable at Earth’s surface conditions.

What factors influence chemical weathering?

Temperature, water availability, reactants like acids and oxygen, and concentration of products.

What is congruent vs. incongruent dissolution?

Congruent dissolves the whole mineral; incongruent dissolves specific elements.

What are common chemical weathering reactions?

Dissolution, hydration, redox, cation exchange, and secondary mineral precipitation.

How do lichens weather rock?

Physically by expanding in cracks and chemically by releasing organic acids.

How do plants contribute to weathering?

Roots pry apart rocks and release acids through decay.

What is pedogenesis?

The process of soil formation.

What are the main soil horizons?

O (organic), A (topsoil), E (eluviated), B (subsoil), C (parent material), R (bedrock) — “On apples every bee climbs red.”

What factors influence soil formation? (CLORPT)

Climate, Organisms, Relief, Parent Material, Time.

How does climate affect soil?

Higher temp & precipitation speed weathering and organic matter accumulation; arid climates lead to salt/carbonate build-up.

How does relief/topography impact soil?

Steep slopes have thinner soils; valleys have deeper, well-developed profiles.

How do organisms affect soil?

Through organic matter input, mixing, weathering, and erosion reduction.

How long does soil formation take?

A horizon: 1–2 years; initial B: ~40 years; well-structured B: 100s of years; mature soil: 100,000s years.

What is residual vs. transported parent material?

Residual forms in place from weathering; transported is moved from another location.

What are the types of peat?

Moss, herbaceous, woody, and sedimentary peat.

What is colluvial debris like?

Coarse, rocky, angular, poorly sorted, and well-drained.

What is alluvial soil?

Nutrient-rich soil deposited by rivers, found in floodplains, deltas, and alluvial fans.

What are the 4 main changes in soils over time?

Additions, losses, transformations, translocations.

How does a soil profile develop over time?

Starts with organic accumulation, followed by biological activity, then horizon differentiation and deepening.

How do forest and grassland soils differ?

Forests: acidic, organic layer on top, more leaching; Grasslands: more organic matter throughout soil, but less developed soil horizon.

What does "effective precipitation" mean in soil science?

Precipitation that contributes to soil moisture, aiding clay and organic matter formation.

How does weathering affect atmospheric CO₂?

Weathering of silicates removes CO₂; weathering of carbonates has no net effec