4.3 The Products of Weathering and Erosion

GEOL1340

The Sedimentary Cycle

The 5-step chronological sequence transforming rock: 1. Weathering -> 2. Erosion -> 3. Deposition -> 4. Burial -> 5. Lithification.

Weathering vs. Erosion

Weathering is the breakdown of rock in place at Earth's surface, whereas erosion is the removal and transport of weathered material by water, wind, ice, or gravity.

Solid Clasts (Detrital Sediment)

Insoluble rock and mineral fragments (gravel, sand, silt, mud) produced by physical disintegration and chemical alteration.

Dissolved Ions (Chemical Sediment)

Soluble ions (Ca2+, Na+, K+, Mg2+, HCO3-, SiO2) leached out during chemical weathering that travel in surface runoff and groundwater.

Soil

An unconsolidated mixture of weathered mineral fragments, clay, and organic matter that sits directly on bedrock.

O Horizon

The uppermost soil layer dominated by organic material and humus.

A Horizon (Topsoil)

An organic-rich soil horizon actively subject to mineral leaching by percolating groundwater.

E Horizon (Zone of Eluviation)

A light-colored, heavily leached soil layer where clays and soluble minerals have been washed downward.

B Horizon (Subsoil)

The zone of accumulation where leached clays, iron oxides, and aluminum oxides collect.

C Horizon

Partially weathered parent rock (regolith) resting directly above solid bedrock.

High Soil Leaching

Heavy rainfall strips away soluble ions (Ca2+, Mg2+, Na+, K+, SiO2), leaving behind insoluble iron and aluminum oxides like Bauxite (aluminum ore).

Low Soil Leaching

Occurs in arid climates where limited rainfall allows soluble ions to accumulate in the upper soil horizons.

Controls on Soil Formation

Climate (primary driver), Parent Rock Composition, Organisms, Relief/Topography, and Time.

Sediment Sorting

The degree of uniformity in particle size; well-sorted sediment indicates long-distance transport or uniform energy (like wind or waves).

Sediment Rounding

The wearing away of sharp edges on clasts during transport; rounded grains indicate longer transport distances or abrasion.

Textural Maturity

Sediment that is well-sorted, well-rounded, and dominated by durable minerals like quartz.

Compaction

The squeezing of buried sediment layers under overlying weight, which reduces pore space and expels trapped water.

Cementation

The precipitation of dissolved mineral ions (typically calcite or quartz) in remaining pore spaces to bind clasts into solid rock.

Feldspar Weathering Products

Clay minerals (kaolinite) plus dissolved potassium, sodium, and calcium ions in solution.

Ferromagnesian Weathering Products (Biotite, Amphibole, Pyroxene, Olivine)

Clay/alteration minerals (chlorite, serpentine, iron oxides) plus dissolved iron and magnesium ions in solution.

Quartz Weathering Products

Durable quartz sand grains, resistant to both chemical acid attack and mechanical abrasion.

Dominance of Quartz Sand

Quartz is abundant in sand deposits because it is hard, lacks cleavage, and is chemically unreactive under surface conditions.