CHAPTER 5: WEATHERING, EROSION, AND SOIL

WEATHERING--the disintegration and decomposition of rock at or near surface.

MASS WASTING--the transfer of rock material downslope under the influence of gravity.

EROSION--the incorporation and transportation of material by a mobile agent: WATER, WIND, or ICE.

Weathering falls into two catagories: MECHANICAL WEATHERING (physical weathering process) and CHEMICAL WEATHERING.

MECHANICAL WEATHERING - "making big pieces into smaller pieces"

Types of Mechanical Weathering:

FROST WEDGING--ice forms in cracks, splits rocks. Expanding ice at a temperature of 22 ^oF ( -7.6^oC) can exert a pressure of 4.3 million pounds per square foot (2,100 kg/cm²) (atmospheric pressure at sealevel ~ 15 lb/ft²)
UNLOADING--removal of rock overburden causes rocks that were under pressure to expand, creating joints, cracks in a rock that have not had appreciable movement of rock along the cracks. The process of expansion by unloading leads to "sheeting" that forms exfoliation domes (such as in Yosemite National Park, CA).
Sheeting in mines due to the release of pressure, by tunneling or quarrying, can cause rock on the sides of shaft to dangerously explode off the walls in rock blasts.
THERMAL EXPANSION--daily heating cycle causes 30^o C variation; this causes expansion pressure on the surface of rocks that creates fractures.
ORGANIC ACTIVITY--plants (roots), burrowing animals, humans. Roots expand after a rainstorm.
Salt wedging--precipitating salts create a pressure as the salt crystals form. This force can break rocks apart. This is most important near the coasts where ocean spray is constantly being deposited on rock surfaces
RESULTS OF MECHANICAL WEATHERING--Mechanical weathering acts to increase the surface area available to chemical attach. For example, a fresh block of granite with a volume of 1 cubic meter has six sides with a total of 6 square meters of surface area. The surface area within a cubic meter of sand approaches a square kilometer! For each cubic meter of sand on the beach about 10 cubic meters of clay are deposited offshore. Clay particles are "flat"- similar to chips of mica, and have up to 100 times the surface area as an equal volume of sand. The greater the surface area, the greater the area to be worked on by chemical weathering.

CHEMICAL WEATHERING--involves chemical transformation into 1 or more new materials.

H₂O + CO₂ = H₂CO₃ (carbonic acid) from atmosphere and plant acids dissolve most rocks through time.
H₂CO₃ + CaCO₃ (calcite) = Ca⁺² + HCO₃^-, a reversible process (land and oceans). Calcite is the primary mineral in limestone. Calcite also acts a a cement in sediments to form sedimentary rocks.
Quartz (SiO₂ ) dissolves and reprecipitates as silica, which acts as a cement in sediments to form sedimentary rocks.
Feldspar weathers to become clay (reversible under metamorphic conditions).
Mafic Minerals become clay, silica, and iron oxide minerals (limonite/hematite).
Cations (such as K⁺, Na⁺, Ca⁺⁺ and anions (such as Cl^-, HSO₄^- ,HCO₃^-) stay in solution and add to the saltiness of the ocean, precipitate as salts, or are incorporated into clays in sediments.

WEATHERING FEATURES

Rock exposures, such as on cliffs or barren hilltops, have characteristic appearances due to Spheroidal Weathering (the "rounding of rough edges") that occurs because chemical attack is more effective on corners (it comes from 3 sides of the corner) compared to faces (chemical attack is only from one direction) and Differential Weathering (some rocks and rock layers resist erosion better than others) creating a rough weathered surface. Hoodoos and arches are the result of differential erosion and the beauty of these features has inspired us to create two national parks Arches and Bryce Canyon National Parks, both in Utah.

As chemical and mechanical weathering proceed, rock material accumulates as a REGOLITH layer of rock and mineral fragments produced by weathering. If water is available, plant and animal activity affect the material, and dead organic matter accumulates. This results in the FORMATION OF SOIL.

SOIL--mineral matter (~45%), HUMUS--organic matter (~5%), air & water (~50%).

CONTROLS OF SOIL FORMATION

PARENT MATERIAL--the original mineral matter can be either: RESIDUAL (regolith that accumulates "in place") or TRANSPORTED mineral material by water, wind, or ice.
TIME--important to all geologic processes; longer exposure = more weathering.
CLIMATE--water and temperature affect weathering processes. Hot & humid climates produce deep weathered soils lacking essential nutrients. Cold & dry climates results in very slow weathering which increases chances of wind erosion before soil can form.
PLANTS & ANIMALS: microscopic organisms do most of the work; the acculation of organic mater forms HUMUS; roots, worms, rodents, and insects rework the regolith and humus to form soil.
SLOPE - soils tend to be thickest on flat surfaces (due to mass wasting & erosion). Sunny slopes tend to be warmer and dry faster.

SOIL PROFILE

O horizon--"fresh" organic matter that accumulates on the ground surface.

A horizon--consists of mixed organic and rock fragments: water washes out fines (by a process called ELUVIATION); mineral nutrients dissolved away by process called LEACHING.

B horizon--zone of "accumulation" of fines; forms a "hardpan"
C horizon--(below "solum" or true soil) is a zone of weathering bedrock above the actual unweathered bedrock--still considered a regolith

THREE GENERAL KINDS OF SOILS

PEDALFER--soils rich in aluminum-rich clays and iron; midlattitudes, with annual precipitation of at least ~25 inches (East Coast region).

PEDOCAL--soils in dry regions (due to high evaporation rates). Enriched with calcium carbonate, caliche (Western United States).

LATERITIC--soils in wet, more tropical climates. All calcite and silica is leached from the soil, leaving red (iron) and aluminum (aluminum can be enriched enough to form the ore mineral Bauxite from which we obtain aluminum metal) (South).

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