WEATHERING - Sites @ WCNR

WEATHERING

the interacting physical, chemical & biological processes that progressively alter the original lithologic character of rocks to produce secondary minerals (e.g. clays) & unconsolidated regolith

Weathering ? breakdown of rock materials Erosion ? transport of broken-down materials

Physical weathering ? disintegration operate simultaneously & Chemical weathering ? decomposition affect each other

Physical weathering: the transformation of rocks into unconsolidated debris; occurs when stress is exerted along zones of weakness within the original material (bedding, fractures, mineral boundaries)

454 lecture 2

Stress field originates via expansion of rocks or minerals, or pressure caused by growth of foreign substance in voids:

thermal expansion: low thermal conductivity of rocks allows them to expand at surface, but not at depth, creating differential stress & spalling

unloading: confining pressure decreases when overlying rocks are removed, & sheets form perpendicular to the direction of pressure release, mirroring surface topography

hydration: hydration of salts or mineral expansion causes

exfoliation at the large scale, spheroidal weathering at

small scale

growth in voids: openings may expand at different rates or in

different directions, creating differential pressure

frost action

plant roots

minerals (percolating fluids evaporate, supersaturate, & precipitate;

e.g. sulfates & carbonates in arid/semiarid regions)

454 lecture 2

Sandstone, Arches National Park, Utah

unloading

Granite, Poudre drainage, Colorado

Granite, Yosemite National Park,

California

454 lecture 2

Granite, Missouri

Granite, central Australia

Sandstone, central Australia454 lecture 2

Physical weathering dominates where mean annual T < 0? C, & there is precipitation

454 lecture 2

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