Geopix UK - Weathering & Erosion

Weathering

Consolidated rock formed by igneous, sedimentary and metamorphic processes will be broken up and broken down by the action of the weather. The broken and altered material is transported by water, wind and ice and deposited as sediment, the foundation for a new generation of sedimentary rocks.

Weathering is the distintegration of rock in situ by the action of elements of the weather, with little or no transport.

Weathering can be divided into Physical and Chemical Weathering.

In physical weathering, mechanical forces from expansion and contraction break solid rock into fragments.

freeze-thaw action or frost shatter:
the force comes form the expansion of water when it freezes to ice
(common enough when the winter
chill freezes water pipes!).

insolation weathering
(or exfoliation weathering):
here the force comes form the expansion of the outer film of rock when exposed to full sunlight in deserts, followed by contraction during the cold nights.

biological weathering:
this can be included in physical weathering because the growth of roots exerts a mechanical force on the crack in which the tree is growing (it works for abandoned roads and buildings!).

salt crystals:
the growth of salt crystals as they evaporate from, for example, sea water provides the force to prize apart the grains of a rock.

For chemical weathering, the composition of the original minerals is altered by the action of carbon dioxide, other acids and oxygen dissolved in rain and soil water. A common example is the alteration of feldpar to clay minerals.

Granite, for example, will change by chemical weathering, to a mixture of clay minerals and sand. Add some decaying vegetation (humus) and soil is formed. Soluble salts drain away to the sea, giving organisms the materials they need to make shells and skeletons.

Spheroidal (onion skin) weathering produces interesting shapes.
A cube is weathered into a sphere as chemical attack is concentrated at the corners and edges of the cube. Usually, the effect is seen on the face of an exposed rock surface. Occasionally, the spheres of granite are piled up like marbles.

Chemical processes involved in the weathering of rocks include

oxidation: reaction with oxygen, usually to form an oxide
reduction: removal of oxygen from the formula
hydration: absorption of water by the mineral lattice (no change in composition)
hydrolysis: reaction between silicates and acidic water to form new minerals
carbonation: reaction between metal ions and carbon dioxide to form carbonates
solution: dissolving of a mineral in water (no change in composition)

The weathering of buildings provides 'experimental' evidence about the rate of weathering, since the age of the building is known. Unfortunately, building stones are not always chosen for their resistance to chemical attack; more unfortunately, the rate of decomposition is accelarated by atmospheric pollution.