Geography Reference
In-Depth Information
Mechanical weathering is the disintegration of rock and other solid earth material by physical
means. Here are the major mechanisms used to accomplish this:
Frost action. Some rocks allow water to occupy space between particles. If the water
freezes, the resulting ice crystals may exert outward pressure that cracks the rock.
Hot-cold fluctuation. In some locations, rock may be exposed to extreme temperature
fluctuations. Over time, alternating heat and cold causes expansion and contraction, respect-
ively, and may result in some parts of the rock breaking off.
Root action. Fine, delicate roots may find their way into cracks or spaces within rock. As
the roots grow, it may exert pressure sufficient to break the rock.
Abrasion. Chips may break off when a hard object scrapes or rubs against rocks. During
the Ice Ages, gigantic glaciers pulverized and ground up untold tons of bedrock, converting it
to pebbles, gravel, and soil particles that are still with us. This action continues with today's
glaciers, but on a much-reduced scale. Similarly, scraping or rubbing occurs when one piece
ofrockstrikes another.This may happen as arockrolls downhill, hitting other rocks as it goes;
or when high winds send sand particles smashing into other rock particles; or when a swiftly
flowing river causes one rock to hit another.
Chemical weathering
Chemical weathering is the disintegration of earth material by chemical means. Here are the
major mechanisms used to accomplish this:
Rusting away (oxidation). Lots of rock contain bits of iron. When oxygen combines with
the iron, rust develops and destabilizes the rock, possibly leading to its break-up.
Dissolution. Direct and prolonged contact between water and certain rock minerals may
cause the latter to decompose and contribute to the ultimate break-up of rock.
Carbonation. Atmospheric carbon dioxide may dissolve in rainwater and create a weak
carbonic acid. Long-term, this may also contribute to dissolution of certain elements in rock
and contribute to its ultimate break-up.
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