the total mass of life pales in comparison to the mass of the outermost crust. Plant
roots can mechanically increase the size of the openings in rocks. Plants can
change the amounts of water in a soil profile and thus alter chemical weathering
via soil moisture. The respiration of plants releases carbon dioxide, which com-
bines with water to increase the acidity of water. Plants are able to shade the soil
thus altering the temperatures and suppressing the rates of chemical reactions.
After plants die and become incorporated in humus, humus keeps the soil moister
by its impressive water-holding capacity. More remarkably, plants produce
organic substances known as chelates capable of chemical alteration of rock by
removal of positively charged ions. Some animals are able to ingest soil materials
and this breaks apart some materials for further weathering. Burrowing animals
move materials to other locations, including down and up within soil profiles,
and instigate further weathering.
Rocks andsoilsreadied by weathering are primed for transportation. Rocks,
rock fragments, and soils on any slope are held against the force of gravity. The
steepest angle of slope for a given type of material is called the angle of repose.
Any material on a slope steeper than the angle of response is subject to downhill
movement impelled by gravity. Such motion may be triggered because of lubrica-
tion by rainfall,earthquakes, the addition of materials onto the slope, the under-
cutting of the slope by a stream, or any number of other environmental events,
including the actions of humans.
Weathering and Mass Wasting 363Angle of Repose
A term fromgeomorphologyreferring to the angle of looseEarthsurface materials with
the horizontal. In a simple way we can observe the angle by digging a hole in beach sand;
steep-sided walls readily collapse and assume the angle of repose. The most prominent exam-
ples of the angle of repose are illustrated in the rock debris called talus (scree) that resides at
the bases of cliffs after having broken off and fallen down the cliff; these steep rock piles can be
many tens of meters tall and active ones are obvious by their lack of vegetation. The maxi-
mum sustainable angle of repose is also known as thecritical angleat which the force of gravity
is balanced with the force of resistance holding the material in place. Resistance is governed
by the cohesiveness of the material, friction between the pieces of the material, its surface
area, and its density. Resistance can be weakened by lubrication by water,earthquakes,
and even digging by humans. Once gravity overcomes resistance the material must move
downslope and assume a new angle of repose. The rate of movement ranges from inexorably
slowly to well over 100 kph (60 mph). The maximum angle of repose varies markedly in differ-
ing materials and differing climatic environments. Dry sand assumes an angle of repose of up
to 35 percent while some rocks repose at angles exceeding 45 percent.