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Weathering and Mass Wasting

The materials on the land surfaces of our planet are not everlasting and do not stay
in the same place. Far from lasting forever, crustal materials are recycled on a
grand scale (seePlate tectonics) and reworked on localscales.Itistheselocal
scales that are addressed in the concepts of weathering and mass wasting. Briefly,
weathering is exposure of crustal materials to the atmospheric environment. This
exposure eventually changes the materials via mechanical, chemical, and biologi-
cal means. These materials are decomposed from large to small pieces and the
pieces can be more readily transported than the original rock. Ultimately, all rock
can be weathered into the tiny pieces called clay (seeSoils) given enough time.
The time is a function of rock type andclimate. Mass wasting is the downslope
movement of materials as caused by gravity. Weathering and mass wasting both
happen on time scales ranging from seconds to thousands of years so that many
of the actual processes are not frequently noted by humans.
Yet, the evidence is plentiful on the planetary surface. First, rock is not as uni-
formly “solid” as might be thought. There are many “nooks and crannies,” espe-
cially in the rocks nearest the surface. These openings include faults, joints,
cavities left by solution, and a range of other large and microscopic openings.
Various rock types are associated with different types of openings. For instance,
limestone tends to have regular horizontal and vertical jointing while this is absent
in basalt that has resulted from lava flows. Basalt frequently has lava vesicles—
small spaces from which gas has escaped during cooling into rock—while this is
not present in limestone. No matter how they are formed, the openings are path-
ways into which the agents of weathering can enter to hasten the weathering pro-
cesses. Rock is weathered from within and this vastly increases the area over
which the weathering agents can act. Sometimes, the manner of rock openings
exerts major control overlandscapesthat are produced. At Bryce Canyon National
Park, located in Utah, there are fantastic forms caused by weathering along closely
spaced vertical joints in limestone while in other places caverns are created by
water dissolving materials along horizontal jointing planes.
Mechanical weathering entails processes that change rock from larger to smaller
parts without chemical alteration. OverEarth, the mechanical process doing the
most massive alteration is known as frost shattering or frost wedging. The greater

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