Geoscience Reference
In-Depth Information
residuals and shells of small mollusks buried millions of years ago. Again, if
increased pressure and heat act upon them, the rocks are transformed into
metamorphic rocks analogously to igneous rock transformation. Slates and marble
are examples. The great majority of the top upper part of the Earth's crust is formed
by sedimentary rocks that originated from transported products of weathering. On
the other hand, a minority of the weathered material originated in place without
transport. Owing to a combination of physical, chemical, and biological processes,
sedimentary rocks were consolidated and bound together by various binding agents
such as compounds of ferric iron or lime (calcium oxide) manifesting, respectively,
colors of rust or light beige.
5.1
Rocks' Weathering
The word weathering indicates that the parent materials, i.e., rocks, are being trans-
formed under the infl uence of weather. Initially, after the entire extent of each solid
rock is cracked or partially crushed, water enters the cracks, and after its long-
lasting direct contact with the rock's minerals, their chemical composition is
changed. The dissolved simple compounds associated with the macerating rocks are
transported away by water leaving a residual of pulverized rocks and transformed
minerals.
Weathering processes are most intensive at the surface of the Earth where it is in
direct contact with the atmosphere and hydrosphere. The intensity decreases with
depth below the surface and depends mainly upon the strength and time span of
numerous factors. The depth of weathering ranges from several millimeters to tens
of meters. Some rocks weather easily, while others are very resistant. For example,
sedimentary rocks weather rapidly to unconsolidated smaller particles of sand and
silt, while volcanic rocks are much more resistant to weathering. One should not
make the mistake of thinking that individual particles of sand and silt are at their
fi nal stage of weathering. Although the weathering of solid rock to small individual
particles takes many, many years, further sequences of weathering continue to pro-
ceed at various rates without ever reaching a true fi nality characterized by well-
defi ned end products. Intensities of weathering are infl uenced by the mineralogy of
individual particles and their layering. It is common to believe that rates of weather-
ing typically decrease with time -
high
at the start and
later
diminish to a legendary
rate of zero. Such a belief is a myth because words like
start
and
later
have impre-
cise meanings when the time scale of geologic processes is not clearly defi ned or
understood. At any time, weathering intensity depends upon the immediate condi-
tion and extent of disruption of the rock and upon the characteristics of the climate.
In the warm, humid climate of the tropics, the intensity rate is high compared with
that in temperate zones.
