Geoscience Reference
In-Depth Information
When we mention dust storms, we cannot neglect the mechanical abrasion of
rocks and all obstacles protruding above the terrain by sand and dust particles car-
ried by the extremely strong winds in arid regions. An instructive example is the
Giza sphinx close to Cairo. Today, the sphinx's features appear hazy and not dis-
tinct. Such an appearance was not the intention of the sculptor about 4,500 years
ago. He did not do it on purpose. We judge in accordance with excavations and
according to statues protected from the outside atmosphere that the sculptor created
all details of the sphinx with great precision and clarity. Its contemporary shape is
the result of dust storms acting for more than 4,000 years. The sphinx had the same
fate as big rocks - bumps and folds of both were smoothed and differed only in the
extent of their abrasion. The surfaces of rocks are more smooth since the abrasive
action of the dust lasted longer, while in the sphinx we still recognize details of the
face. We are not speaking about several injuries or wounds caused by men, soldiers
in the last two or three centuries. Since the sphinx was carved out of a sedimentary
rock having certain layers softer than others, the softer layers have been more
abraded to create horizontal stripes not as a desirable decoration but a simple prod-
uct of nature.
5.1.2
Chemical Weathering
Chemical weathering causes distinctive changes of rocks and minerals. Principally,
it destroys crystal lattices of minerals and dissolves cementing materials of sedi-
mentary rocks that “glue” various mineral particles together. The weathering action
on the crystal lattice of a mineral can be demonstrated from an example and experi-
ence of our practical life:
The owner of an old house sells it to a developer who decides to increase the
strength of its external framework and simultaneously improve its appearance with
an attractive façade. All the way around the house, the workers fi rst construct a scaf-
folding made of tubes that were linked and fi xed together with screws and special
connections. With the tubular framework in place, the workers carried out the task
of annexing a continuous grid to the scaffolding. After adding water to a dry mixture
of plaster, they covered the grid and its framework with the moist mixture to form
an attractive façade. Having completed their work in a perfect manner, they eagerly
awaited the next day to view the dried texture and color of their handicraft. The
crystal lattice is represented in our story by the entirety of the constructed scaffold-
ing with its atoms modeled by the special, screwed connections. The scaffolding
tubes represent the forces binding the atoms together within the lattice. The dry
plaster mixture is an assortment of products from earlier weathering. During the
following night, a strong windstorm engulfs the modernized house. Although the
initial waves of wind vigorously vibrate the scaffolding, its tubular internal structure
cannot endure the repetitive strikes of wind pressure and, as a result, suddenly col-
lapses. This external action of forces imitates in our model the external factors caus-
ing weathering. The workers are disappointed the next morning to fi nd a big pile of
