A variety of
natural processes break down exposed rock on the planet’s surface. Known as
weathering, this phenomenon contributes to the formation of soil, and it can
eventually lead to the creation of geological wonders such as the Grand Canyon.
Today I would like to talk about the two most common forms of weathering.
The first of
these is physical
weathering. It involves
the application of sufficient physical stress to cause a rock to fracture into
smaller pieces. One cause of physical weathering is thermal expansion.
You see, when a slab of rock is exposed to sunlight, its surface temperature increases. But rock isn’t a good conductor of thermal energy, so its interior remains cool. Now, as the surface of a rock formation gets warmer, it begins to expand. And of course, when temperatures drop at night, it shrinks back to its original size. This constant cycle of expansion and contraction creates areas of extreme physical tension within the rock, which eventually causes the upper layers to break off.
The
other process I want to talk about today is chemical weathering. It occurs as a result of the
interaction between chemicals in the atmosphere and rainwater. Gasses such as
carbon dioxide and sulfur dioxide create acids when combined with water. And
these gasses are found in the atmosphere . . . particularly in regions with
high levels of pollution. This means that rainwater is often mildly acidic. So, when it falls on an exposed
rock formation, it can actually dissolve the surface layers. The process is accelerated when
rainwater encounters rocks such as limestone and marble that are made up of
minerals containing calcium carbonate . . . Um, this compound is highly
reactive to acid, meaning that formations that contain large amounts of it are
usually eaten away very quickly.
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