Geoscience Reference
In-Depth Information
5.1.1
Physical Weathering
We are going to start the description of weathering with
physical weathering
that
looks like the simplest among all soil-forming processes. When a mineralogical
heterogeneous rock surface is radiated by the sun, the dark minerals absorb more
radiation than the light-colored minerals that strongly refl ect the incoming radia-
tion. We know it from our own experience since we dress in whitish clothing during
summer time in order to avoid our own overheating. The minerals composing the
rock increase their volume when their temperature rises. As the dark minerals are
warmer than the light minerals due to the absorption of sun radiation, they have the
tendency to increase their volume more than whitish minerals. We have to consider
also the fact that each mineral has its own value of volume increase with a specifi c
rise of temperature. Accordingly, we know that each mineral has its own coeffi cient
of volume extension. However, the compactness of the rock prevents the irregulari-
ties in volume extension to be realized and causes an internal stress. During the rela-
tively cold night, the volume also changes, but in the opposite direction. With the
minerals having the tendency to shrink, an internal stress is again produced. When
the tendencies are many times repeated, the individual minerals start to be separated
and cause cracks in the surface of the initially compact rock. The cracks are micro-
scopic, not visible by the naked eye. Sometimes the process continues to greater and
greater depth, while the individual minerals keep their original position. We have
observed this cracking in the dry climate of deserts and semideserts. We described
the occasion of discovering such a product of physical weathering in our topic,
Facts About Global Warming: Rational or Emotional Issue?
- Catena, 2010:
I (the fi rst author, MK) took my students of the Khartoum University to an excursion to the
north to visit the landscape of the sixth Nile cataract. The students sat on the bed of the truck
while I sat next to the local driver with one of the students as an interpreter. During the fi rst
half-hour we drove in the direction of rutty wheel tracks. When there were no more tracks
or any obvious trail to follow, we continued to take an unmarked route roughly to the north,
which only the driver knew that eventually led to the Nile cataract. The driver's name,
translated from Arabic, was Crocodile. Since I knew that animals have a better instinct for
direction, I trusted the driver mainly because of his name from the animal kingdom. The
students and I had hiking boots while the driver Crocodile was wearing homemade sandals
with soles cut from an old tire. I decided to stop close to an apparently unweathered coarse
granite rock. Knowing that visual teaching is the most effective, I walked to the rock where
I swung my right foot to the rear and then I kicked with all my force as if I were kicking a
penalty during a football game. The students were silent for a second and then they exploded
in a cheerful ovation when they saw that my entire boot penetrated into the “rock” like in a
heap of sand. Everybody tried the kick, even the driver Crocodile. He did it with less force -
his foot was in a sandal. My explanation on physical weathering of coarse-grained rocks
followed in English. I also did not forget to mention that fi ne-grained rocks do not all
weather in the same way. In the dry hot climate, they are sometimes covered by a compact
glassy coating as we had the chance to see later on at the Nile cataract. All of my “lecture”
was in English. With the driver knowing only a couple of English words, he defi nitely never
heard nor even understood terms like weathering and glassy compact coating.
We continued on our drive to the cataract where the Nile valley narrowed and the stream
was very swift and fi lled with rapids as its water fl owed with diffi culty around the obstacle
of large, fi ne-grained igneous rocks of Sabaloka mountains. Here we stopped again, and
