Geoscience Reference
In-Depth Information
even before I could say a single word through the interpreter, the driver Crocodile, smiling
with pride, quickly ran to the rock to be the fi rst to kick it. He kicked with all his might, but
the solid glassy rock did not budge and left his foot bleeding and swelling with excruciating
pain. I had to drive on our return to the university, with the driver Crocodile only being able
to navigate me with the help of my student interpreter. I am absolutely confi dent that those
students kept in mind for the rest of their entire lives that vivid lesson on physical weather-
ing, and on the role of climate, which is modifi ed by the specifi c nature of weathering rock.
And the driver Crocodile is still probably thinking that the white “khavadia” is not worth
trusting.
In regions outside of deserts, there are zones with rainy seasons where water is
fi lling fi ne cracks freshly appearing on the surface of otherwise compact rocks. If it
happens in zones of a mild climate, frosty days and weeks having temperatures
below freezing are prevalent during the winter. Water in the cracks freezes, and
since ice has a bigger volume, the freezing of water acts like a chisel expanding the
original thin cracks. As the freezing, or combination of freezing-melting-freezing
repeats for many years, the individual minerals are fi nally released. A similar but
less intensive effect transpires when clay and soluble products of chemical weather-
ing fi ll the cracks. Water running across the mineral surface after a thaw or rain
loosens and transports unbound minerals away. As the suspended liberated particles
hit each other as well as other obstacles, they are scraped and abraded. When they
fi nally subside, they are rounded and free of their original shape.
The fi ne roots of plants act in a similar way when they penetrate into tiny cracks
where they are searching for water and nutrients released from the rock by chemical
weathering. The fi ne hairlike roots are the extension of bigger roots, and both are
growing to produce pressures that split microscopic cracks and pores. The dead
roots remain in cracks, and as they are decomposed and chemically transformed,
additional stresses begin.
A special type of physical weathering is the action of glaciers. Glacial ice fl ows
at a very slow pace. The force driving the ice is the resultant of the continual force
of gravity being linked to the variable action of other forces. Rather than describing
the physics of ice fl ow, we accept the fact that the driving force exists and glaciers
move slowly and continuously. The slowly moving ice causes abrasion of the rock
at the front, bottom, and sides of the glacier. Slowly but surely, a mound or dam
consisting of boulders, small stones, sand, and clay is eventually formed at its
exposed peripheral locations. These mounds of accumulated glacial debris are
called moraines. A terminal moraine is formed at the front part of the glacier, and if
the glacier has partly melted, the moraine is no longer connected with the glacier
and marks the greatest advance of the glacier during the last thousands of years.
According to a nearby town and village or a well-known location, geologists have
given names to these moraines that are typical for individual glacial periods of the
Pleistocene. Today, the majority of all well-known moraines are now on dry land,
signifi cantly distant from the glacier. Their distant locations are caused by the fact
that during the past 11,500 years, interglacial temperatures were 5-9 °C higher than
those in glacial periods and lasted long enough to allow a partial melting of the
glacier that originated during 100,000 years of the last glacial period or by the fact
