Geoscience Reference
In-Depth Information
continents and islands. And, as we show two pages later on, the confi guration of
continents was never constant in the past. Supercontinents existing in early stages of
the Earth's history broke up into continents and islands which later rejoined at least
partly into other continents only to break up and partially join together again. The
wandering of their ever-changing positions, shapes, and sizes was so fascinating
that the South African geologist Alexander du Toit wrote in 1937 the topic
Our
Wandering Continents.
This geological unrest lasting for hundreds of millions of
years infl uenced soil genesis a countless number of times.
The pioneer plants mentioned above acquired a new habit of tremendous global
importance - they gained their energy directly from solar radiation, extending what
eukaryotic microorganisms and marine algae were doing earlier, but with smaller
impact upon the Earth system when we compare it to the role of terrestrial plants.
With the solar energy the plants transformed water and atmospheric CO
2
into
organic compounds. They also required nutrients such as phosphorus, potassium,
and others that became available during the occurrence of chemical weathering of
minerals composing the original rocks. Upon being absorbed by plant roots, those
nutrients were distributed into all organs of the plant's body. The carbon cycle origi-
nated from the same sequence. Carbon as a simple gas CO
2
is accepted from the
atmosphere and transformed into more complicated organic compounds that pro-
vide nourishment for terrestrial animals and alimentation for microorganisms. In
both cases, this part of the C cycle is closed inasmuch as they are again decomposed
to CO
2
. If we intend to balance the whole cycle, we must include within the cycle
the carbon kept in bodies of animals and in their secretions. Many of the compounds
formed in both processes of composition and decomposition have decisive infl uence
upon soil physical and chemical properties. The most remarkable characteristic of
all initial processes of soil formation was the presence of living organisms, since
without them soils could not originate and nowadays they cannot exist. Just super-
briefl y: no terrestrial life - no soil and vice versa: no soil - no life on continents
and islands.
The mineral material from which soil originates is called parent rock. It could be
a solid rock, or a silty sediment called loess blown by winds tens of thousands of
years ago and deposited, or sediments transported and deposited by rivers, or large
areas of sea bottom originally muddy-like and dried out when the seawater level
sank by tens of meters. Rocks have their own history of origin. Since it is not our
intention to offer a complete family tree starting at the origin of our planet Earth and
since this detailed knowledge is not required in soil science, we start with the solidi-
fi cation of the melted magma on or close to the Earth's surface. When the hot lava
is cooling and solidifi es, a strange transformation occurs. The hot liquid or plastic
material appearing rather like a homogeneous corn mush solidifi es into a mixture of
small or bigger minerals fi xed fi rmly together. There are two types of rocks differing
one from the other because of the size of their minerals. The fi rst example is granite
with whitish, gray, and black minerals that can be recognized and seen by the naked
eye. The second example is basalt that looks like a dark gray to black monolithic
stone and only a more detailed study optimally made with the aid of a magnifying
glass will reveal individual minerals having different colors. Compared with granite,
