Soil-forming factor 1: Parent material
Parent material refers to material from which the soil has been
derived and, in most cases, is of geological origin (see peat on
page 30). The nature of the parent material can have a profound
influence on the characteristics of the soil. For example, the
texture of sandy soils is determined largely by the parent materials,
which in turn controls the movement of water through the soil.
The mineralogy of the parent material is mirrored in the soil and
can determine the weathering process and control the natural
vegetation composition. For example, lime-rich soils are generally
derived from calcareous rocks (e.g. limestone, chalk) or sediments
derived from such deposits. In turn, lime-rich soils can offset the
development of acidic conditions but may not support plants that
are not tolerant of alkaline soil conditions (e.g. rhododendrons).
Two types of parent material are recognised: a) unconsolidated
deposits or loose sediments that have been transported by ice,
water, wind or gravity and b) weathered materials directly overlying
the consolidated hard rock from which they originate. In both cases,
the parent material can be weathered through physical destruction of
rock (freezing or drying cycles) or chemical reactions (dissolution of
elements). Weathered parent material is often referred to as saprolite.
While the forces created by the expansion and contraction of
minerals, induced by daily temperature variations, cause rocks to
shatter and exfoliate (especially in hot deserts), in most cases water
is the dominant agent in weathering processes. Water can cause
rocks to shatter through repeated freezing and thawing of water
trapped in rock cavities. Water also initiates solution and hydrolysis
(the destruction of a compound through a reaction with water that
produces an acid and a base) that liberate minerals contained within
the rock. Water also supports life which, in certain situations, is
a major contributor to the weathering process. Plant roots can
cause physical weathering as they grow and expand inside cracks
in the rocks. Roots and decaying vegetation also produce organic
compounds such as solvents, acids and alkalines that enhance the
actions of percolating rainwater.
The degree of weathering depends on a number of environmental
factors such as temperature (determined by climate, exposure and
altitude), the rate of water percolation (determined by texture,
relief, climate), the presence of oxygen (again texture and climate),
the surface area of the parent material (largely determined by the
geological structure) and the mineralogy of the parent material (for
example, quartz is much more stable than olivine).
Examples of soil formations on continuous, hard bedrock in Tunisia (left; GT)
and transported sediments in East Africa (right; EM), in this case, of fluvial
origin. Where consolidated parent material lies close to the surface, soil
depth is generally shallow and horizon development is weak. Unconsolidated
sediments can completely mask the characteristics of the underlying bedrock.
The map below shows the lithological properties of the surface geology
of Africa. Lithology describes the mineral composition and structure of
geological material which is based on rock formation (i.e. whether it is
igneous, sedimentary, metamorphic) and mineralogy (e.g. carbonate, silicic,
mafic). This map is a good proxy for soil parent material as it only reflects
surface conditions and not the underlying bedrock. It should be noted that
the general nature of this map means that at a local level, the conditions may
be quite different to that shown.
Other than the terms alluvium (deposited by water), aeolian (deposited by
wind), organic (peat deposits) and colluvium (transported by gravity), all of
which denote recent deposition, the age of material is not indicated. The
preponderance of wind-blown sediments across Africa is striking as are
the volcanic areas. Please consult the glossary at the end of the atlas, the
adjacent text box and page 28 for an explanation of the geological terms.
(USGS/JRC) [8a]
Weathering continues in the soil following a sequence from the least
to the most stable minerals. For example,
•
Early weathering: clay fraction of young soils, often
characterising dry conditions
Gypsum, Calcite, Hornblende, Biotite
•
Intermediate weathering: temperate soils, often with natural
grass or forest vegetation
Quartz, Muscovite, Vermiculite, Montmorillonite
•
Advanced weathering: clay fraction of highly weathered soils
of humid/tropical regions
Kaolinite, Gibbsite, Hematite, Goethite
Minerals undergo changes that cause the formation of secondary
minerals and other compounds that are soluble in water (to varying
degrees).
The distinctive features for Africa from a geological point of view
are four-fold:
•
most of Africa is made up of very old rock, often exposed
to the surface. In most cases it is crystalline and derived from
igneous sources (i.e. magma) and has often been altered
chemically (metamorphised);
•
the continent is slowly being torn in two along the great East
African Rift Valley - volcanic activity is common;
•
large parts of the African land surface have been geologically
stable for millions of years which gives rise to deep weathering
profiles;
•
large parts of Africa are covered by recent sediments, in
particular, wind blown sand.
A thorough summary of the geology of Africa is given in [8].
Surface lithology
Carbonates
Non-carbonate sedimentary
Metamorphic sedimentary
Basic igneous rock
Acid igneous rock
Metamorphic igneous rocks
Extrusive volcanic rocks and ejecta
Colluvium
Organic material
Aeolian sediments
Alluvium
Water
14
Soil Atlas of Africa | Introduction
Rock types
Igneous rocks are formed by the solidification of molten magma and
are the original source of all other rocks. They include rocks such as
granite, dolerite and basalt and are generally divided according to the
presence of the mineral quartz.
Sedimentary rocks are formed by the deposition of weathered
material by wind or water. Shales are deposited on ocean floors.
Conglomerates and sandstones are composed of resistant fragments
of other rocks while limestones and chalk are created through the
precipitation of calcium carbonate from solution.
Metamorphic rocks are igneous or sedimentary rocks that have
been transformed by intense heat or pressure resulting in changes in
mineralogy and structure. Examples include gneiss, marble and slate.