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Soil Genesis
SPS 350
Soil Genesis
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Soil genesis consists of two main steps:
1. Accumulation of parent materials
2. Differentiation of horizons in the profile
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These two steps are not clear cut and
distinct rather they merge and overlap so
that we cannot really tell where one
stops and the other begins.
Accumulation of parent material
– This deals mainly with the different rock types
and the weathering of these rocks into
materials from which soils form
Rocks and Rock weathering:
• Of more than 90 elements that are known to
comprise matter, 8 make up approximately 98%
of the earth’s crust.
• These elements exist in various chemical
combinations called minerals, which in turn are
combined to form rocks.
• An understanding of the characteristics of the
rocks and their transformations from one
another is basic knowledge required for the
study of clay mineralogy, soil formation, and the
mineral nutrition of plants.
Rocks important in soils
• Rocks may be classified in three groups
depending upon their mode of formation,
although there are many rocks transitional
between these groups which defy a simple
scheme of classification. The three groups
are:
– Igneous
– Metamorphic
– Sedimentary
Igneous rocks
• Igneous rocks are formed by the cooling and
subsequent solidification of molten material called
magma.
• A magma is a molten rock mass. A magma
consists not only of molten rock matter, but also
suspended solids and dissolved gases called
volatiles.
• The kind of igneous rock formed depends on four
factors:
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The cooling rate magma
Chemical composition of magma
Water content of magma
Depth of solidification of magma
Cooling rate:
• The rate of cooling determines the texture of the
igneous rock.
• Texture-refers to the sizes, size range, shapes
etc of mineral grains in the igneous rock.
• If the cooling is slow the resulting igneous rock
will be coarse textured
– plutonic or intrusive.
• where magma cools suddenly as in the case
where it is extruded onto the earths surface by
volcanic action, the igneous rock will be fine
grained
– volcanic or extrusive igneous rocks.
Chemical composition:
• Chemical analyses of many" igneous rocks have shown that
the silica content contributes to the type of igneous rock
formed.
• A magma which is rich in silica (SiO2) is more likely to form
glass if cooled rapidly at or near the surface of the earth.
• Silica-rich magmas are very viscous, slowing down the rate
at which crystals can grow thereby leading to fine textured
or glassy material.
• Silica content is also responsible for the acidity of rocks.
Magma containing 60% or more silica solidifies to form what
is termed acidic igneous rocks, those containing less than
50% silica produce basic igneous rocks, while magmas
consisting of 50 to 60% silica give rise to intermediate
igneous rocks.
Water content:
• In general, as the amount of water
contained in a magma increases, so does
the size of the crystals which grow during
solidification.
Depth of solidification:
• The depth at which a magma solidifies indirectly
influences the way in which a magma solidifies
because:
• Magmas cool more slowly at great depth than at
the earth's surface because the magma at depth
is insulated by the surrounding rocks
• Water is lost rapidly at or near the earth’s
surface in form of vapor. It is retained at great
depth because of the confounding pressure.
Examples of Igneous Rocks
• Because of their very common
occurrence, granite and basalt will be used
to represent the igneous rocks
Granite:
• Granite is the most extensively occurring igneous
rock. It constitutes the central core of many great
mountain ranges, where it has been exposed by
long-continued erosion through past ages.
• Granite is the typical bedrock of the continents
and is associated with some of the most important
ore deposits.
• Most granite have the appearance of an even
textured, coarse grained, light colored rock which
consists chiefly of the potassium feldspars and
quartz with smaller amounts of amphiboles and
micas, especially biotite.
• Granite can have many colors depending on the
color of the feldspar and other minerals present.
Basalt:
• Basalt is an extrusive rock having formed
mostly in widespread lava flows.
• Basalts are dense, fine grained and are
composed chiefly of the plagioclase
feldspars and pyroxenes (augite) and
smaller amounts of the amphiboles
(hornblend), iron oxides and olivine.
• Many modern volcanoes consist largely of
basalt.
Classification of Igneous rocks
• Many different schemes have been proposed for
classifying igneous rocks. Most classifications
make use of at least 2 kinds of information:
• The occurrence
– Occurrence refers to where the rock formed i.e
surface (volcanic) or at depth (plutonic). The best
evidence we can use when studying a specimen is
the texture the rock. The texture is an indication of:
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the geological conditions of temperature,
pressure,
depth and chemical composition.
a reflection of the rate of cooling. Coarse texture from slow
cooling, fine texture from rapid cooling, porphyritic texture
probably from a combination of both,
• Mineralogy of the rocks
– The mineral composition, depends upon the chemical
composition of the molten material from which igneous
rocks are formed.
– If the minerals in a rock are known, the chemical
composition is not difficult to calculate; but the reverse is
not true, because different minerals in various
proportions may crystalize out of the same magma.
• Granite and obsidian look very unlike –one containing quartz,
feldspar, and other minerals, the other consisting almost solely of
volcanic glass, yet they may have virtually identical amounts of
silica, aluminum, and other chemical constituents.
• The difference being at what depth each was formed. Hence
texture and composition must be considered together in other to
determine the name of the rock.
Sedimentary Rocks
• Sedimentary rocks are secondary in origin i.e the
materials of which they are composed are derived by a
chain of processes including weathering, erosion,
transportation and deposition of pre-existing rocks.
• Sedimentary rocks are the final products of several
complex and interacting geological processes and are
important for several reasons:
– these rocks form a thin layering covering about 75% of the land
surface of the earth.
– sedimentary rocks contain virtually our entire record of life
– they also contain virtually all of our petroleum, natural gas, and
coal, and even much of our uranium.
Sedimentary materials and their
origin:
• Even the most deeply buried igneous rock's will some
day be exposed by the processes of erosion.
• Once exposed, weathering sets in and the products will
be carried by water and wind and deposited elsewhere in
layers, one upon another like pages in a book
•
• Water is by far the most important factor in transporting
this material and the water-borne sediment may:
– settle out of the water simply because of its weighty or
– it may be precipitated chemically from a state of solution.
• When the settled-out material is converted into firm rock,
it constitutes sedimentary rocks.
Examples
• Iindividual sand grains may accumulate as sediment until they
are changed to sandstone.
• Coarser pieces called gravel will make a conglomerate.
• Finer particles, clay or mud, will become shale.
• The rock waste or the sediments may have been derived from
an extensive area, or it may have come from a limited source.
• The fragments can exist in a wide range of particle sizes, and
the particles may be rounded or angular and well sorted
(narrow range of particle sizes) or poorly sorted (wide range of
particle sizes).
• These characteristics of sedimentary rocks provide useful
information
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about the sources of the sediments,
their means of transportation and
the kind of environment in which they were deposited
these information are helpful in unraveling the history of a given part of
the earth.
Another example
• The other type of sedimentary rock
consists of dissolved matter that has
precipitated out of solution just as salt is
deposited when a pan of salty water is
evaporated.
• Minerals formed in this way are called
evaporites and include halite (NaC.l),
sylvite, gypsum, and anhydrite.
Sandstone
• These are composed or sand size grains whose mineralogy is variable
depending upon the nature of the source of the sediments, although
quarts predominates.
• The sand grains are cemented together by iron oxides, silica or
carbonates and the color of the rock is largely dependent upon the
nature of the cement.
• The gritty nature of sandstone is indicative of a coarsely porous
material and because of this property, petroleum and water are usually
preserved in sandstone.
• Carbonate cements are relatively soft so that when the rock is broken
the grains tend to stay intact while the cement breaks; hence, the
surface of the rock tends to be grainy like a piece of sandpaper.
• Siliceous cements are among the toughest of all cements. The
cement is often as tough as the particles it holds together. A rock that
is cemented with silica will often break right through the grains rather
than around them. Ferruginous cements are typically dark colored
(reddish, brownish, yellowish brown. Iron oxide cements can be fairly
tough but their colors give them away.
Shale
• Shale is the most abundant of the sedimentary rocks in
all continents.
• They are very fine grained sedimentary rocks which
have been formed by the consolidation of mud
sediments consisting of mixtures of silt and clay.
• They are composed chiefly of clay minerals, feldspars,
micas and the like; however, those mineral constituents
are too small in size to permit their recognition by the
unaided eye.
• Shales are soft and easily scratched. Their color is
usually gray, although brown to black shales containing
appreciable quantities of organic matter occur.
Limestone
• Calcium carbonate comes out of solution to become
limestone and it is the chief among the sedimentary rocks
that are formed in this way rather than by settling of actual
fragments?
• The mineral CaCO3 is converted into a deposit of limestone
in two main ways:
– By saturation of the solution and the resultant precipitation of the
excess CaCO3 which can no longer stay dissolved.
– By the action of plants and animals.
– These organisms remove CaCO3 from the water in which they live
and use it to reinforce their skeletons or construct their shells e.g.
corals.
– Aquatic green plants can extract CO2 from the water causing
CaCO3 to precipitate and leading to the deposition of thick layers of
limestone.
• Limestones are usually fine grained, even textured and
consist largely of carbonates with admixtures of clay
minerals, iron oxides, and quartz as impurities.
• When the carbonate is calcite the limestone is referred to
as calcitic limestone and when the carbonate is dolomite
the rock is called dolomitic limestone.
• The color of limestone Is usually some tint of gray,
although white, yellow and browns are also common.
• Their appearance, however, is no indication of their purity,
but as long as the non-carbonate material present as
impurity is less than 50%, these rocks are still considered
to be carbonate rocks.
Uses of Limestone
• Limestone is one of the most important building stones in
some countries and stone quarrying is good business.
• Cement industries are based upon limestone as its most
necessary raw material.
• Soils derived from limestone are noted for their fertility
and pulverized limestone is usually added to acid soils to
offset -their deficiency in lime.
• Chalk is a very soft, fine grained calcitic limestone
composed of the shells of minute sea animals called
foraminifera.
• Marl is a loose, earthy material consisting of a carbonate
mixed with clays and silts in variable amounts.
Conglomerate:
• This consists of a mixture of materials ranging in size from
colloidal clay particles to good sized rock fragments. e.g,
– Gravel is the loose aggregation of material coarser in size than
sand. It eventually becomes conglomerate when it turns to rock
and sand grains fill the spaces between the pebbles and boulders
and all these are held together by cementing agents.
• Conglomerates are distinguished from one another by
shape of the particles.
– Those with angular edges being called breccia
– Those with rounded edges are called conglomerate.
– When conglomerates weather into soil forming deposits, the
cements give way yielding a coarse debris which is of little
agricultural value.
Other sedimentary rocks
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Coal
Gypsum
Rock salt
Chert –sedimentary rock weathered from
limestone
• Phosphate rock
Classification of Sedimentary rocks
• The classification of sedimentary rocks may be based on
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texture,
size,
composition,
origin,
structure or to
environment.
• Size range refers to the particle sizes according to the
wentworth scale.
• The amount of particulate material refers to weather the
rocks are detrital i.e. having a mixture of mineral and rock
fragments and therefore a broken texture, or non detrital
i.e. having a crystalline texture with little mineral and rock
fragments.
Metamorphic Rocks:
• Igneous and sedimentary rocks when exposed to
circumstances drastically different from those under which
they originated, can change into metamorphic rocks.
• The metamorphic rocks are rocks which have been formed
by marked transformations of igneous and sedimentary
rocks as a result of heat, pressure, movement of the crust,
and the chemical action of liquids, and gases.
• The final result which marks a metamorphic rock is a
significant change in texture or mineral composition, or
both.
• As a result of this change, metamorphic rocks are
distinguish from their sedimentary counterparts by their
crystalline texture, and from their igneous counterparts by a
banded or laminated appearance.
Metamorphic rocks and their origin.
• In terms of origin, metamorphic rocks more closely
resemble igneous rocks than sedimentary rocks.
• This is true because metamorphism takes place as the
temperature of a rock mass and the pressure applied to it
increase.
– If the temperature is high enough, part or all of the rock mass may
melt generating new magma.
• Both igneous and metamorphic rocks tend to have a
crystalline texture.
• The main difference is that igneous rocks develop their
crystalline textures as solid mineral phase from liquid
magma while metamorphic rocks develop their crystalline
texture as previously formed solid phase recrystallize to
form new mineral phases without passing through the
liquid state.
Quartzite
• Quartzite is formed from the metamorphism of sandstone.
– During the metamorphism, the original quartz grains are
recrystallized and therefore there is fusion between the quartz and
the cementing agent such that the individual quartz grains cannot
be seen.
– It is for this reason that quartzite is very coherent and hard and
breaks across both grains and matrix (conchoidal) whereas
sandstone breaks around the individual grains.
• Quartzite can be differentiated from marble, which
resembles it, by its failure to effervesce in acid and by
being too hard to scratch with a knife.
• However, like marble, it is massive and does not show any
lamination or foliation.
• Quartzite is very durable and wherever it occurs, it
supports hills and ridges, preserving them from erosion
while the adjacent topography succumbs.
Slate
• Slates are formed by intense pressure acting on shale
which is a sedimentary rock.
• The intense pressure changes shale to slate by rotating
the flaty grains until they lie with their flat surfaces facing
the direction of the force.
• Slates are very fine grained and are harder, more dense
than shales.
• Slates are commonly gray to black in color and usually
consist of fine white mica, quartz, graphite or
carbonaceous matter which - provides the usual darkgray color.
• Owing to its resistance to fire, its durability, and its
strength, as well as its cleavage, slate is the main stone
used in roofing in many countries.
Marble
• Marble is unquestionably the most beautiful of
all rocks and is prized for its inexhaustible
variety.
• It ranges in color from the snow-white type used
in statuary to the variegated ornamental type
and the uniformly colored architectural marble.
• Marble is merely metamorphosed limestone and
the colors are due to the presence of impurities
such as clay, sand, and iron oxide.
• Marble has a crystalline texture and is
composed of grains of calcite (rarely dolomite).
Gneiss
• Gneiss is a coarse grained metamorphic rock
having a banded appearance due to the
alternation of layers of unlike mineral
composition.
• The minerals in it are mostly feldspar, quartz,
biotite and muscovite mica and hornblende.
• Usually the quartz and feldspar occur together in
layers which are separated by thin bands of
mica.
• Gneisses are commonly derived by the
metamorphism of granites and some may form
by the transformation of conglomerates.
Schist
• Some schist rocks are derived by the metamorphism of
basic igneous rocks like basalts, and some from a
sedimentary rock.
• The schists are next to gneisses as the most common
metamorphic rocks.
• Of the schists, mica schist is the most frequently
occurring.
• It is composed essentially of quartz and either
muscovite or biotite mica.
• The mica plates all lie together parallel to one another,
giving the rock a striking laminated appearance.