Soil Genesis

1. Soil Genesis SPS 350

2. Soil Genesis • Soil genesis consists of two main steps: • Accumulation of parent materials • Differentiation of horizons in the profile • 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.

3. Accumulation of parent material • This deals mainly with the different rock types and the weathering of these rocks into materials from which soils form

4. 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.

5. 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

6. 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: • The cooling rate magma • Chemical composition of magma • Water content of magma • Depth of solidification of magma

7. 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.

8. 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.

9. 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.

10. 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.

11. Examples of Igneous Rocks • Because of their very common occurrence, granite and basalt will be used to represent the igneous rocks

12. 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.

13. 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.

14. 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: • 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,

15. 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.

16. 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.

17. 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.

18. 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 • 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. Other sedimentary rocks • Coal • Gypsum • Rock salt • Chert –sedimentary rock weathered from limestone • Phosphate rock

27. Classification of Sedimentary rocks • The classification of sedimentary rocks may be based on • 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.

28. 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.

29. 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.

30. 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.

31. 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 dark-gray 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.

32. 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).

33. 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.

34. 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.