Chapter two Origin of Soil and Grain Size

2.1 INTRODUCTION In general, soils are formed by weathering of rocks. The physical properties of soil are dictated primarily by the minerals that constitute the soil particles and, hence, the rock from which it is derived.

On the basis of their mode of origin, rocks can be divided into three basic types: Igneous, Sedimentary, and Metamorphic.

GEOLOGICAL (ROCK) CYCLE SOIL is the final product of rock weathering

2.2 ROCK TYPES Three major groups:

2.2.1 Igneous rocks Igneous rocks are formed by the solidification of molten magma ejected from deep within the earth’s mantle. After ejection by either fissure eruption or volcanic eruption. Extrusive rocks are the result of magma cooling at the surface(Rapid cooling),

and they are characterized by a fine grained structure Intrusive Rocks are the result of magma cooling within the earth’s crust (slow

cooling), and they are characterized by large crystalline structure.

Thus, depending on the proportions of minerals available, different types of igneous rock are formed. Granite, gabbro, and basalt are some of the common types of igneous rock generally encountered in the field.

Main Igneous rocks used for construction

• Granite: Often used for buildings and architectural construction • Gabbro: Used in crushed stone for concrete aggregate, road.. • Pumice: Used as an abrasive material in hand soup • Basalt: Used for concrete aggregate and for road base, asphalt pavement

aggregate

Pumice

Granite

Extrusive Intrusive

• Other physical agents that help disintegrate rocks are, wind, running water of streams and rivers, and ocean waves.

In chemical weathering, the original rock minerals are transformed into new minerals by chemical reaction. Water and carbon dioxide from the atmosphere form carbonic acid, which reacts with the existing rock minerals to form new minerals and soluble salts.

In mechanical weathering, large rocks are broken down into smaller pieces without any change in the chemical composition, it may be caused by: • the expansion and contraction of rocks from the continuous gain and loss of

heat, which results in ultimate disintegration. water seeps into the pores and existing cracks in rocks, as the temperature drops, the water freezes and expands.

2.2.1.1 Weathering

Weathering is the process of breaking down rocks by mechanical and chemical processes into smaller pieces.

2.2.1.2 Transportation of Weathering Products The products of weathering may stay in the same place or may be moved to other places by ice, water, wind, and gravity. The soils formed by the weathered products at their place of origin are called residual soils.

2.2.2 Sedimentary Rocks Detrital sedimentary rocks: The deposits of gravel, sand, silt, and clay formed by weathering may become: - compacted by overburden pressure - cemented by agents like iron oxide, calcite, dolomite, and quartz. Cementing agents are generally carried in solution by ground-water. They fill the spaces between particles and form sedimentary rock. (ex. Conglomerate Sandstone, Mudstone and shale)

Chemical Sedimentary rock also can be formed by chemical processes. (limestone, chalk, dolomite, gypsum…)

Conglomerate

Sandstone

Limestone

Lithification of Clay

CLAY

SHALE

Lithification

2.2.3 Metamorphic Rock Metamorphism is the process of changing the composition and texture of rocks (without melting) by heat and pressure. During metamorphism, new minerals are formed, and mineral grains are sheared to give a foliated-texture to metamorphic rock. (Marble, granite, gabbro, and diorite).

Under extreme heat and pressure, metamorphic rocks may melt to form magma, and the cycle is repeated.

2.3 Soil-Particle Size Soils generally are called gravel, sand, silt, or clay, depending on the predominant size of particles within the soil. To describe soils by their particle size, several organizations have developed particle-size classifications.

2.3.1 Clay 2.3.1.1 Definition • clays generally are defined as particles smaller than 0.002 mm. However, in

some cases, particles between 0.002 and 0.005 mm in size also are referred to as clay. Particles classified as clay on the basis of their size may not necessarily contain clay minerals.

• Clays have been defined as those particles “which develop plasticity when mixed with a limited amount of water”

2.3.1.2 Clay minerals Clay minerals are complex aluminum silicates composed of two basic units:

(1) silica tetrahedron (2) alumina octahedron.

• The three important clay minerals are: kaolinite consists of repeating layers of elemental silica-gibbsite .Each layer is about 7.2 Å thick. The layers are held together by hydrogen bonding. Illite consists of a gibbsite sheet bonded to two silica ,the layers are bonded by

potassium ions. Montmorillonite consists of one gibbsite sheet sandwiched between two silica

sheets.

• The clay particles carry a net negative charge on their surfaces, cations (+ve ions) in the double layer.

• Water molecules are polar. Hydrogen atoms are not axisymmetric around an oxygen atom; instead, they occur at a bonded angle of 1050 .As a result, a water molecule has a positive charge at one side and a negative charge at the other side. It is known as a dipole.

• Dipolar water is attracted both by the negatively charged surface of the clay particles and by the cations in the double layer.

• the water held to clay particles by force of attraction is known as double-layer water. The innermost layer of double-layer water, which is held very strongly by clay, is known as adsorbed water. This water is more viscous than free water is. This orientation of water around the clay particles gives clay soils their plastic properties.

2.3.1.3 Double layer