Rock can be defined as a compact, semi-hard to hard mass of natural material composed of one or more minerals. The rocks that are encountered at the surface of the earth or beneath are commonly classified into three groups according to their modes of origin. They are igneous, sedimentary and metamorphic rocks. The mineral grains that form the solid phase of a soil aggregate are the product of rock weathering. The size of the individual grains varies over a wide range. Many of the physical properties of soil are dictated by the size, shape and chemical composition of the grains. Fig.1 shows a diagram of the formation cycle of different types of rock and the processes associated with them. This is called the rock cycle.
Classification of Rocks
Rocks used for engineering works may be classified in the following three ways.
1) Geological
2) Physical
3) Chemical
I) Geological Classification of Rocks
Based on their origin of formation rocks are classified into three main groups - Igneous, sedimentary and metamorphic rocks.
1) Igneous Rock
Igneous rocks are considered to be the primary rocks formed by the cooling of molten magma or by the recrystallization of older rocks under heat and pressure great enough to render them fluid. 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, some of the molten magma cools on the surface of the earth. They have been formed on or at various depths below the earth surface. There are two main classes of igneous rocks. They are
1. Extrusive (poured out at the surface), and
2. Intrusive (large rock masses which have not been formed in contact with the atmosphere).
Intrusive rocks formed in the past may be exposed at the surface as a result of the continuous process of erosion of the materials that once covered them.
Initially both classes of rocks were in a molten state. Their present state results directly from the way in which they solidified. Due to violent volcanic eruptions in the past, some of the molten materials were emitted into the atmosphere with gaseous extrusions. These cooled quickly and eventually fell on the earth's surface as volcanic ash and dust. Extrusive rocks are distinguished by their glass like structure. Intrusive rocks, cooling and solidifying at great depths and under pressure containing entrapped gases, are wholly crystalline in texture. Such rocks occur in masses of great extent, often going to unknown depths. Table 1 shows the mineral composition of igneous rocks.
Table 1 Mineral Composition of Igneous Rocks
|
Mineral |
Percentage |
|
Quartz |
12-20 |
|
Feldspar |
50-60 |
|
Ca, Fe and Mg, Silicates |
14-17 |
|
Micas |
4-8 |
|
Others |
7-8 |
Feldspars are the most common rock minerals, which account for the abundance of clays derived from the feldspars on the earth's surface. Quartz comes next in order of frequency. Most sands are composed of quartz.
Some of the important rocks that belong to the igneous group are granite and basalt. Granite is primarily composed of feldspar, quartz and mica and possesses a massive structure. Basalt is a dark coloured fine grained rock. It is characterized by the predominance of plagioclase, the presence of considerable amounts of pyroxene and some olivine and the absence of quartz. The colour varies from dark grey to black. Both granite and basalt are used as building stones. Generally igneous rocks are strong and durable.
Example: Basalt, Trap, Andesite, Rhyolite, Diorite, Granite
2) Sedimentary Rock
When the products of the disintegration and decomposition of any rock type are transported, redeposited and partly or fully consolidated or cemented into a new rock type, the resulting material is classified as a sedimentary rock. Due to weathering action of water, wind and frost; existing rocks disintegrates. The disintegrated material is carried by wind and water. Flowing water deposits its suspended materials at some points of obstacles to its flow. These deposited layers of materials get consolidated under pressure and by heat. Chemical agents also contribute to the cementing of the deposits. The rocks thus formed are more uniform, fine grained and compact in their nature.
The sedimentary rocks generally formed in quite definitely arranged beds or strata, which can be seen to have been horizontal at one time although sometimes displaced through angles up to 90 degrees. Sedimentary rocks are generally classified on the basis of grain size, texture and structure. From an engineering point of view, the most important rocks that belong to the group are sandstones, limestones and shales.
The deposits of gravel, sand, silt and clay formed by weathering may become compacted by overburden pressure and cemented by agents like iron oxide, calcite, dolomite and quartz. Cementing agents are generally carried in solution by groundwater. They fill the spaces between particles and form sedimentary rock. Rocks formed in this way are called detrital sedimentary rocks. All detrital rocks have a clastic texture. Sedimentary rock also can be formed by chemical processes. Rocks of this type are classified as chemical sedimentary rock. These rocks can have clastic or non-clastic texture. Table 2 shows some examples of chemical sedimentary rock.
Table 2 Chemical Composition of Sedimentary Rock
|
Composition |
Rock |
|
Calcite (CaCO3) |
Limestone |
|
Halite (NaCl) |
Rock salt |
|
Dolomite [CaMg(CO3)] |
Dolomite |
|
Gypsum (CaSO4 - 2H2O) |
Gypsum |
Limestone is formed mostly of calcium carbonate deposited either by organisms or by an inorganic process. Most limestone has a clastic texture; however, non-clastic textures also are found commonly. Chalk is a sedimentary rock made in part from biochemically derived calcite, which are skeletal fragments of microscopic plants and animals. Dolomite is formed either by chemical deposition of mixed carbonates or by the reaction of magnesium in water with limestone. Gypsum and anhydrite result from the precipitation of soluble CaSO4 due to evaporation of ocean water. They belong to a class of rocks generally referred to as evaporites. Rock salt (NaCl) is another example of an evaporite that originates from the salt deposits of seawater. Sedimentary rock may undergo weathering to form sediments or may be subjected to the process of metamorphism to become metamorphic rock.
Example: Limestone, Sandstone, Dolomite and Slate
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. Rocks formed by the complete or incomplete recrystallization of igneous or sedimentary rocks by high temperatures, high pressures and/or high shearing stresses are metamorphic rocks. The rocks so produced may display features varying from complete and distinct foliation of a crystalline structure to a fine fragmentary partially crystalline state caused by direct compressive stress, including also the cementation of sediment particles by siliceous matter. Metamorphic rocks formed without intense shear action have a massive structure. Some of the important rocks that belong to this group are gneiss, schist, slate and marble.
Gneiss is a metamorphic rock derived from high grade regional metamorphism of igneous rocks, such as granite, gabbro and diorite. Low grade metamorphism of shales and mudstones results in slate. Slate is a dark coloured, platy rock with extremely fine texture and easy cleavage. Because of this easy cleavage, slate is split into very thin sheets and used as a roofing material. The clay minerals in the shale become chlorite and mica by heat; hence, slate is composed primarily of mica flakes and chlorite. Phyllite is a metamorphic rock, which is derived from slate with further metamorphism being subjected to heat greater than 250 to 300°C. Schist is a type of metamorphic rock derived from several igneous, sedimentary and low grade metamorphic rocks with a well foliated texture and visible flakes of platy and micaceous minerals.
Metamorphic rock generally contains large quantities of quartz and feldspar as well. Marble is formed from calcite and dolomite by recrystallization. The mineral grains in marble are larger than those present in the original rock. Green marbles are coloured by hornblends, serpentine, or talc. Black marbles contain bituminous material and brown marbles contain iron oxide and limonite. Quartzite is a metamorphic rock formed from quartz rich sandstones. Silica enters into the void spaces between the quartz and sand grains and acts as a cementing agent. Quartzite is one of the hardest rocks. Under extreme heat and pressure, metamorphic rocks may melt to form magma and the cycle is repeated.
Example : Due to metamorphic action granite becomes gneiss, trap and basalt change to schist and laterite, lime stone changes to marble, sand stone becomes quartzite and mud stone becomes slate.
II) Physical Classification of Rocks
Based on the structure, the rocks may be classified as:
1) Stratified Rocks
These rocks are having layered structure. They possess planes of stratification or cleavage. They can be easily split along these planes.
Example : Sand stones, lime stones, slate etc.
2) Unstratified Rocks
These rocks are not stratified. They possess crystalline and compact grains. They cannot be split in to thin slab.
Example : Granite, trap, marble etc.
3) Foliated Rocks
These rocks have a tendency to split along a definite direction only. The direction need not be parallel to each other as in case of stratified rocks. This type of structure is very common in case of metamorphic rocks.
III) Chemical Classification of Rocks
On the basis of their chemical composition rocks are classified as:
1) Silicious Rocks
The main content of these rocks is silica. They are hard and durable.
Example : Granite, trap, sand stones etc.
2) Argillaceous Rocks
The main constituent of these rocks is argil i.e., clay. These stones are hard and durable but they are brittle. They cannot withstand shock.
Example : Slate and laterite
3) Calcareous Rocks
The main constituent of these rocks is calcium carbonate. Limestone is a calcareous rock of sedimentary origin while marble is a calcareous rock of metamorphic origin.
Example : Limestone, marble, kankar, dolomite and gravel
Rock Minerals
It is essential to examine the properties of the rock forming minerals since all soils are derived through the disintegration or decomposition of some parent rock. A 'mineral' is a natural inorganic substance of a definite structure and chemical composition. Some of the very important physical properties of minerals are crystal form, colour, hardness, cleavage, luster, fracture and specific gravity. Out of these only two, specific gravity and hardness are of foundation engineering interest. The specific gravity of the minerals affects the specific gravity of soils derived from them. The specific gravity of most rock and soil forming minerals varies from 2.50 (some feldspars) and 2.65 (quartz) to 3.5 (augite or olivine). Gypsum has a smaller value of 2.3 and salt (NaCl) has 2.1. Some iron minerals may have higher values, for instance, magnetite has 5.2. It is reported that about 95 percent of the known part of the lithosphere consists of igneous rocks and only 5 percent of sedimentary rocks. Soil formation is mostly due to the disintegration of igneous rock which may be termed as a parent rock.
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