The origin and conditions under which rocks form fundamentally influence their chemical and mineralogical composition, as well as their crystalline structure and texture. These characteristics determine not only the strength of rocks but also their decorative appeal, making them key factors in the engineering and economic assessment of rocks for use in various buildings and infrastructure projects.
The rocks may be classified on the basis of their geological formation, physical characteristics and chemical composition as shown in Figure below
Classification of Rocks
-
Geological
- Sedimentary
- Metamorphic
- Igneous
-
Physical
- Stratified
- Unstratified
- Foliated
-
Chemical
- Argillaceous
- Silicious
- Calcarious
Based on Geological Formation
This classification is based upon the mode of the formation. The rock cycle is shown in Fig. below. On the basis of geological classification, rocks are classified as igneous, sedimentary and metamorphic.
1. Igneous Rocks
Igneous rocks, also known as primary, unstratified, or eruptive rocks, are of volcanic origin and form as a result of the solidification of molten masses lying below or above the Earth’s surface. The high temperature in the inner layers of the Earth causes silicate masses to melt. This molten mass, known as magma, is forced up through volcanic eruptions and spreads over the Earth’s surface, where it solidifies to form basalt and trap, commonly known as effusive rocks.
When magma solidifies below the Earth’s surface, it forms solid crystalline rocks termed as deep-seated plutonic rocks. Examples of these include granite, syenite, diorite, and gabbro. If the magma solidifies at a relatively shallow depth, the resulting rock possesses a finely grained crystalline structure and is termed as hypabyssal. Dolerite is an example of such a rock. The principal constituents of magma are quartz, mica, and feldspar. The texture of the rock is greatly influenced by the rate of cooling of the magma.
| Type | SiO2 | Na2O + K2O | FeO + MgO | CaO | Other |
|---|---|---|---|---|---|
| Ultra basic rocks | 40-45 | 0-5 | 40-50 | ||
| Basic rocks | 45-52 | 5-12 | 5-10 | 12-18 | |
| Intermediate rocks | 52-66 | 12-14 | 10-15 | 18-21 | |
| Acidic rocks | 66-76 | 14-17 | 2-10 | 0-5 | |
| Ultra acidic rocks | 76-77 | 17-19 | 0-2 | 0-1 |
Note (i): When magma cools and solidifies, it forms igneous rocks. The size of the crystals in the rock depends on the rate of cooling. Slow cooling results in larger crystals, while rapid cooling results in smaller crystals. The crystals in the rock are held together by a cementing material.
Note (ii): During volcanic eruptions, lava cools and solidifies to form igneous rocks. The size of the crystals in the rock depends on the rate of cooling. Slow cooling results in larger crystals, while rapid cooling results in smaller crystals. The crystals in the rock are held together by a cementing material
2. Sedimentary Rocks
Sedimentary rocks, also known as aqueous or stratified rocks, form through the breakdown of the Earth’s surface by various weathering agents such as rain, sun, air, and frost. These agents disintegrate the surface, and rainwater carries the resulting fragments to rivers. As these rivers flow towards lower elevations, their velocity decreases, allowing sediments (including disintegrated rock pieces, sand, silt, clay, debris, etc.) in the water to settle. This settling occurs in layers due to seasonal variations. Over time, the sediments consolidate into horizontal beds under the pressure from overlying material.
The properties of sedimentary rocks vary depending on the nature of the sediment and the type of bond between the sediment and grains. Typically, these rocks exhibit clear stratification and well-defined bedding planes. They are relatively soft and easily split along bedding as well as normal planes. Sedimentary rocks form through different processes. For instance, chemical deposits like gypsum, anhydrite, magnesite, dolomite, and lime tufas result from the precipitation of salts in drying water basins. Organogenous rocks, such as limestone, shale, chalk, diatomite, and tripoli, form from the accumulation of plant or animal remains.The examples of rocks resulting from the deterioration of massive magmatic or sedimentary rocks (fragmental rocks) are sandstone, laterite, sand, gravel, carbonate conglomerate and breccia.
3. Metamorphic Rocks
The formation of metamorphic rocks occurs when either igneous or sedimentary rocks undergo transformation due to the effects of earth movements, changes in temperature, liquid pressures, and other geological processes. This transformation leads to the creation of rocks with distinct characteristics. Metamorphic rocks may exhibit a foliated structure, as seen in slate, gneiss, schist, and phyllite, or a non-foliated structure, as seen in marble, quartzite, and serpentine.
| Rock Type | Metamorphic Rock |
|---|---|
| Granite | Gneiss |
| Syenite | Gneiss |
| Sandstone | Quartzite |
| Marl | Marble |
| Shale | Slate,Schist,Phyallite |
| Mudstone | Slate |
| Limestone | Marble,Schist |
| Felsite, Tuff | Schist, Slate |
| Dolomite | Marble |
Based on Physical Properties
The rocks may be classified as stratified, unstratified and foliated.
- Stratified Rocks show distinct layers along which the rocks can be split. The examples are sandstone, limestone, shale, slate, marble, etc
- Unstratified Rocks do not show any stratification and cannot be easily split into thin layers. The examples of such rocks are granite, basalt, trap, etc
- Foliated Rocks have a tendency to split up only in a definite direction. Most of the metamorphic rocks have a foliated structure, except for quartzite and marble which have granulose structure.
Based on Chemical Properties
The rock may be classified as argillaceous, silicious and calcarious.
- Argillaceous :The principal constituent is clay (Al2O3). The rocks are hard and brittle, e.g. slate, laterite, etc.
- Silicious: The principal constituent is silica (SiO2), i.e. sand. The rocks are very hard and durable, e.g. granite, basalt, trap, quartzite, gneiss, syenite, etc.
- Calcarious : The principal constituent is lime, e.g. limestone, marble, dolomite, etc
In summary, this article eloquently unpacks the world of rocks, emphasizing their pivotal role in civil engineering. By exploring the origins, classifications of rocks, and diverse characteristics of igneous, sedimentary, and metamorphic rocks, it offers engineers a crucial roadmap for leveraging these natural formations in constructing our world’s infrastructure. Understanding the complexities of rocks equips engineers with vital knowledge to create resilient structures that endure the test of time on Earth’s solid ground.
