Rocks

UNIT – II

Chapter - 4 (B)

STONES

 

ROCKS

Building Stone is a naturally available material of construction. It is obtained from rock in irregular shape and size by quarrying. Rock is a mixture of two or more minerals, such as Quartz, Silica, Felspar, Mica, etc. Mineral is a substance formed by the natural inorganic process. It possesses a definite chemical composition and molecular structure.

 

1. CLASSIFICATION OF ROCKS

(i) GEOLOGICAL CLASSIFICATION

Depending upon the method of formation, i.e., Geologically, rocks are classified as:

1. Igneous or Un-stratified Rocks: In its formative stage, Earth was a molten mass. Slowly it began to cool, starting from the surface. This solidified portion of the liquid mass is known as Igneous Rocks. Examples: Granite, Basalt, Trap, etc.

2. Sedimentary Rocks or Aqueous or Stratified Rocks: These are formed by the deposition of natural agencies of weather such as wind, frost and rain acting on the existing rocks and disintegrating them in the form of layers called Sediments. Sediments are subjected to heavy pressure and undergo compaction resulting in Sedimentary Rocks.

Examples: Gravel, Limestone, Sandstone, Gypsum, Shale, Peat, etc.

3. Metamorphic Rocks: Both the igneous and sedimentary rocks undergo changes due to enormous heat or pressure found inside the Earth resulting in the formation of Metamorphic Rocks. Examples: Marble, Slate, Laterite, Gneiss, Quartzite.

(ii) PHYSICAL CLASSIFICATION (STRUCTURE)

It is based on general structure of rocks. It is the manner in which its mass is arranged.

1. Un-stratified Rocks: These are of granular or crystalline structure. They have become solid mass by cooling. Examples: Igneous Rocks like Granite, etc.

2. Stratified Rocks: The stratified structure is due to the splitting of a series of parallel layers of sedimentary rocks. Examples: Sedimentary Rocks like Slate, etc.

3. Foliated Rocks: It is a parallel rearrangement of minerals showing tendency to split in definite direction. Examples: Metamorphic Rocks like Gneisses, etc.

(iii) CHEMICAL CLASSIFICATION

1. Siliceous Rocks: In these rocks, Silica is main constituent. e.g., Granite, Sandstone.

2. Argillaceous Rocks: In these rocks, Clay is main constituent. e.g., Slate, Laterite.

3. Calcareous Rocks: In these rocks, Calcium Carbonate is the main constituent.

Examples: Limestone, Marble.

 

2.  QUARRYING AND DRESSING OF BUILDING STONES

Quarrying of Stones: Quarrying is the process of extracting stone blocks from existing rocks. It is done at some depth below the top surface of rocks where the effects of weathering are not found. Quarrying of soft and hard rocks is done by the following methods:

1. Digging, Heating or Wedging: In soft rocks like limestone and marble, stones are obtained by digging, heating or wedging using hand tools, namely, pick-axes, hammers, chisels, etc.

2. Blasting: In hard and dense rocks, stones are obtained by blasting using explosives.

Dressing of Stones: Stones obtained after quarrying have rough surfaces. They are irregular in shape and size. Dressing is the process of cutting the stones to a regular shape and size and the required surface finish. The purposes of dressing are:

1. Suitable Size: To prepare the stones for a suitable size for easy handling and transport;

2. Regular Shape: To prepare the stones into a regular shape and pleasing appearance with neat horizontal and vertical mortar joints between the adjacent stones;

3. Particular Purpose: To make hammer-dressed surface, tooled surface, polished surface, rubbed surface or cut-stone surface to suit a particular stone masonry;

4. Proper Bedding: To secure proper bedding in stone masonry.