ROCK is a relatively hard, naturally occurring mineral material. It consists of a single mineral or of several minerals that are either tightly compacted or held together by a cement like mineral matrix.
6.1 Types of rock
The three main types of rock are Igneous, Sedimentary, and Metamorphic.
6.1 .1 Igneous Rock
These types of rocks are formed by the solidification of magma either under the surface of earth or over it. These are of three types according to the solidification process.
Plutonic Rock :- The rocks, which are formed underneath the surface of the earth, are called the plutonic rocks.
Volcanic Rock :- The rocks, which are solidified on the surface of earth, are called volcanic rocks.
Hypabyssal Rock :- The rocks, which are solidified on the way of extrusion process, are called hypo basal rocks.
Igneous rocks are commonly identified in the field by the study of their interlocking crystallization of a number of mineral grains. Igneous rocks are generally hard, massive, compact, having no bedding plane and interlocked grains. It usually contains much feldspar. By the study of these properties we identified the igneous rocks in the field. e.g.,
Granite - generally equigranular texture, light colour appearance
Gabbro - coarse grain and dark colour
Dolerite - containing dark minerals in good proportion with medium
6.1.2 Metamorphic Rock
These are formed by alteration of alignment, texture, structure, chemical composition etc. in pre-existing rocks due to the action of temperature, pressure and chemical reaction. Metamorphic rocks are generally hard and having of interlocking grains.
· Contact Metamorphic
· Dynamic Metamorphic
· Dynamo Thermal
Metamorphic rocks are distinctly distinguished from other types of rocks by the development of features like cleavage, foliation, schistosity and by the presence of such minerals which are known to be of metamorphic origin. Metamorphic rocks often exhibit an interlocking texture of the constituent minerals grains. Thus these rocks are basically identified in the field on the basis of colour, texture as well as structure. e.g.,
Slate - slate cleavage
Phyllite - soapy feeling
Schist - rough, irregular and undulated plane, unequal growth of minerals
Gneiss - band of contrast colour, composition and texture (mica band)
6.1.3 Sedimentary Rock
These rocks are derived from pre-existing rocks through the process of erosion, transportation and deposition by various natural agencies such as wind, water, glacier etc. The loose sediments undergo compaction and form resulting products as sedimentary rocks. According to the mode of transportation the sedimentary rocks are divided into three different types.
Clastic Rocks :- The rocks which are formed by deposited mechanically are known as clastic rocks.
Chemical Deposits :-The rocks which are formed by chemical precipitation are known as chemical deposits.
Organic Deposits :-The rocks which are formed by deposited organically are called organic deposits.
Sedimentary rocks are identified in the field by the study of different layers, which are originally bedded or laminated under suitable conditions. There may be the impression of fossils in sedimentary rocks. The colour and the grain size of the rocks may be different in different layers. By the study of these properties also the rocks may be identified. e.g.,
5.2 Major Rock Types of the Field Study Area
Following are the different types of rock that we found during our study in malekhu area.
Day 2nd Date:-2067/11/16
Following types of rock were identified in the Malekhu area
6.2.1 Metamorphic rocks
It is a medium to fine-grained metamorphic rock of complex silicate composition. The rock shows a foliated structure and represents an intermediate stage in the metamorphic transformation of slates to schist with the unaided eye. In the field the presence of muscovite was found in the rock. This is found at the south from the highway along the stream.Dark green type of phyllite was found in the Robang formation (rb) .
Schists are megascopically crystalline metamorphic rock characterized by a typial schistose structure. The constituent platy and flaky mineral and are arranged in irregular parallel layers of bands.
Marble: Marble is essentially a granular rock composed chiefly of recrystallised calcite. It is characterized by a granular structure. Marble often show banded structure. Marvels are commonly used in building constructions in the form of blocks, slabs, aches and as chip for flooring. It is also used as decorative and ornamental purpose. In Malekhu it was found at a distance of 3275m south from the highway.
Amphibolite is a grouping of rocks composed mainly of amphibole (as hornblende) and plagioclase feldspars, with little or no quartz. It is typically dark-colored and heavy, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
In Malekhu it was found at a distance of 4650m south from the highway. The gneiss was found in the boulder as an eye structure and so called Augen Gneiss. This was found on the Kulekhani formation (ku) of Bhimphedi group.
Quartzites are granular metamorphic rocks composed chiefly of intersatured grains of quartz. The name Orthoquartzite is used for a sedimentary rock of similar composition but of different origin; it has siliceous cement. It contains subordinate amount of micas, feldspars, garnet and some amphiboles which result from the impurities of the original sandstone during the process of metamorphism. The rock is generally very hard, strong, dens and uniformly grained. It finds extensive use in building and road construction.
6.2.2 Sedimentary Rocks
These are the most abundant sedimentary rock formed from the non-elastic group and are composed chiefly of calcium carbonate. However the presence of dolomite, quartz, feldspar and iron oxides is rather a common texture. Limestone is a primary source of material for the manufacture of Portland cement and for a wide variety of limes. Its other uses are in the metallurgical industries as a flux. It was found along the Prithivi highway towards Gajuri as the Robang formation (RB) as a Malekhu limestone formation (ML) in the upper Nawakot Group and
Sandstone is a mechanically formed sedimentary rock. it is formed by the cementation of the sand particles between 1/10 mm to 2 mm in diameter. Sandstone is, perhaps, the most familiar of all the rocks as it is usually quarried and used more than any other rock, for all types of buildings. The various varieties of sandstones are named after the cementing material. e.g. Siliceous sandstone, ferruginous sandstone, calcareous sandstone, argillaceous sandstone.
Dolomite (pronounced /ˈdɒləmaɪt/) is the name of a sedimentary carbonate rock and a mineral, both composed of calcium magnesium carbonate CaMg(CO3)2 found in crystals.
Dolomite rock (also dolostone) is composed predominantly of the mineral dolomite. Limestone that is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologic literature as magnesian limestone. Dolomite was first described in 1791 as the rock by the French naturalist and geologist, Déodat Gratet de Dolomieu (1750–1801) for exposures in what are now known as the Dolomite Alps of northern Italy.
The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, gray to pink, commonly curved crystals, although it is usually massive. It has physical properties similar to those of the mineral calcite, but does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid unless it is scratched or in powdered form. The Mohs hardness is 3.5 to 4 and the specific gravity is 2.85. Refractive index values are nω = 1.679 – 1.681 and nε = 1.500. Crystal twinning is common. A solid solution series exists between dolomite and iron rich ankerite. Small amounts of iron in the structure give the crystals a yellow to brown tint. Manganese substitutes in the structure also up to about three percent MnO. A high manganese content gives the crystals a rosy pink color noted in the image above. A series with the manganese rich kutnohorite may exist. Lead and zinc also substitute in the structure for magnesium.
6.2.3 Igneous Rocks