ROCK CLASSIFICATION

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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.
Igneous rocks are considered to be the primary rocks formed by the cooling of molten
magmas, or by the recrystallization of older rocks under heat and pressure great enough to render
them fluid. 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).

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, in
general, 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. 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-colored 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 color varies from dark-grey to black. Both granite and basalt are used as building
stones.
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. The sedimentary rocks generally are
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.
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. The characteristic feature of gneiss is its
structure, the mineral grains are elongated, or platy, and banding prevails. Generally gneiss is a
good engineering material. Schist is a finely foliated rock containing a high percentage of mica.
Depending upon the amount of pressure applied by the metamorphic forces, schist may be a very
good building material. Slate is a dark colored, 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. Marble is the end product of the metamorphism of limestone and other sedimentary rocks
composed of calcium or magnesium carbonate. It is very dense and exhibits a wide variety of
colors. In construction, marble is used for facing concrete or masonry exterior and interior walls
and floors.

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, color, 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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