Classification of Rocks: 3 Types

After reading this article you will learn about the three types of rocks. The types are: 1. Igneous Rocks 2. Sedimentary Rocks 3. Metamorphic Rocks.

1. Igneous Rocks:

The earth cooled down some 4600 million years ago to form molten silicate material called magma that contained varying amounts of water and highly reactive fluids. The magma rapidly cooled down at the surface of the earth to form extrusive igneous rocks of fine texture because the molecules of the minerals did not get enough time to form large crystals of minerals with will define faces.

Hence an insulating mat of extrusive igneous rock was formed at the surface of the earth. Below this pressure was exerted and therefore the magma slowly cooled down to form intrusive or abyssal igneous rocks of coarse texture because the gases could not escape and the minerals got enough time to arrange themselves to form large crystals of minerals with well-defined faces.

The intrusive rocks formed due to the cooling down and solidification of the magma within the vertical cracks between the existing rocks below the surface of the earth is called the dykes.

Similarly, intrusive rocks formed within the horizontal cracks and narrow and irregular cracks are known as sills and veins respectively. The magma cooled at a very great depth below the surface of the earth to form the plutonic igneous rock.

Igneous rocks were buried under high pressure and temperature very deep in the earth. When the pressure was decreased, the hot solid rock melted to form the magma, which was forced out to the surface of the earth in the form of lava which cooled down rapidly to form volcanic or extrusive rocks of a fine texture. Extrusive and intrusive rocks have not been clearly demarcated. The Hypabyssal rocks are intermediate in origin between the extrusive and the intrusive rocks. They are of intermediate texture.

Soil Forming Igneous Rocks:

Hot molten matter must contract on cooling and solidifying. So all igneous rock have cracks or joints. The fabric of an igneous rock is determined by the physical state and arrangement of the material that solidified from the magma to form the rock. An igneous rock in which minerals form an interlocking system is a crystalline igneous rock.

The texture of igneous rocks means the sizes of grains or crystals of minerals present in them. If they can be seen with the naked eye, the rock is said to have a coarse medium or fine grained texture depending on the size of their crystal.

If the mineral crystal of the rock can only be seen with the help of a microscope, then the concerned rock possesses the aphanite texture. Some igneous rock possess a porphyritic texture, which means that large perfectly developed crystals called euthedral crystals, are enclosed within a ground mass of very fine grains.

In the past, lighter coloured igneous rocks containing more than 66 per cent silica and relatively lower amount of basic elements were called acidic rocks, and darker igneous rocks containing 45-52 per cent silica and relatively higher amount of basic elements were called basic rocks because the former weathered to form relatively coarser textured soils poorer in basic elements and acidic in reaction and the latter weathered to form relatively finer textured soils, richer in basic elements and alkaline in reaction. Today, acidic rocks are also called felsic rocks and basic rocks are also called mafic rocks.

A few most important soil forming igneous rocks are as follows:

(i) Granite is a coarse to medium textured plutonic or intrusive acidic rock. It mainly consists of orthoclase and quartz, and minor amounts of biotite or muscovite and hornblende, rarely Augite. The minerals occur as interlocking irregular crystals but linear and graphic fabrics are sometimes found. As orthoclase and quartz is the most abundant mineral in granite, the rock is white light gray or pink like the minerals.

(ii) Rhyolite is made up of the same minerals as granite but has a fine, glassy or porphyritic texture, Rhyolite is usually light, may be white, light grey or various shades of red. A characteristic textural feature is a streaked pattern known as flow banding which results from the concentration of coloured material or glass in layers during the flow of highly viscous lava.

(iii) Syenite and Trachyte’s are almost entirely composed of orthoclase and a small amount of dark minerals like hornblende, Augite and biotite. So their colour is blackish grey or blackish pink. Syenite is a coarse to medium textured equiangular plutonic rock, but trachyte is a fine textured volcanic rock.

(iv) Diorite is a coarse to medium textured dark grey, plutonic rock whose chief constituent is plagioclase feldspar. It also contains hornblende or biotite. These dark minerals may be as abundant as plagioclase.

(v) Andesite: It is a grey to greyish black fine grained volcanic rock of porphyritic texture. It contains plagioclase, Augite, hornblende and biotite.

(vi) Gabbro is coarse textured plutonic basic rocks which consist of coarse-grained inter growth of crystals of anorthite, hornblende or Augite. Many gabbro varieties contain olivine and are therefore coloured greenish black. Typical gabbro contains both more Augite and hornblende than anorthite or equal amounts of Augite and /or hornblende and anorthite and is therefore dark grey in colour.

(vii) Anorthosite consists almost entirely of anorthite, the interlocking coarse textured and therefore rendering it white or grey in colour. It is a volcanic basic rock.

(viii) Pyroxenite is a coarse textured plutonic ultrabasic rock that consists almost entirely of pyroxene, and is therefore black in colour.

(ix) Hornblendite is a coarse textured plutonic ultrabasic rock consisting almost entirely of hornblende. It is black in colour.

(x) Peridotite is coarse, medium textured ultrabasic equiangular rock that consists almost entirely of olivine and a little pyroxene and is therefore greyish green in colour.

(xi) Dunite is a coarse textured plutonic ultrabasic rock that consists of only olivine and therefore is green in colour.

(xii) Basalt is a volcanic basic igneous rock that consists of anorthite, Augite and hornblende. It is fine grained to aphinitic in texture and coal black to dark grey in colour. Many varieties also contain olivine. Basalt is commonly frothy and cellular and is filled with numerous holes called vesicles where gas bubbles were trapped in the molten lava when it solidified. Such a structure is known as secoriaceous.

2. Sedimentary Rocks:

The term sediment means to anything that settles down in water or wind. Sediments originated by mechanical, chemical and organic processes. In the mechanical process, rocks were gradually disintegrated to smaller pieces and ultimately to the constituent minerals, which were carried away by running water or a blowing wind elsewhere, where they were deposited in layers according to the densities, which means that the heavier particles settled down first followed, by lighter particles.

Then these layers were cemented or lithified at a relatively lower temperature and pressure to form the sedimentary rocks. Lithification is a process that converts newly deposited sedimentary layers into compact sedimentary rocks by the combined effects of pressure, heat and cementing action.

The sedimentary rocks of mechanical origin are more or less similar to the original igneous rocks in mineralogical and chemical composition, although some proportion of easily decomposable minerals of the igneous rocks had dissolved, increasing the proportion of resistant minerals in the sedimentary rocks. For example, quartz is the chief constituent of both granite, and igneous rock and sandstone, sedimentary rocks. Granite contains about 66 per cent silica whereas sandstone contains about 79 per cent silica.

In the chemical process, minerals were gradually dissolved in water and carried away elsewhere where the solution became concentrated by evaporation when the minerals crystallized, and settled down in layers that were cemented or lithified to form sedimentary rocks e.g. limestone. In the organic process, certain kinds of marine plants and animals and their excreta were deposited at the bottom of the ocean bed in layer which were lithified to form sedimentary rocks e.g. chalk.

Soil Forming Sedimentary Rocks:

One of the outstanding characteristics of sedimentary rocks is their arrangement into layers where sediments of different textured and composition was deposited when the velocity and direction of the stream and the wind changed.

Sedimentary rocks usually contain fossils. Layers maybe inclined or horizontal. The most abundant minerals in the sedimentary rock are quartz, kaolin and calcite, which are white or colourness when pure. Sedimentary rocks may also be of different shades of yellow, brown, red, blue, green, grey and black, Red and yellow colours are due to hematite and limonite respectively.

The blue green colour is due to the presence of copper oxide. The deep black colour is due to the presence of manganese oxide. Most shades of grey and black, very common in sedimentary rocks, especially shale’s, are due to the admixing of various amounts of carbon derived from organic matter in the sediment.

Concretions are formed by the precipitation of minor inorganic constituents from solution during or after deposition of sediments. While moving through sedimentary beds the ground water may dissolve a minor constituent of a large mass of rocks and redeposit the accumulated material in a concentrated form around certain nuclei to form concretions, calcite, limonite, pyrite and other concretions occur in sandstone and shale’s. Concretions vary from a fraction of an inch to several feet in diameter.

A few most common soil forming sedimentary rocks are described below:

(i) Conglomerate and Brecia:

Conglomerate consists of a cemented mixture of rounded particles of gravels and sands. Gravels are usually composed of quartz, but may also contain other minerals. But brecia consists of cemented angular gravel and sand particles.

(ii) Sandstone:

Sand particles have been cemented together to form sandstone. Sand particles are chiefly quartz grains but other minerals like calcite, gypsum, limo-mite, hematite, magnetic, olivine, garnet etc. may occur as impurities.

The cement is usually silica or calcium carbonate in the white, lighter coloured rocks while iron oxide is responsible for the cementing of sand grains in rocks with a yellowish, brownish or reddish colour. Sandstone which has unequally sized quartz grains is called grit. The massive and somewhat impure variety of quartz is called Flint. Chert is an impure dark, flint-like rock.

(iii) Shales:

Mostly Kaolinites usually contaminated with impurities like silt, iron oxide, organic material and others, have been cemented together to form the layered beds called shale’s or mudstone. Most shale usually split along the bedding planes. Red, brown and yellow varieties contain iron oxide while black varieties contain organic matter.

(iv) Silt-stone:

Siltstones are an intermediate between the sandstone and shales because they are all gradations in texture between sandstone and shales. They consist of fine sand, silt and some clay and include such varieties as shaly sandstone and sandy shales.

(v) Limestone:

Limestone’s mainly consist of calcite but some impurities like clay, iron oxide, fine sand, silt etc. may also occur in some. Most limestone’s consist of materials that were once the skeletal parts of living organism. Limestone effervesces in dilute hydro-chlorine acid.

(vi) Chalk:

Chalk is a fine grained, soft and porous variety of limestone. Mixtures of microscopic shales of foraminifera, (marine protozoa that had been enclosed within the perforated calcareous shale), pulverized fragments of other shale and other calcareous matter have been cemented together usually with calcareous cements to form the chalk.

(vii) Marl:

It is an impure variety of limestone containing calcite and clayey material as impurities.

(viii) Dolomite:

It contains considerable amounts of magnesium carbonate and calcium carbonate.

3. Metamorphic Rocks:

Rocks were buried several thousand feet deep below the surface of the earth where they were subjected to a high temperature and the chemical action of magmatic fluids. Consequently, existing minerals decomposed to form new minerals that arranged themselves parallel to the plane of application of high pressure. This parallelism in texture is known as schistosity. New kinds of rocks were formed.

These were known as schistose rock of metamorphic rock. The banding produced in the schistose rocks known as foliation. During the course of formation of metamorphic rocks, less stable minerals were gradually converted to more stable minerals.

Metamorphic rocks might have undergone several cycles of metamorphism each time, making new adjustments in the constituent minerals and texture until they finally became stable although the total chemical composition might not have changed. For example, a bed of limestone or dolomite might have changed into marble, shale into pliyllite and slate to mica schist depending upon the intensity of heat and pressure.

Soil Forming Metamorphic Rocks:

Metamorphic rocks may be foliated or un-foliated. Foliated metamorphic rocks have a tendency to break along parallel planes. Un-foliated metamorphic rocks break down irregularly. Foliated metamorphic rocks are made up of leaf like layers and their constituent minerals are flattened and oriented parallel with the folia.

There are three kinds of foliation. In some rocks like gneiss, lighter and dark coloured minerals occur in alternate layers which give a banded appearance to the rocks. This kind of banded appearance is not found in some other kinds of rocks like schistoses because they contain only one mineral. Fine grained foliated rocks like slate are said to have a slaty cleavage because they can be split in smooth thin plates.

The foliated rocks are often folded, meshed and injected with igneous materials and crumpled into intricate and complex patterns due to movements of rocks under high temperature and pressure. Some of the minerals which usually occur in igneous and sedimentary rocks also occur in metamorphic rocks.

For example, quartz occurs in igneous, sedimentary and metamorphic rocks. Feldspar, pyroxenes, micas, and hornblende commonly occur in igneous and metamorphic rocks. Similarly, calcite and dolomite occur in both limestone and marble.

A few common soil forming metamorphic rocks are as follows:

(i) Slate:

Slate is the most perfectly foliated metamorphic rock. It is of such a fine texture that the crystals of minerals cannot be seen with the naked eye. It has a slaty cleavage i.e. it can be broken along closely spaced parallel planes. It is usually black or grey coloured, but red coloured slates have also been found.

(ii) Phyllite:

Phyllite is intermediate in the size of grains between slate and schist. It is coarser than slate and finer than schist. It has glimmering lustre which is due to the presence of small flakes of lustrous muscovite mica. Probably most phyllites are formed from metamorphosis of shales.

(iii) Schist:

Slate grade into schist with increasing grain size. The slaty minerals of schist can be recognized with the naked eye whereas those of slate cannot be recognized with the naked eye. All schists include tabular, flaky or even fibrous minerals.

The individual folia laminations of schist are regularly spaced at a distance of up to about half a centimeter. This is the only features by which schists can be identified from more closely layered gneisses. Mineral grains of schist are larger than those of phyllite.

Sometimes one band may be composed of mica, horn blended, chlorite etc. and the adjacent band composed of quartz. So they are called mica schist, hornblende, schist, chlorite schist etc. Generally schists contain more than 50 per cent of the flaky minerals.

(iv) Gneiss:

Gneiss is a coarse-grained banded metamorphic rock. The bands are commonly folded and twisted in a complex manner. It consists of alternate layers or bands of dark coloured minerals like hornblende, biotite and light coloured minerals quartz and feldspar.

(v) Quartzite:

The sandstone had been metamorphosed to form the quartzite. The pore spaces which separated sand grains from each other have been filled up with newly crystallized quartz. The non-foliated quartzite may be stained red, yellow or other colours due to the presence of impurities.

(vi) Marble:

Marble is a non-foliated course to fine textured metamorphic rock which consists mainly of calcite and/or dolomite. Pure marble is white in colour, but may be of different colours due to presence of impurities.