Petrology is a branch of Geology, which deals with the study of rocks, and includes



It is a branch of Geology, which deals with the study of rocks, and includes:

(a) Pedogenesis, i.e., origin and mode of occurrence as well as natural history of rocks.

(b) Petrography, i.e., dealing with classification and description of rocks.

The branch of petrology dealing with the study of stones alone is called 'lithology'. Stones include the rocks that are necessarily hard, tough and compact.

As we know, rocks are necessarily the constituents of the earth's crust. Rocks are composed of minerals. Some rocks are monomineralic, composed of one mineral only while most of the rocks are multiminerallic consisting of more than one mineral species as essential constituents.

Igneous and metaigneous rocks constitute 95% of all the rocks of the earth's crust. A sedimentary and metasedimentary rock constitutes 5% of the rocks of the earth's crust.

Classification of Rocks :

According to the mode of origin, all rocks are categorised into three major groups:

I. Igneous Rocks or Primary Rocks.

II. Sedimentary Rocks or Secondary Rocks.

III. Metamorphic Rocks.

I. Igneous rocks:

These are the rocks formed by the solidification of magma either underneath the surface or above it; accord­ingly they are divided into two groups:

(a) Intrusive bodies:

Which are formed underneath the surface of the earth?

(b) Extrusive bodies:

These are due to the consolidation of magma above the surface of the earth. These are also known as Volcanic-rocks.

On the basis of the depth of formation, intrusive rocks are of two types:

(i) Plutonic rocks, which are formed at very great depths.

(ii) Hypabyssal rocks, which are formed at shallow depth.

Important Features of Igneous Rocks:

1. Generally hard, massive, compact with interlocking grains.

2. Entire absence of fossils.

3. Absence of bedding planes.

4. Enclosing rocks are baked.

5. Usually contain much feldspar.

II. Sedimentary rocks:

These rocks have been derived from the pre-existing rocks, through the processes of erosion, transporta­tion and deposition by various natural agencies like, wind, water, glacier, etc. The loose sediments, which are deposited, undergo the processes of compaction and the resulting products are known as sedimentary rocks.

On the basis of place of formation, sedimentary rocks are of two types:

(i) Sedentary rocks, that are the residual deposits, formed at the site of the pre-existing rocks from which they have been derived. These arc not formed by the process of transportation.

(ii) Transported, in which case the disintegrated and decom­posed rock materials are transported from the place of their origin and get deposited at a suitable site. According to the mode of transportation of the deposits, these rocks are sub-divided into three types as;

(a) Mechanically deposited. Clastic rocks.

(b) Chemical precipitation. Chemical deposits.

(c) Organically deposited. Organic deposits.

Important Features of Sedimentary Rocks:

1. Generally soft, stratified, i.e., characteristically bedded.

2. Fossils common.

3. Stratification, lamination, cross-bedding, ripple marks mud cracks, etc. are the usual structures.

4. No effect on the enclosing or the top and bottom rocks.

5. Quartz, clay minerals, calcite, dolomite, hematite is the common minerals.

III. Metamorphic rocks:

These are formed by the alteration of pre-existing rocks by the action of temperature, pressure aided by sub-terranean fluids (magmatic or non-magmatic).

Important Features of Metamorphic Rocks:

1. Generally hard, interlocking grains and bedded (if derived from stratified rocks).

2. Fossils are rarely preserved in rocks of sedimentary origin except slates.

3. Foliated, gneissose, schistose, granulose, slaty, etc., are the common structures.

4. Common minerals are andalusite, sillimanite, Kyanite, cordierite, wollastonite, garnet, graphite, etc.

1. Igneous Rocks Types:

(i) Granite-with its volcanic equivalent, i.e., Rhyolite.

(ii) Syenite-with its volcanic equivalent 'Trachyte',

(iii) Nepheline-Syenite (and phonolite).

(iv) Anorthosite.

(v) Granodiorite and Monzonite.

(vi) Gabbro, diorite and norite, and their volcanic equivalents basalts (deccantraps), andesite etc.

(vii) Peridotite.

(viii) Carbonatite.

2. Sedimentary Rocks:

(i) Sandstone

(ii) Shale

(iii) Limestone and dolomite

(iv) Saline rocks

(v) Laterite.

3. Metamorphic Rocks:

(i) Gneiss


(iii) Quartzite

(iv) khondalite

(v) Charnockite

(vi) Marble

(vii) Gondite

(viii) Kodurite

(ix) Slate

(x) Phyllies

Mode of Occurrence or Forms of Igneous-Rocks:

The form, i.e., the size, shape of the igneous bodies, depends mostly on the following factors:

(i) Mode of formation.

(ii) Viscosity of magma, which in turn depends on the

(a) temperature, and

(b) composition of the magma.

(iii) Relation with the surrounding country-rocks, i.e.,

(a) physical characters of the invaded rocks,

(b) weight of the overlying rockmass in case intrusive bodies

(c) structure.

The intrusive and the extrusive rocks exhibit typical forms, which are characteristic to them.

The forms assumed by intrusive bodies depend upon major geological structures as faults, folds, bedding planes, etc. Accord­ingly there are two major categories of forms of the intrusive bodies

1. Discordant-bodies.

2. Concordant-bodies.

1. Discordant-bodies:

In this case an intrusive mass happens to cut across the structures of the pre-existing rocks of the country. There are different types of discordant forms in unfolded regions as well as in highly folded regions.

(a) In unfolded regions:

(i) Dykes:

These discordant igneous bodies exhibit a cross- cutting relationship with the country rocks. Dykes commonly occur in groups and such group may be of radiating, arcuate or any other pattern.

Since for the formation of dykes the magma is to be sufficiently mobile, the composition of the dykes are mostly basic, i.e., doleritic. Dykes are evidences of regional tension in the crust within the area of igneous activities. Larger dikes produce baking and hardening; effect on either side.

(ii) Ring dyke:

A dyke of arcuate out crop; occurring more or less in the form of a complete or nearly complete circle.

(iii) Cone-sheets:

These are inwardly dipping (in the form of inverted co-axial cones) dyke-like masses with circular out crop.

(b) In high folded regions:

(i) Batholiths:

These are the largest intrusive bodies. Most batholiths are found in belts of deformation within the earth's crust and are granitic in composition. These are widening downwards to unknown depths.

Batholiths of comparatively smaller dimensions are called 'stocks' and stocks of circular outcrop upon the surface are known as bosses. The remnants of the country-rock occurring upon or near the top surface of such intrusive masses are known as 'roof- pendants'.

(ii) Ethmolith:

These are funnel-shaped basic bodies with circular outcrop.

(iii) Harpolith:

Sickle-shaped basic bodies formed by stretch­ing of the strata after or during injection.

(iv) Chonolith:

Any irregular intrusive body.

Any deep-seated intrusive body, irrespective of its shape and size, is known as 'pluton'.

2. Concordant bodies:

These are intrusive bodies that run parallel to the structures of the country-rocks in which they occur.

(a) In anfolded regions:

(i) Sills:

These are thin parallel sided tabular sheet of magma that has penetrated along bedding planes, planes of schistosity, unconformities, etc. These are also doleritic in composition. They may attain any orientation in space depending upon the attitude of the rock beds in which they occur.

(ii) Laccolith:

These intrusive bodies have their lower surface "at and have a convex top. It is due to accumulation of viscous magma which is usually acidic in composition, which pushes the Overlying rocks upwards to make room for the mass.

(iii) Lopoliths:

These are saucer shaped bodies, which are of dimensions and are of basic to ultrabasic in composition.

(iv) Bysmalith:

Sometimes the magma breaks through the overlying rock beds and the igneous mass after consolidation is known as bysmalith.

(b) In highly folded regions:

(i) Phacoliths:

These are crescentic shaped igneous bodies occurring along the crests and troughs of folds of the country rocks. They are basaltic in composition.

Forms of Extrusive Bodies:

(ii) Concordant:

Lava flows and pyroclastics which are the products of volcanic activities are the usual forms of extrusive igneous bodies.

(ii) Discordant:

Volcanic neck. It is a mass of igneous rock produced by the consolidation of lava and pyroclastic materials in the channel of eruption of an extinct volcano.