Notes on The Classification of Igneous rocks

The word 'igneous' implies fire derived from the Latin word 'ignis' or Sanskrit word 'agni'. Igneous rocks are, therefore, of thermal origin formed through the solidi­fication of molten material (magma) origi­nating within the earth's crust.

Such magma sometimes has been poured out upon the surface of the earth during volcanic erup­tions, and on reaching the surface gets solidified by cooling. There are occasions when magma is solidified deep down be­neath the surface.

Sometimes the molten rock material gets solidified in the channels which connected the magma reservoir with the earth's surface. Thus, cooling and solidi­fication are the two distinct processes invol­ved in the formation of igneous rocks.

These rocks are referred to as the primary rocks, because they represent the rocks which directly or indirectly provided materials for the formation of other types of rocks.

The term primary rock is associated with the theory that the earth was originally in a completely molten condition. As the earth solidified through cooling, the first rocks were igneous.

The igneous rocks being formed from the solidification of magma, it is clear that they have been formed at all ages in earth history, and they are being formed even at present, and they will continue to form in future too.

Igneous rocks are of many varieties. As regards their colour, mode of occurrence, and mine­ral composition, there is great variation in these rocks.

Some common igneous rock types

Subclass

Rock type

Composition

Intrusive

Granite

Felsic minerals, quartz, feldspars

(Cooling at depth producing

Diorite

Felsic minerals without quartz, inclu-ding

course crystal texture)

plagioclase, feldspar and amphi-bole

Mafic minerals, plagiolase feldspar, pyroxene

Gabbro

and olivine

An ultramafic rock of pyroxene and olivine

Peridotite

Extrusive

Rhyolite,

Same as granite

(Cooling at the surface,

Andesite

Same as diorite

producing fine crystal

Basalt

same as gabbro

texture)

Modes of occurrence- On the basis of the mode of occurrence, igneous rocks are divided into two broad groups:

1. Intrusive Igneous Rocks

2. Extrusive Igneous Rocks

1. Intrusive Igneous Rocks:

Intrusive rocks are those that were formed through the solidification of molten rock material beneath the earth's surface. However, the depth at which these rocks form varies from one place to the other.

If the magma solidified at great depth, the intrusive rocks thus formed are called plutonic rocks (Pluto, the Greek god of the underworld). Some intrusions which fail to reach at the surface, and solidify at intermediate depth, are called hypabyssal rocks.

(a) Plutonic rocks have coarse grain size, because being formed at great depth their cooling and crystallisation occurred rather slowly. Since the great depth causes the cooling process to be slow, the resulting rocks are compact, coarse in texture, and large crystalled.

Typical examples of plutonic rocks are granite and gabbro. In addition, diorite, peridotite and syenite are also referred to as plutonic rocks. Granite is a medium-to coarse-grained rock composed of quartz, feldspar and mica. It is important to note that in a plutonic rock the individual crystals can easily be distinguished with the unaided eye.

(b) Hypabyssal igneous rocks:

When magma fails to escape to the surface, it enters into the cracks, pores and between the beds of overlying rocks during volcanic activity. It is in these places that molten rock cools and solidifies beneath the earth's surface.

In such cases cooling is relatively more rapid than in a large mass as in case of plutonic rocks, but it is definitely slower than that on the surface. This produces a very variable and intermediate category of hypabyssal rocks.

Some of these consist of well-formed crystals of a variety of minerals. These are known as porphyries, granophyres etc. The hypabyssal rocks thus formed are both in large masses and in thin sheets. Such rock types are the result of the small scale igneous intrusions such as a sill or a dyke.

Besides the above two major forms of intrusive igneous rocks, solidified magmas assume various shapes and forms which are as under:

(i) Dykes (Dikes):

When the magma rises through vertical fissures, it soli­difies to form walls of rock and it cuts across the bedding planes of sedimen­tary rocks. In such cases the cooling is more rapid than in the plutonic rocks.

In dykes the crystals are small. Dykes vary in thickness from a few centi­meters to hundreds of meters. In length these rocks vary from a few meters to many kilometres. The typi­cal example is the great Dyke of Zim­babwe, which are 10 km wide and 600 km long.

Dyke rocks are usually less resistant than the surrounding country rocks. However, there are some of the dyke rocks which are much more resistant than the rocks into which they were intruded.

After the overlying rocks have been worm away, dykes appear as great walls of rock. In certain cases dykes form the cores of long ridges. After its erosion, a dyke appears to be a long narrow ridge.

In case a dyke is eroded faster than the surrounding rock, a long narrow trench is formed. A dyke may have branches in the form of smaller dykes.

(ii) Sills:

When magma is injected between the bedding planes of a sedimentary rock, the result is the solidified horizontal sheets which are of varying thickness. Such sheets may extend for many square kilometers.

Sills are formed from highly fluid magma which is basaltic in its chemical composition. Remember that the horizontal extent of a sill is much greater than its thickness.

There are some sills which are several hundred centimeters thick and cover an area of many square kilometers. Sills are more resistant to erosion than are the surrounding rocks. Thus, they are very important in the formation of landforms.

(iii) Batholiths:

These are very large, irregular but elongated masses of intrusive igneous rock. Batholiths, the large masses of rock, invariably occur in the heart of mountain ranges. These are large crystalled rocks formed by the slow cooling of the masses of magma.

When exposed by prolonged denudation, the batholiths are exposed as massive upland areas. At the time of their formation they were deeply buried by overlying rocks. Batholiths are generally dome-shaped and their edges are almost vertical.

It is in­teresting to learn that none has ever seen the base of a batholith, because they are deeply within the earth. It may extend down to many kilometers and it's a real extent Exposed by Denudation may be several thousand kilo-meters.

However, they are limited, to the thickness of the earth's crust. The largest known batholith is found in British Columbia which is 2400 km long and 160 km wide.

(iv) Laccolith:

Laccoliths are formed where viscous magma has forced the overlying strata of rocks into a dome or an arch. They are lenticular masses of igneous rock which have been injected between the layers of stratified rock.

Because of the very rapid movement of magma, the beds just above the central part of the intrusion get arched up several thousands of centimeters. The bottoms of the laccoliths are flat, while the tops are irregular and dome-shaped. Remem­ber that laccoliths are found only in stratified rocks.

(v) Stocks:

Stocks are found in any kind of rock, but they are small igneous intrusions. They are rounded in shape.

Stocks are of great economic signifi­cance because deposits of many pre­cious metals like gold, silver, copper and zinc etc. are found it their (stocks) fractures which in the form of veins extends from a stock in the rocks surrounding it.

Stocks are generally composed of granitic rocks. The area exposed at the surface is usually less than 100 km2. Stocks are found in great numbers in the mineral-rich area of the southern Rocky Mountains.

(vi) Volcanic necks or plugs:

In the dying stage of a volcano, magma sometimes solidifies in the conduit at varying depths. After the overlying rocks have been comple­tely eroded, these circular masses of igneous rocks become visible. Such intrusions of molten rock are known as volcanic necks or plugs.

(vii) Lopolith:

Lopolith is a saucer- shaped intrusive igneous rock which, as opposed to a phacolith, is concave upwards. It shows a banded structure. It is more funnel-shaped than saucer- shaped; in other words, a lopolith has a shape that resembles a funnel.

According to some, Lopolith's shape is produced when the intruded magma pushes down the underlying strata, while others opine that the funnel shape is produced when the Lopolith occupies a tectonic basin.

(viii) Phacolith:

An igneous intrusion is called a phacolith when both its roof and floor are convex upward. The igneous rock of a phacolith is usually associated with the highest region of a large fold.

2. Extrusive igneous rocks:

Those igneous rocks which have formed on the surface of the earth as a result of the accumulation and solidification of molten rock material ejected from a volcano are known as extrusive rocks.

Extrusive rocks are further subdivided on the basis of their mode of origin. They are classified as explosive types and quiet types.

Explosive types:

When the volcanic erup­tions are of the explosive type, accumulations of gases and steams throw the lava violently into the air. Here, it may be pointed out that magma that reaches the surface of the earth is called Lava.

In explosive eruption, lava is blown into small pieces that fall back to the earth's surface as solid particles. The large pieces are called bombs; those of smaller size are known as lapilli; while very fine particles are called 'volcanic ash' or volcanic dust.

Beds of such fine volcanic material accumulated in water bodies present a stratified appearance. This is known as tuff. When there are mixtures of coarse and fine angular materials they are known as breccias or agglomerates.

Quiet types:

In the quiet types of eruption lava comes out through small cracks and fissures. After cooling and solidification, lava becomes basalt type of rock.

Sometimes, lava comes out in quick succession and this gives rise to layers of lava piled one upon another. In many countries of the world such lava flows cover very large areas, say, thousands of square kilometers with thickness of several hundred meters.

The upper and lower surfaces of individual flows are characterized by glassy texture, vesicular structure and ropy, stringy appearance.

Several lava flows during fissure eruption form extensive lava plateau and lava plains. Peninsular India offers a fine example of lava plateau which covers an area of about 7, 74,000 km2.

Chemical Composition of Igneous Rocks - Acidic and Basic Igneous rocks

This dual classification of igneous rocks is based on the basis of the presence of silica (SiOz). As a matter of fact, the nature of igneous rocks is dependent on the type of magma and the depth at which it cools.

In acidic igneous rock the percentage of silica in the magma varies from 60% to 70% or more. On the contrary, the basic igneous rocks have less percentage of Silica, e.g. about 50% or less. On the basis of silica percentage, the igneous rocks are classified as under:

Silica percentage Rock type Basic oxides over 65 Acid 35 35

65-55 Intermediate 35-45

55-45 Basic or mafic 45-55

Less than 45% Ultra basic over 55

Acid rocks include granite and obsidian. They are light in colour and in weight. Intermediate rocks are represented by diorite and andesite. Basic rocks are darker and heavier, and include basalt and gabbro. Intermediate rocks include diorite and andesite. Peridotite is the representative of ultra basic rocks.

Mineral Composition of Igneous Rocks

Igneous rocks are also classified on the basis of their mineral composition. It is worthwhile to remember that the great bulk of igneous rocks consist of silicate minerals, which are compounds containing silicon and oxygen atoms.

However, majority of silicate minerals consist of two or more of the metallic elements, such as aluminum, iron, calcium, sodium, potassium, and magnesium. The igneous rocks are divided into two broad categories on the basis of the kinds and proportions of their constituent minerals.

The broad groups are: (i) felsic igneous rocks, (ii) mafic igneous rocks, and (iii) ultramafic igneous rocks. The felsic igneous rocks are composed of quartz and the feldspars. Such rocks are light in colour varying from white, pink or grayish, and are low in density. The term felsic has been derived from two minerals i.e. feldspars (fe) and silicate (si). This mineral group is rich in silica.

The mafic igneous rocks, on the contrary, consist of the minerals of the dark group such as pyroxene, amphiboles and olivines. All these minerals are rich in magnesium and iron. The term mafic has been derived from magnesium (ma) and ferrous (f) which stands for iron.

In mafic group of igneous rocks the two elements, magnesium and iron, dominate. The mafic minerals are not only dark or black in colour, but are also denser than the felsic minerals; the ultramafic igneous rocks are rich in such minerals as pyroxene and olivine.

Classification of Igneous Rocks on the basis of Textures

On the basis of texture alone igneous rocks are classified as under:

(1) Granitoid or Phaneritic:

In this group of igneous rocks, their grains can be recognised with the unaided eye. Most of the plutonic rocks fall into this category and are coarse-grained.

(2) Pegmatitic:

Very coarse grained rocks belong to this group. Such rocks are characterised by very large crystals. Actually the rocks of this group belong to the granite family in which there are large and irregular masses of quartz, feldspar and a few other minerals. Pegmatitic syenites, pegmatitic diorites, etc. are the representative rocks of this group.

(3) Aphanitic:

Rocks of this group are highly granular, but generally the grains are so small that they cannot be recognised with the aid of a microscope. Basalts and felsites found mainly in dykes or sills and non-glassy lavas are the typical examples.

(4) Glassy:

Many extrusivte igneous rocks like pumice, obsidian and pitchstone are composed of natural glass. However, these rocks may contain some grains.

(5) Porphyritic:

Such rocks contain grains of various sizes. The chief characteristic of this group of rocks is the great variation in the size of grains.

(6) Fragmental:

This term refers to the condition in which various extrusive volcanic rocks occur. For example, volcanic breccia, dust and tuff represent this category.

Chief Characteristics of Igneous Rocks:

1. Igneous rocks were formed by the solidification of molten rock material called magma or lava.

2. There is almost complete absence of stratification or beds.

3. Fossils are not found in igneous rocks since they are solidified form of molten magma. Thus, they are not fossiliferous.

4. Since igneous rocks are massive and very hard, water percolates with great difficulty along the joints found in them.

5. Igneous rocks are storehouse of valuable minerals like iron ore, aluminium, manganese, gold, silver, zinc, lead, etc.

6. Igneous rocks are crystalline or granular. However, the difference in the rate and place of cooling causes great variations in the size, form and texture of grains in different kinds of igneous rocks. Granites are coarse grained, but basalts are glassy or fine grained.

7. Igneous rocks are not easily affected by chemical weathering, but basalts, when they come into contact with rain water, are easily weathered. Coarse grained intrusive igneous rocks like granite are largely affected by the process of mechanical weathering.

8. There is abundance of silicate minerals in igneous rocks.

9. Most of the igneous rocks are highly complicated in their chemical composition.

10. Igneous rocks are usually associated with the volcanic activity.