Groups of Igneous Rocks: Intrusive and Extrusive | Rocks | Geography

The igneous rocks are more commonly classified on the basis of the mode of occurrence into two major groups:- 1. Intrusive Igneous Rocks 2. Extrusive Igneous Rocks.

Group # 1. Intrusive Igneous Rocks:

When the rising magmas during a volcanic activity do not reach the earth’s surface rather they are cooled and solidfied below the surface of the earth, the resultant igneous rocks are called intrusive igneous rocks. These rocks are further subdivided into two major groups of plutonic intrusive igneous rocks and hypabyssal intrusive igneous rocks on the basis of the depth of the place of cooling of magmas from the earth’s surface.

When the magmas are cooled and solidified very deep within the earth, the resultant rocks become plutonic but when the magmas are cooled just below the earth’s surface, the rocks are called as hypabyssal igneous rocks.

(i) Plutonic igneous rocks are formed due to cooling of magmas very deep inside the earth. Since the rate of cooling of magmas is exceedingly slow because of high temperature prevailing there and hence there is sufficient time for the full development of large grains. Thus, the plutonic igneous rocks are very coarse­-grained (pegmatites) rocks. Granite is best representa­tive example of this category.

(ii) Hypabyssal igneous rocks are formed due to cooling and solidification of rising magma during volcanic activity in the cracks, pores, crevices, and hollow places just beneath the earth’s surface, the resultant rocks are called as hypabyssal igneous rocks. The magmas are solidified in different forms depend­ing upon the hollow places such as batholiths, laccoliths, phacoliths, lopoliths, sills, dikes etc.

It should be remembered that these should not be taken as the types of igneous rocks because these are different shapes of solidified magmas:

(A) Batholiths are long irregular and undulating forms of solidified intruded magmas. They are usually dome-shaped and their side walls are very steep, al­most vertical. The upper portions of batholiths are seen when the superincumbent cover is removed due to continued denudation but their bases are never seen (fig. 8.1) because they are buried deep within the earth.

When exposed to the surface they are subjected to intense weathering and erosion and hence their sur­faces become highly irregular and corrugated. Numer­ous batholithic domes were intruded below the Dharwarian sedimentaries in many parts of the penin­sular India during pre-Cambrian period.

Many of such batholithic domes have now been exposed well above the surface in many parts of the Chotanagpur plateau of India mainly Ranchi plateau where such batholithic domes are called as Ranchi batholiths. Murha pahar near Pithauriya village, to the north-west of Ranchi city, is a typical example of exposed Ranchi batholithic domes.

(B) Laccoliths:

The world laccolith has been derived from German word, ‘laccos’ meaning thereby ‘lithos’ or rocks. Laccoliths are formed due to injection (intrusion) of magmas along the bedding planes of horizontally bedded sendimentary rocks. Loccoliths are of mushroom shape having convex summital form.

The ascending gases during a volcanic eruption force the upper starta of the fiat layered sedimentary rocks to arch up in the form of a convex arch or a dome. Consequently, the gap between the arched up or domed upper starta and the horizontal lower starta is injected with magma and other volcanic materials (fig. 8.2).

(c) Phacoliths are formed due to injection of magma along the anticlines and synclines in the re­gions of folded mountains (fig. 8.3).

(D) Lopoliths:

The word lopolith has been de­rived from German word ‘lopas’ meaning thereby a shallow basin or bowl shape body. When magma is injected and solidified in a concave shallow basin whose central part is sagged downward, the resultant form of solidified magmas is called a lopolith. The rocks of lopoliths are generally coarse-grained be­cause of slow process of cooling of magmas.

(E) Sills:

The word ‘sill’ has been derived from an Anglo-Saxon word ‘syl’ meaning thereby a ledge. The sills are usually parallel to the bedding planes of sedimentary rocks. In fact, sills are formed due to injection and solidification of magmas between the bedding planes of sendimentary rocks. Thick beds of magmas are called sills whereas thin beds of magma are termed as ‘sheets’.

The thickness of sills ranges between a few centimetres to several metres. When sills are tilted together with the sendimentary beds due to earth movements and are exposed to exogenous denudational processes, they form significant landforms like cuesta, hogbacks and ridges (figs. 8.4 and 8.8).

(F) Dykes represent wall-like formation of so­lidified magmas. These are mostly perpendicular to the beds of sedimentary rocks. The thickness of dykes ranges from a few centimetres to several hundred metres but the length extends from a few metres to several kilometres. A well-defined dyke is observable across the palaeochannel and valley of the Narmada river near Dhunwadhar Falls (Bheraghat) near Jabalpur city.

The relative resistance of dykes in comparison to the surrounding country rocks gives birth to a few interesting landforms e.g.:

(i) If the rocks of dykes are weaker and less resistant than the country rocks, the upper portion of dykes is more eroded than the country rocks, with the result a depression is formed, which, when filled up with water, is called a ‘dyke lake’ (fig. 8.5);

(ii) If the rocks of dykes are more resistant than the country rocks, upstanding ridges and hills are formed because of more erosion of the country rocks (fig. 8.6) and

(iii) If the rocks of dykes and country rocks are of uniform resistance, both are uniformly dissected and hence no significant landform is developed but the height is gradually reduced (fig. 8.7).

Group # 2. Extrusive Igneous Rocks:

The igneous rocks formed due to cooling and solidification of hot and molten lavas at the earth’s surface are called extrusive igneous rocks. Generally, extrusive igneous rocks are formed during fissure eruption of volcanoes resulting into flood basalts. These rocks are also called as volcanic rocks.

Extrusive igne­ous rocks are generally fine-grained or glassy basalts because lavas after coming over the earth’s surface are quickly cooled and solidified due to comparatively extremely low temperature of the atmosphere and thus there is no enough time for the development of grains or crystals. Basalt is the most significant representa­tive example of extrusive igneous rocks. Gabbro and obsidian are the other important examples of this group.

Extrusive igneous rocks are further divided into two major subcategories on the basis of the nature of the appearance of lavas on the earth’s surface e.g.:

(i) Explosive type and

(ii) Quiet type.

(i) Explosive type:

The igneous rocks formed due to mixture of volcanic materials ejected during explosive type of violent volcanic eruptions are called explosive type of extrusive igneous rocks. Volcanic materials include ‘bombs’ (big fragments of rocks), ‘lapilli’ (fragments of the size of a peas) and volcanic dusts and ashes.

Fine volcanic materials, when depos­ited in aquatic condition, are called ‘tuffs’. The mix­ture of larger and smaller particles after deposition is called ‘breccia’ or ‘agglomerate.’ These are more sus­ceptible to erosion because these are not well consoli­dated.

(ii) Quiet type:

The appearance of lavas through minor cracks and openings on the earth’s surface is called ‘lava flow’. These lavas after being cooled and solidified form basaltic igneous rocks. Flood basalts resulting from several episodes of lava flow during fissure flows of volcanic eruption form extensive ‘lava plateau’ and ‘lava plains’ wherein several layers of basalts are deposited one upon another.

The thickness of lavas of the Columbia plateau of the states of Washington and Oregon (USA), spread over an area of about 6,45,000 km² (2,50,000 square miles), measures about 1,216 m (4,000 feet). The extensive lava flows during Cretaceous period covered an area of about 7,74,000 km² (3,00,000 square miles) of Peninsular India. Several beds of basaltic lavas are clearly observable all along the exposed sections of the Western Ghats mainly near Khandala (between Mumbai and Pune) and over Mahabaleshwar plateau.