FORMS OF IGNEOUS ROCKS
An igneous mass will acquire on cooling depends on a number of factors such as
(a) the structural disposition of the host rock (also called the country rock)
(b) the viscosity of the magma or lava
(c) the composition of the magma or lava
(d) the environment in which injection of magma or eruption of lava takes place.
It is possible to divide the various forms of igneous intrusions into two broad classes:
All those intrusions in which the magma has been injected and cooled along or parallel to the structural planes of the host rocks are grouped as concordant bodies.
Forms of concordant bodies Sills
Ø The igneous intrusions that have been injected along or between the bedding planes or sedimentary sequence are known as sills.
Ø It is typical of sills that their thickness is much small than their width and length. Moreover, this body commonly thins out or tapers along its outer margins.
The upper and lower margins of sills commo11ly show a comparatively finer
grain size than their interior portions. This is explained by relatively faster cooling of magmatic injection at
Ø In length, sills may vary from a few centimeters to hundreds of meters
Sills are commonly subdivided into following types:
(a) Simple Sills: formed of a single intrusion of magma;
(b) Multiple Sills: which consist of two or more injections, which are essentially of the same kind of magma;
(c) Composite Sills: which result from two or more injections of different types of magma;
(d) Differentiated Sills: these are exceptionally large, sheet-like injections of magma in which there has been segregation of minerals formed at various stages of crystallisation into separate layers or zones.
(e)Interformational Sheets: the sheet of magma injected along or in between the planes of unconformity in a sequence are specially termed as interformational sheets. These resemble the sills in all other general details.
Ø These arecordant, small sized intrusive that occupy positions in the troughs and crests of bends called folds. In outline, these bodies are doubly convex and appear crescents or half-moon shaped in cross-section.
Ø As regards their origin, it is thought that when magma is injected into a folded sequence of rocks, it passes to the crests and troughs almost passively i.e. without exerting much pressure.
Ø These are concordant intrusions due to which the invaded strata have been arched up or deformed into a dome.
Ø The igneous mass itself has a flat or concave base and a dome shaped top.
Ø Laccoliths are formed when the magma being injected is considerably viscous so that it is unable to flow and spread for greater distances.
Instead, it gets collected in the form of a heap about the orifice of eruption. As the magma is injected with sufficient pressure, it makes room for itself by arching up the overlying strata.
Ø Extreme types of laccoliths are called bysmaliths and in these the overlying strata get ultimately fractured at the top of the dome because of continuous injections from below.
Ø Those igneous intrusions, which are associated with structural basins, that are sedimentary beds inclined towards a common centre, are termed as lopoliths.
Ø It is believed that in the origin of the lopoliths, the formation of structural basin and the injection of magma are "contemporaneous", that is, broadly simultaneous.
Ø All those intrusive bodies that have been injected into the strata without being influenced by their structural disposition (dip and strike) and thus traverse across or oblique to the bedding planes etc. are grouped as discordant bodies.
Ø Important types of discordant intrusions are dykes, volcanic necks and batholiths.
Ø These may be defined as columnar bodies of igneous rocks that cut across the bedding plane or unconformities or cleavage planes and similar structures.
Ø Dykes are formed by the intrusion of magma into pre-existing fractures.
Ø It depends on the nature of magma and the character of the invaded rock whether the walls of the fracture are pushed apart, that is, it is widened or not.
Ø Dykes show great variations in their thickness, length, texture and composition.
Ø They may be only few centimeters or many hundreds of metes thick.
Ø In composition, dykes are generally made up of hypabyssal rocks like dolerites, porphyries and lamprophyres, showing all textures between glassy and phaneritic types.
Ø Cone sheets and Ring Dykes may be considered as the special types of dykes.
Ø The cone sheets are defined as assemblages of dyke-like injections, which are generally inclined towards common centres.
Ø Their outcrops are arcuate in outline and their inclination is generally between 30 o - 40 o .
Ø The outer sheets tend to dip more gently as compared to the inner ones
Ø Ring Dykes are characterised by typically arcuate, closed and ring shaped outcrops.
Ø These may be arranged in concentric series, each separated from the other by a screen of country rock.
They show a great variation in their diameter; their average diameter is around 7 kilometers. Few ring dykes with diameters ranging up to 25 kms are also known.
Ø Origin of dykes It has been already mentioned that dykes are intrusions of magma into pre- existing fractures present in the rocks of the crust.
Ø These original fractures are generally caused due to tension.
Ø Their original width might have been much less than the present thickness of the dykes.
Ø This indicates widening of the cracks under the hydrostatic pressure of magmatic injection.
Ø In some cases vents of quiet volcanoes have become sealed with the intrusions.
Ø Such congealed intrusions are termed volcanic necks or volcanic plugs.
Ø In outline these masses may be circular, semicircular, or irregular and show considerable variation in their diameter. The country rock generally shows an inwardly dipping contact.
Ø These are huge bodies of igneous masses that show both concordant and discordant relations with the country rock.
Ø Their dimensions vary considerably but it is generally agreed that to qualify as a batholith the igneous mass should be greater than 100 square kilometers in area and its depth should not be
traceable. This is typical of batholiths: they show extensive downward enlargement
Ø In composition, batholiths may be made of any type of igneous rock.
Ø They also exhibit many types of textures and structures. But as, a matter of observation, majority of batholiths shows predominantly granitic composition, texture and structure.