Forms of Igneous Bodies

Extrusive igneous bodies – form when magma pours out on the surface of the earth crust.

Intrusive igneous bodies – form when magma consolidates at some depth below the earth crust. Such rock bodies show variation in size and shape. Ex. Batholith, stock, dyke & sill.

Discordant bodies – those rock which cuts through the overlaying strata. Ex. Batholith, stock, dyke.

Concordant bodies – those rock which lie between rocks bed. They do not show transgressive relation to the rock they invade. Ex. Sills, lopoliths and laccoliths.

Description of igneous rocks

Batholith – large intrusive igneous bodies which have transgressive relation with adjacent country rocks. Diameter usually exceeds 100 km and their outcrop at the surface is roughly circular or oval.In cross-section batholiths posses steep outwardly dipping and is bottomless. Their composition is granite or granodioritic. 

Stock and bass – Irregular igneous mass of batholithic habit are called stocks. They are small in size and diameter is between 10 to 20 km. Stocks with circular outcrop are called boss.

Lopolith – Saucer shaped concordant igneous body which is bent downward into a basin like shape. Its diameter is 10 to 20 times it thickness. They are much larger than laccoliths. It’s commonly of basic. 

Laccolith – lens shaped intrusive igneous bodies which cause overlaying beds to arch in the form of doom. It has flat bass and doom top, diameter is 2 to 3 km and several hundred meters in thickness.

Phacolith – Crescent shaped body of igneous rock which occupy crest and trough of folded strata due to minimum stress. They are formed when igneous materials invade the folded region.

Sill – Sheet like structure which run parallel to the bedding plane of pre exciting strata. They may be parallel, inclined or horizontal depending on the attitude of strata in which they are intruded. They vary in thickness from few centimeters to several meters but they are thin compare to their length. It is composed of dolerites and basalts.
 

Dyke – Wall like igneous body that cut across the strata of the pre exciting rocks in vertical of steep incline. Thickness varies from few centimeters to several meters. They occur in group and run parallel to one direction or in radial to a center. Dyke with circular outcrop and conical form is called Ring Dyke and those with inverted conical form is called Cone Sheet. 

Volcanic Plug – Vertical or cylindrical shaped igneous body.

Lavaflows – Volcanic igneous rock occurs as lavaflow and it has tabular in shape. 
Volcanic pipes are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes.