Classification

Igneous rocks show a wide variation in mineral composition and texture due to conditions of crystallization. The most useful classification is based on mineralogy and texture.

Relative silica content – presence of quartz indicates excess of silica and felspathoids indicate deficiency of it.

Why is magma less dense than the surrounding rock

In Geology, Volcanoes, Plate Tectonics 
Frida Inta 

Magma is less dense because it is a fluid, is hot, and it is full of gases.Ultimately a volcano is an expression of the lightest minerals in a magma, mixed with volatile gases. It is true that molten rock takes up more space than cold rock, because the ions involved are in a greater state of excitement.

Fold

When the compressional force applied under high confining pressure, rock beds tend to yield folding. Fording result in shorting and thickening of rock bed
Terminology
Anticline, Syncline, Limb, Axial plain, Axis of fold,Plunge of fold
Types of fold
Symmetrical fold - Two limbs dip in same angle but in opposite direction, axial plain is vertical and passing through crest and tough.

Structure of Igneous Rock

Flow Structure - parallel band or streaks which is caused by flow of magma or lava during cooling and crytallization.

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.