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.

Igenous Rock 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.

Igneous Rocks

Igneous rocks are formed by cooling and solidification of magma.

  90% of the earth crust is composed of igneous rock.

  SiO₂ (40% - 75%), Al₂O₃ (10% - 20%) and oxides of Ca, Mg & Fe
less than 10%.

  Acid magma is rich in Si, Na & K.

  Basics magma is rich in Ca, Mg & Fe.

  Based on the silica percentage present in the rocks, they are classified into four.

Silicate Structure

Nesosilicates  (Island Silicates)  

Oxygens atoms are not shared with other SiO₄^-4 tetrahedrons. Ratio between Si & O is 1:4. The basic structural unit is SiO₄^-4. Eg. Olivine and Garnet group (Mg,Fe)₂SiO₄.

The atomic structure of nesosilicates are generally dense which cause the mineral of this group to have high specific gravity and hardness. Crystal system is generally equidimensional and have poor cleavage.

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Pumices and Scoria

                                     
Pumice /ˈpʌmɨs/, called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough textured volcanic glass, which may or may not contain crystals. It is typically light colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles and thicker vesicle walls and being dark colored and denser.