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.

1.      Ultra-basicRocks – (Si ≤ 45%), Peridotite.

2.      Basic Rocks­­ – (45% - 55%), Gabbros & Basalt.

3.      Intermediate Rocks – (55%-65%), Diorite.

4.      Acid Rocks – (Si ≥65%), Granites.

  Quartz is an essential mineral in acid rocks, accessory minerals in intermediate and basic rocks.

  Alkali felspar (Potash felspars with sodium rich plagioclase) is an essential mineral in acid rocks.

  It is either absent or found in intermediate, basic or ultrabasic rocks.

  Calcium rich plagioclases are found mostly in basic rock and andesine in intermediate.

  Olivine is essential minerals in ultrabasic and basic rock; amphiboles and pyroxenes are also important constituent. However it occurs as accessory minerals in acid and intermediate.

Composition	Alkali feldspar	Alkali feldspar-Plagioclase	Plagioclase
Ultrabasic (No felspar)			Dunite, peridotite, pyroxenite.  Komatite
Basic (dark mineral ˃60%)	Alkali-Gabbro Alkali-Basalt	Syeno-Gabbro Trachy-basalt	Gabbro Basalt, Dolerite
Intermediate (LM 60-30%)	Syenite Trachyte	Monzonite Trachy-andesite	Diorite Andesite
Acid (light mineral˃60%)	Granite Rhyolite	Adamellite Rhyodacite	Granodiorite Dacite

       I.            Acid Rocks (also known as Felsic rock)

Ø  Generally light in color and low specific gravity (≤2.7).

Ø  High proportion of quartz and alkali feldspar.

Ø  Silica percentage is high.

     II.      Basic Rocks (also known as Mafic Rock)

Ø  Dark in color and usually high specific gravity (≥3.2).

Ø  Low percentage of silica content.

Ø  Olivine, pyroxene, hornblende & biotite, little or no quartz.

Igneous Rocks can be also classified as;

1.      Oversaturated Rocks – This rock crystallized from Melts. Silica content is high with abundant quartz and alkali feldspars.

2.      Saturated Rocks – This rock are formed when the amount of silica present in the Melts is just enough to form silicate minerals. They do not content Quartz.

3.   Undersaturated Rocks – This rock crystallized from Melt which is deficient in Silica and high in Alkalis and Aluminum Oxides such as felspathoids.

Occurrence of Igneous Rocks

Magma originated at the temperature of about 900˚- 1600˚C. On consolidation it produces two types of rocks.

     i.         Extrusive Rocks (volcanic rock)­­­­_­­­­­­

Ø  Formed by solidification of Lava which is produced due to volcanic eruption.

Ø  As lava cool and crystallize rapidly, the texture of volcanic rock is generally fine grain or glassy.

Ø  They often content gas cavities called Vesicles which are forms due to escape of volatiles upon cooling of lava.

Ø  It shows Flow Structures which is due to movement of viscous lava.

      ii.            Intrusive Rocks

Ø  It is formed when magma crystallizes beneath the earth’s crust.

Ø  Depending on the depth of formation it is classified into two;

v  Plutonic Rock

v  Hypabyssal Rock

1.      Plutonic Rock

·         Magma which is deeply buried in the earth’s crust cools slowly with the retention of volatiles. As a result the minerals constituents crystallizing from it has time to grow to considerable size giving the rock a coarse grained texture.

2.      Hypabyssal Rock

·         These are formed by solidification of magma close to the earth’s surface, and textures are usually finer grained but coarser than volcanic rocks.

·         It occurs as injections within the country rocks.

·         They commonly show porphyritic texture.