Formation of Igneous Rocks, Types & Examples

Formation of Igneous Rocks

The igneous rocks are among the oldest and fascinating materials on Earth. They are produced as molten rock in the deep-rooted areas of the earth and cool and settle to form the, hard granite rocks that we currently observe in mountains, landscapes and even in the buildings we construct nowadays. The term igneous was derived in the Latin ignis, signifying fire, and this term is a perfect fit since the derivation of the word denotes a fiery nature of their origin. Due to their power, different textures and the mineral structure, igneous rocks contribute significantly to geology and engineering, as well as the natural development of the Earth crust.

 

Rock Cycle

 

How are igneous rocks formed?

Igneous rocks are formed from the cooling and hardening of molten rock, either below the Earth’s surface (magma) or after erupting onto the surface as lava.

There are two types of igneous rocks: Intrusive igneous rocks and extrusive igneous rocks.

                         

Intrusive igneous rocks

When magma loses its mobility before reaching the surface it eventually crystallizes to form intrusive igneous rocks. These are also known as plutonic rocks. These rocks take a long time to cool therefore large crystals have time to grow, this is why igneous rocks often look coarse-grained.

Examples of igneous rocks are Granite, diorite, gabbro, and peridotite.

                       

                                                                    Figure 1: Diorite

                       

Extrusive igneous rocks

Igneous rocks that form when molten rock solidifies at the surface are classified as extrusive igneous rocks. These are called volcanic rocks. These rocks form on the Earth’s surface therefore they cool quickly and develop fine grains. Basalt and rhyolite are common examples of Extrusive igneous rocks.

                        

                        

Intrusive igneous features

 Batholith

• Intrusive body greater than 100 km is called batholith

• Largest intrusive body (sometimes consist of numbers of plutons), hundreds of km long

• Majorly felsic/granitic

• A Large irregular mass that extends deep into surrounding rock layers. 

 Stocks

• An intrusive body with similar characteristics to batholiths but smaller than 100 km.

                      

Extrusive Structure

• Volcanic Neck; Lava; Ash:

• A volcanic neck forms when magma solidifies inside the vent of an extinct volcano.

                      

Bowen’s reaction series

Canadian geologist Norman L. Bowen explained that minerals crystallize from magma in two pathways  known as continuous and discontinuous series.

The cooling and crystallization of magma is a long and complex process in which various silicate minerals crystallize in a definite order and at a particular temperature. The sequence of separation of minerals from a silicate melt is now well established. If a mineral remained in the melt after it crystallized, it would react with the remaining melt and produce the next mineral in the sequence.

Continuous reaction series

Minerals in a solid-solution series form continuous reaction series. Plagioclase members show this type of reaction. In this composition of minerals is continuously being changed and variation in composition is perfectly continuous. In this series we can see the evolution of plagioclase as they changed from Ca-rich to Na-rich.

Discontinuous reaction series

These occur at definite temperature. It takes place at a lower temperature when an early crystallized magma(mineral) reacts with the remaining magma to form a mineral of new composition.

Olivine reacts with the melt at lower temperature and form pyroxene, amphibole, biotite and others. Minerals that crystallize at high temperature are less weathering resistant. While mineral with low temperature is high weathering resistant.

 

                      

Importance of igneous rocks in the field of civil engineering

·     Igneous rocks are important in civil engineering because they are strong, durable and highly resistant to weathering.

·     They have tightly interlocked crystals.

·     Igneous rocks are used in foundations, bridges and railway ballast such as (basalt and granite).

·    These rocks have high compressive strength.

·     Igneous rocks typically have low water absorption.

·     Igneous rocks can withstand freeze–thaw conditions without cracking.

·     Massive igneous formations offer stable ground.

·    They Support heavy structures like dams and high-rise buildings.

·     Reduce risk of settlement and structural failure.