Igneous Rocks: Types, Properties and Uses

Key Concepts

• Igneous rocks

Introduction:  

 

Magma is a layer of melted rock present below the Earth’s crust. Sometimes, magma flows to the Earth’s surface. When magma reaches the surface of the Earth is termed as lava. Magma may reach Earth’s surface either by volcano, i.e., huge explosion or slowly come out of cracks in Earth’s crust. When a melted rock cools and hardens inside or outside the Earth’s crust, it forms igneous rock. So, igneous rocks are formed from melted rock. Igneous means fire made. 

Types of igneous rocks 

There are two types of igneous rocks based on their formation. They are intrusive and extrusive igneous rocks. 

Intrusive igneous rocks: 

When magma cools and hardens below the Earth’s crust, rocks form. It is called intrusive igneous rocks. The process of cooling down of magma below the Earth’s crust takes a very long time, maybe thousands of years. Magma cools very slowly and forms large mineral crystals of rocks. Examples: Granite, gabbro, pegmatite, diorite  

Extrusive igneous rocks: 

When lava comes out of the Earth’s crust and cools down, it forms extrusive igneous rocks. Above Earth’s crust, lava cools comparatively quickly or sometimes in a few hours. The rocks formed by this process are small. Examples: Basalt, andesite, rhyolite, scoria, pumice, obsidian. 

Properties of igneous rocks: 

  

The properties of an igneous rock depend upon its formation and the minerals present in it. Some igneous rocks are hard, whereas others fall apart easily. The texture of the rock is decided by the size of the mineral crystals within a rock. 

Properties of granite: 

• Granite is intrusive igneous rock. 
• It contains large mineral crystals due, to which it a has coarse texture. 
• There are many minerals in the granite that gives granite various colors. 

Properties of pumice: 

• Pumice is an extrusive rock, also called as lava rock. The small holes in pumice rock are caused by gases that escape as lava cools down.  
• Pumice is a lightweight rock.    

Properties of obsidian: 

• It is an extrusive rock. Lava cools very quickly, so mineral crystals do not have enough time to form. 
• Hence obsidian rocks are very smooth and look like shiny black glass. 

Uses of igneous rocks :

• Granite is very strong and long-lasting, used for making kitchen countertops, building materials. 
• Gabbro is another igneous rock usually mixed with concrete and used for making bridges, buildings and sidewalks. 
• The lightweight and rough-textured pumice stone is used to scrub off the dirt. 

Summary:

• Magma is a layer of melted rock present below the Earth’s crust.
• When a melted rock cools and hardens inside or outside the Earth’s crust, igneous rocks
• are formed. When magma cools and hardens below the Earth’s crust, intrusive igneous rocks form.
• Granite, gabbro, pegmatite, diorite are examples of intrusive igneous rocks.
• When lava comes out of the Earth’s crust and cools down, extrusive igneous rocks are formed.
• Basalt, andesite, rhyolite, scoria, pumice, obsidian are examples of extrusive igneous rocks.
• Granite is very strong and long-lasting, used for making kitchen countertops, building materials.