Igneous rocks

Rocks are naturally formed when tiny grains of different minerals are compressed due to the pressure exerted on them. The process of rock formation varies for different types of rocks. 

Sedimentary rocks are formed through the accumulation or deposition of mineral or organic particles at the Earth’s surface, while metamorphic rocks are created when existing rocks are exposed to high heat, pressure, and watery hot fluids. Lastly, igneous rocks are formed when molten lava cools and solidifies. In this article, we will explore igneous rocks and their formation in detail.

What are Igneous Rocks?

Igneous rocks are one of the three main types of rocks found on Earth. They are formed through the cooling and solidification of molten rock material known as magma (beneath the surface) or lava (after a volcanic eruption).

These rocks are often referred to as "primary rocks" because they are the first to form directly from Earth's molten state. They create the foundation for the Earth's crust and are rich in minerals like quartz, feldspar, and mica.

Igneous rocks can form in different environments — deep underground, at the Earth's surface, or even underwater — and the conditions under which they form affect their texture, color, and mineral content.

Types of Igneous Rocks

Igneous rocks are mainly classified into two major types based on where and how they cool:

1. Intrusive (Plutonic) Igneous Rocks

• Formed from magma that cools slowly beneath the Earth's surface.
• The slow cooling allows large crystals to develop.
• These rocks are usually coarse-grained (you can see the crystals with the naked eye).
• Examples: Granite, Diorite, Gabbro.

2. Extrusive (Volcanic) Igneous Rocks

• Formed when lava erupts onto the surface and cools rapidly.
• Fast cooling results in small or no visible crystals, making them fine-grained or even glassy.
• Examples: Basalt, Pumice, Obsidian.

Also Check: Types of Rocks

Characteristics of Igneous Rocks

The properties of igneous rocks vary based on their composition and formation process. Some common characteristics of igneous rocks include:

• Texture:
  • Coarse-grained (intrusive) or fine-grained (extrusive).
  • Sometimes glassy or porous, depending on cooling rate.
• Color:
  • Light-colored rocks (felsic) like granite have more silica.
  • Dark-colored rocks (mafic) like basalt have more iron and magnesium.
• Hardness and Durability:
  • Igneous rocks are usually very hard and resistant to weathering and erosion.
• Mineral Composition:
  • They often contain minerals like quartz, feldspar, mica, amphiboles, and pyroxenes.
• Density:
  • Mafic rocks tend to be denser than felsic rocks.

Classification of Igneous Rocks

The classification of igneous rocks is based on three main factors:

1. Texture

• Determined by the size and arrangement of mineral crystals.
• Coarse-grained (slow cooling), fine-grained (rapid cooling), glassy (very rapid cooling), or vesicular (presence of gas bubbles).

2. Mineral Composition

• Rocks rich in silica (over 65%) are called felsic rocks (e.g., granite).
• Rocks with intermediate silica content (55–65%) are called intermediate rocks (e.g., diorite).
• Rocks low in silica (45–55%) are mafic (e.g., basalt).
• Rocks with very low silica (less than 45%) are ultramafic (e.g., peridotite).

3. Origin

• Plutonic rocks: Form deep underground (e.g., granite, gabbro).
• Volcanic rocks: Form at or near the surface (e.g., basalt, pumice).

Igneous Rocks Examples

Here are some common igneous rocks examples you should know: