Igneous Rocks Definition
We have all seen the eruption of volcanoes on television or computers. With the eruption of the volcanoes, lava starts flowing on the surface. Magma is usually a molten rock liquid that is found below the surface when the crust of the earth melts. The formation of igneous rocks starts taking place when the molten lava or magma begins to cool down and solidify. Igneous rock meaning relates to the solidification process that leads to the creation of rocks. The features of the igneous rock can be identified by its texture, mineral composition, density, and colour. These rocks are formed either with or without crystallization.
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Cooling Process of Igneous Rock
In the intrusive igneous rock, the process of cooling is usually slow that allows the growth of large mineral crystals within the rocks. The feature of igneous rocks having coarse minerals is due to the crystals of the intrusive rocks. Examples of igneous rocks include granite, peridotite, diorite, and gabbro.
Next comes the extrusive type of igneous rocks that don’t allow crystallisation to take place. Thus, the final appearance is the fine-grained, glassy and vesicular rock formation. Examples of igneous rocks in the extrusive category include basalt, andesite, and rhyolite.
Types of Igneous Rocks
The igneous rocks definition includes two categories of rock formation. These two types are discussed below in detail.
Intrusive Igneous Rocks: When the molten lava cools slowly below the earth’s surface, the crystallization results in the formation of large crystals. These typically have a large amount of silica content within them and based on it they are known as diorite, gabbro, pegmatite, and granite. Actually, most of the magma available in the crust is never able to reach the earth’s surface.
Extrusive Igneous Rocks: The extrusive igneous rocks are also known as volcanic rocks due to their formation from volcanoes. During the volcanic eruption when the magma reaches the earth’s surface, these are known as lava or volcanic rocks. The features of the igneous rocks in the extrusive category also have a silica content in a higher amount. Some of these rocks cool down so instantly that these form an amorphous glass. Examples of extrusive igneous rocks are basalt, pumice, tuff, etc.
Types of Magma and Formation of Igneous Rocks
There are types of magma and these igneous rocks meaning comes from the type of magma they originate from. We will see the classification and properties of igneous rocks based on the magma.
Intermediate Igneous Rocks: The composition of magma between felsic and mafic leads to the formation of intermediate igneous rocks. These are typically formed in the subduction zones that also include the oceanic plates. The structure of the rock includes examples like feldspar, pyroxene, biotite, quartz, and amphibole.
Ultramafic Igneous Rocks: The characteristics of igneous rocks are mainly ferromagnesian and olivine in nature. For example, a slow cooking rock peridotite is a perfect example in this category. These igneous rocks cool down very slowly and are rare in nature.
Mafic Igneous Rocks: When magma cools down, the ferromagnetic minerals dominate this type of rock formation. Typically, it is found prevalent in oceanic divergent zones. It contains minerals such as magnesium and iron silicate. Moreover, these rocks also have other minerals like olivine, pyroxene and others.
Felsic Igneous Rocks: This rock formation by magma contains aluminium and silicon. The formation takes place in the continental crust having high gas content. Besides, it also has mineral contents like biotite, quartz, potassium, and more. Examples of rock in this category include rhyolite and granite.
Identification Process of Igneous Rocks
What we generally understand by the meaning of igneous rocks is that it is formed from the cooling and solidification of molten rocks or magma. When the molten lava starts to cool a new layer of minerals and textures are formed with the addition of new chemical components surrounding the rocks. The pressure and temperature at which the lava begins to cool and the time is taken depends on several factors. Due to this, we can see every igneous rock vary in its texture and composition. However, it is the textural and compositional properties that help us to identify the igneous rocks and determine the cooling process and magma formation.
Did You Know
The first type of rocks formed on the earth surface was igneous rocks. These are also known as the primary rocks.
FAQs on Igneous Rocks
1. What is an igneous rock and what are some common examples?
An igneous rock is one of the three main rock types, formed through the cooling and solidification of molten rock material called magma (below the surface) or lava (on the surface). Because they are formed from molten material, they are often called "fire rocks" and are considered primary rocks. Common examples include Granite, Basalt, Pumice, and Obsidian.
2. How are intrusive and extrusive igneous rocks different?
The primary difference between intrusive and extrusive igneous rocks lies in where and how quickly they cool:
Intrusive igneous rocks (or plutonic rocks) form when magma cools and solidifies slowly beneath the Earth's surface. This slow cooling allows large, visible crystals to form, resulting in a coarse-grained texture. A common example is Granite.
Extrusive igneous rocks (or volcanic rocks) form when lava erupts onto the Earth's surface and cools and solidifies quickly. This rapid cooling results in very small crystals (fine-grained texture) or no crystals at all (glassy texture). A common example is Basalt.
3. What are the main characteristics of igneous rocks?
Igneous rocks have several distinct characteristics based on their formation from magma or lava:
They are crystalline in nature, with textures ranging from coarse-grained (like Granite) to fine-grained (like Basalt) or glassy (like Obsidian).
They generally do not contain fossils, as the intense heat of the magma would destroy any organic remains.
They are typically hard and non-porous, although some types like Pumice are highly porous due to trapped gas bubbles.
They are composed of various silicate minerals and do not react with acids.
4. Why is the cooling rate of magma crucial for the texture of an igneous rock?
The cooling rate of magma or lava directly controls the size of the mineral crystals that form, which defines the rock's texture. When magma cools very slowly deep within the Earth's crust, ions have ample time to migrate and form large, visible crystals, creating a coarse-grained (phaneritic) texture like in Granite. Conversely, when lava cools very rapidly on the surface, there is insufficient time for large crystals to grow, leading to a fine-grained (aphanitic) texture. An extremely rapid cooling, or quenching, prevents crystal formation altogether, resulting in a glassy texture like Obsidian.
5. How are igneous rocks classified based on their chemical composition?
Igneous rocks are often classified into four main types based on their silica (SiO₂) content and mineral composition:
Felsic: High in silica (>65%), light in colour, and rich in minerals like quartz and feldspar. Example: Granite.
Intermediate: Moderate silica content (55-65%), with a mix of light and dark minerals. Example: Diorite.
Mafic: Low in silica (45-55%), dark in colour, and rich in magnesium and iron. Example: Basalt.
Ultramafic: Very low in silica (<45%), very dark, and composed mainly of olivine and pyroxene. Example: Peridotite.
6. What is the importance of igneous rocks in everyday life?
Igneous rocks have significant economic and practical importance. Granite, known for its durability and beauty, is widely used for kitchen countertops, flooring, and as a building stone for monuments. Pumice is used as a lightweight abrasive in cleaning products and personal care items like exfoliating stones. Basalt is crushed and used as aggregate in construction projects, such as for road building and railway ballast.
7. How do igneous rocks fundamentally differ from sedimentary and metamorphic rocks?
The fundamental difference lies in their origin, as explained by the rock cycle:
Igneous Rocks are formed from the cooling and solidification of molten magma or lava.
Sedimentary Rocks are formed from the accumulation, compaction, and cementation of sediments (particles of pre-existing rocks, minerals, or organic matter) over a long period.
Metamorphic Rocks are formed when existing igneous or sedimentary rocks are transformed by intense heat, pressure, or chemical reactions, without melting completely.
8. Can an igneous rock turn into another type of igneous rock?
Yes, this is possible through the rock cycle. An existing igneous rock, whether intrusive or extrusive, can be subjected to intense heat and pressure deep within the Earth, causing it to melt back into magma. If this new magma then cools and solidifies, it will form a new igneous rock. For example, a piece of Granite on a tectonic plate could be subducted, melt, and later erupt as Rhyolite lava, forming a new extrusive igneous rock.
