Igneous Rocks [Definition, Classification, Types and Formation]

​Igneous rocks are formed from the cooling of magma (a boiling liquid found underneath the Earth’s surface). The name ‘igneous’ comes from the Latin word ‘igneous,’ which means fire.

In that notion, igneous rocks occur when molten rock turns into solid below the earth’s crust to create intrusive igneous rocks or on the earth’s surface to create extrusive igneous rocks.

They are rocks that are produced via heating followed by solidification through cooling.

The heated component is the magma, which is partially or fully melting material of original rocks in the earth’s crust.

These rocks are continuously subjected to extreme heat, intense pressure changes, and distortion in composition.

How Are Igneous Rocks Classified? 

Igneous rocks are grouped according to:

• Chemical composition
• Mode of occurrence
• Mineralogy
• The geometric setting of the igneous structure
• Texture

The classification of the igneous rocks may give us vital information regarding the conditions under which they were created. Particles and size are variables applied in the classification of these rocks.

The particles and size of rock are greatly dependent on the cooling history and the mineral properties of the rock.

Some basic minerals vital in igneous rocks’ formation include quartz, olivine, feldspars, micas, pyroxenes, and amphiboles. All other minerals that are also present are considered unnecessary in almost all types of igneous rocks. These unnecessary minerals are known as ‘accessory minerals.’

Geologists normally use igneous textures to know the processes involved in forming a particular type of igneous rock. The main types of igneous textures are aphinitic, glassy, pegmatite, phaneritic, porphyritic, and pegmatitic.

Types of Igneous Textures

Aphinitic Texture

Igneous rocks with this texture usually occur from the rapid crystallization of lava.

The extrusive rocks cool very quickly; therefore, their minerals form fine crystals that the naked eye cannot see and distinguish. Andesite, basalt, and rhyolite possess this type of texture.

 

Glassy Texture

This texture forms when lava from a volcanic eruption cools very rapidly such that no crystallization occurs.

This results in an amorphous glass that has little or no crystals. Obsidian and pumice rocks have this type of texture.

 

Pegmatitic Texture

This type of texture is formed when magma cools, and some minerals increase in size extensively. The sizes may range from some centimeters to quite a several meters. Pegmatite displays this texture.

 

Phaneritic Texture

This texture is seen in plutonic igneous rocks, which underwent slow crystallization underneath the earth’s surface. When magma cools at a slow pace, the minerals can increase in size and have large crystals.

The crystals can be seen and distinguished by the naked eye. Diorite, gabbro, and granite possess this type of texture.

 

Porphyritic Texture

The rapid change of conditions causes this texture as the magma continues to cool down. The minerals that had been created earlier, by the slow cooling magma, will possess large crystals.

The remaining melt will form a fine-grained matrix due to the sudden cooling. This results in an aphinitic rock with several bigger crystals enclosed in its matrix.

The porphyritic texture can also form when magma is crystallized under a volcano, but eruption occurs before the crystallization is complete. As a result, the lava formed crystallizes much faster with smaller-sized crystals.

Pyroclastic Texture

Pyroclastic textures form when violent volcanic eruptions throw the lava into the atmosphere creating fragmental and glassy materials. These materials eventually fall to the surface as lapilli, volcanic ash, and volcanic bombs.

In a less complicated classification, igneous rocks are distinguished based on the kind of feldspar they contain. Rocks that contain feldspar are further distinguished according to the existence or absence of quartz.

Rocks that lack feldspar or quartz are further distinguished on the kind of iron or magnesium element present.

Rocks with quartz with silica are termed ‘silica-oversaturated.’ Those with feldspathoids are termed as ‘silica-under saturated’ since feldspathoids cannot coexist in a stable state with quartz.

Igneous rocks that contain crystals huge enough to be viewed by the naked eye are known as ‘phaneritic.’ Those with too tiny crystals to be viewed are known as ‘phanitic.’

In general, phaneritic suggests an intrusive origin, whereas phanitic suggests an extrusive one.

Igneous rocks that have bigger and clearly detectable crystals enclosed in a fine-grained matrix are called porphyry.

Porphyritic texture arises when the number of the crystals increases significantly in size before the main mass of the molten rock crystalizing as a fine-grained uniform component.

Types of Igneous Rock

There are mainly two types of igneous rock:

Intrusive/ plutonic Rocks

These are formed when the hot molten rock (magma) cools down slowly beneath the earth’s crust and solidifies into rocks. Intrusive rocks are naturally tough and are usually coarse-grained. Examples are gabbro and granite rocks.

Extrusive/ volcanic Rocks

These are formed when the hot molten rock (magma) spills over to the earth’s surface due to volcanic eruption. The magma, now termed lava, cools quicker on the earth’s surface to create fine-grained igneous rocks. Examples of such rocks are basalt, pumice, and obsidian.

How is Igneous Rock Formed? 

Molten substances exist underneath the earth’s crust and are usually subjected to intense pressure and high temperatures (up to 1200°C).

As a result of the immense pressure and temperature changes, the molten substances may flow up to the surface leading to a volcanic eruption.

Eventually, they cool and solidify to form extrusive (volcanic) igneous rocks. On the other hand, some of the molten substances may cool and solidify beneath the earth’s surface slowly to form intrusive (plutonic) igneous rocks.

It is due to the immense temperatures and pressure changes that igneous rocks lack organic materials.

The molten substances interlock and undergo crystallization as the melt cools down to become solid in nature.

As time passes, the melt becomes a hard rock consisting of crystals with no pores. Furthermore, it does not show any uniform grain alignment.

The rock can be composed fully of a single mineral or a variety of minerals. The size of the rock is dependent on the cooling process.

Quick cooling leads to smaller-sized crystals, whereas slow cooling leads to larger crystals.

Some Examples of Igneous Rocks

There exist at least 700 known kinds of igneous rocks. Many of them are created underneath the earth’s crust because volcanic eruptions do not occur very frequently.

Andesite

Andesites are fine-grained extrusive indigenous rocks that are light gray in color. They consist mainly of plagioclase minerals that are mixed with hornblende, biotite, and pyroxene.

Basalt

Basalts are fine-grained volcanic rocks. They are dense and appear in dark-gray color. They mainly consist of plagioclase and pyroxene.

Basalts are the most common form of solidified lava and are utilized in building and construction processes.

Diorite

Diorites are coarse-grained intrusive igneous rocks. They are made up of a mixture of minerals such as pyroxene, hornblende, feldspar, and at times, quartz. Diorites are light in color with some darkened spots.

Fire opal

Fire opals are flamingo orange, yellow or red-colored igneous rocks. They are seen as fascinating geological rocks, and they occur in rhyolite.

They are created long after rhyolite has cooled, and water-rich in silica may flow inside the rocks. The water usually leaves gems like agate, Jasper, topaz, and opal in the openings of the rocks.

Gabbro

Gabbros are coarse-grained intrusive igneous rocks that are dark in color. They consist of mineral elements such as feldspar, pyroxene, and at times, olivine.

They are usually gray in color and have small spots. Gabbros are used to form concrete aggregate, ballast for railroads, and road metal.

Granite

Granites are coarse-grained intrusive igneous rocks that are light in color. They consist of 3 major minerals, which are: mica, feldspar, and quartz.

They may be gray, tan, or pinkish, which depends on the size of the grains and the concentration of the 3 minerals. Granite is largely used in architectural construction and making ornaments due to its strength and availability in large quantities.

Obsidian

Obsidians are dense extrusive igneous rocks that are dark in color. They are created when lava cools rapidly without crystallizing. They are dark but are clear in slim pieces.

Pegmatite

This is an igneous rock that results from the crystallization of molten rock full of rare minerals. It is made up of granite and is really coarse.

Pegmatite usually consists of elements that are not in the other areas of the magma chamber. It is light in color.

Peridotite

This is a coarse-grained intrusive igneous rock that is made up mainly of olivine. It may also have little amounts of elements such as amphiboles, feldspar, pyroxene, and quartz.

Pumice

This is a porous and vesicular igneous rock that results from the rapid solidification of magma. The porousness and vascularity of its texture are caused by the gas trapped in the molten rock undergoes solidification.

Pumice stones are commonly used as abrasive components in products such as Emery boards and hand soaps.

Rhyolite

These are fine-grained extrusive igneous rocks that are light in color. They are usually made up of quartz and feldspar minerals. They normally possess a smooth surface.

Scoria

Just like pumice, scoria is a porous and vesicular extrusive igneous rock. Its porousness and vascularity seen in its texture are caused by gas trapped in the magma while it is solidified.

It typically forms as a bubbly crust above lava as it moves down from the volcano and settles down with some gas trapped within.

Welded Tuff

This is a volcanic igneous rock composed of the components that are discharged from a volcanic eruption. The discharged components fall on the earth’s surface and turn into rocks through lithification.

Welded tuffs are mainly made up of volcanic ash, and sometimes they have huge particles like clinkers.

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Why igneous rocks are geologically necessary

• Their minerals and overall chemistry provide needed information regarding the components of the mantle from which igneous rocks originate, the heat and pressure levels that led to their formation, and the original rock that disappeared.
• Their exact ages can be obtained from different types of radiometric dating and can therefore be compared to the nearby geological elements. This generates a time series of events.
• Their properties are typical characteristics of a particular tectonic surrounding. This allows geologists to conduct tectonic reconstituting.
• In some situations, they carry vital metal ores. For example, andorites and granites carry uranium, tin, and tungsten, while gabbros carry chromium and platinum ores.

That was all about igneous rocks here today. Let us know if you want more info on any specific realm. Your interaction matters, cheers!