Igneous Rock Textures

In geological studies, rocks are categorized into three types - igneous, sedimentary and metamorphic rocks. Going by the definition, igneous rocks are crystalline rocks which are formed as a result of cooling of the magma. These rocks are further categorized into extrusive igneous rocks and intrusive igneous rocks on the basis of where the formation takes place. The texture is perhaps the most important, but sadly the most underrated attribute of this rock type. In geology, the textures of these rocks plays a crucial role in determining its origin. More importantly, it also helps the geologists in classification of different types of igneous rocks.

Factors Determining the Texture of Igneous Rocks

In igneous rocks, the texture is determined by various factors - and cooling rate is by far the most important among them. The diffusion rate, i.e. the rate at which molecules move through the magma, also plays a crucial role in determining the textures of igneous rocks. More importantly, it is the diffusion rate of molecules which determines the rate of the growth of crystals. Yet another important factor which determines the texture of igneous rocks is the rate of nucleation of new crystals - which ensures that the chemical constituents of a crystal come together instead of drifting apart. Other than these factors, the conditions prevailing in the surrounding also have a crucial role to play in the entire process. Owing to the difference in temperature the rate of cooling is faster at the surface of the Earth as compared to the same beneath the surface.

Igneous Rock Textures

As we mentioned earlier, the texture of igneous rock play an important role when it comes to rock identification. Other than the shape and size, the texture of a rock also includes the distribution of mineral grains and inter-grain relationships. Like we said earlier, igneous rocks are categorized into two types:

• Those rocks which are formed on the surface of the Earth are known as extrusive igneous rocks or volcanic rocks.
• Those rocks which are formed beneath the surface of the Earth are known as intrusive igneous rocks or plutonic rocks.

The texture of igneous rocks differs in either of these types as there is drastic difference in conditions prevailing at the surface of the Earth and beneath it.

In case of extrusive igneous rocks, the magma cools down rapidly as it comes in contact with the atmosphere. In case of intrusive igneous rocks, on the other hand, magma cools down slowly - thus giving ample time for the process of crystallization. While extrusive igneous rocks are typically characterized by small crystals which are not visible to the naked eye, the intrusive igneous rocks are characterized by large crystals which are easily seen by naked eye. The six most commonly occurring textures that can be seen in igneous rocks are phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. The details of each of these sic different igneous rock textures are given below.

Texture	Formation	Examples
Phaneritic	In this case rocks the magma is crystallized slowly below the Earth's surface owing to which large crystals - visible to the naked eye, are formed.	Granite, dorite, etc.
Aphanitic	In this case, the magma cools rapidly as its comes in contact with the atmosphere thus giving very little time for formation of crystals.	Basalt, andesite, etc.
Porphyritic	This texture is most often occurred when conditions change rapidly in course of magma cooling. (For instance, magma starts cooling beneath the surface of the Earth, but suddenly it is thrown out as a result of eruption.) You can see both large and small crystals in these rocks.	Porphyritic basalt, porphyritic granite, etc.
Glassy	An igneous rock gets this texture when cooling of magma happens so fast that there is absolutely no time for the formation of crystals.	Obsidian rock, pumice, etc.
Pyroclastic or Fragmental	When volcanic eruption blasts lava into the air it results in fragmentation of the same. The rock which are formed as a result of cooling of this fragmented magma sport the pyroclastic texture.	Welded tuff
Pegmatitic	This texture can be attributed to its formation process wherein some minerals become very large in course of cooling of magma.	Granite pegmatites, diorite pegmatites, etc.

Though the mineral composition if igneous rocks can help in their identification, there are certain limitations to this. For instance - in case of extrusive igneous rocks wherein the crystals are very small it is difficult to identify the mineral composition of the rock. In such cases, the texture of these rocks can be of significant help in identification and classification of the said rock type.