Igneous Rock Textures

Rocks are categorized into three types: igneous, sedimentary, and metamorphic. Going by their definition, igneous rocks are crystalline rocks which are formed as a result of cooling of the magma. These rocks are further categorized into extrusive 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 attributes of this rock type. It plays a crucial role in determining their origin. More importantly, it helps the geologists classify these rocks into different types.

In igneous rocks, the texture is determined by various factors, including the cooling rate, diffusion rate, and the rate of nucleation of new crystals, which ensures that the chemical constituents of a crystal come together instead of drifting apart. It is the diffusion rate of molecules that determines the rate of the growth of crystals.

Besides 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.

As we mentioned earlier, the texture plays 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 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 volcanic rocks, the magma cools down rapidly as it comes in contact with the atmosphere. In case of plutonic rocks, magma cools down slowly, thus giving ample time for the process of crystallization.

While extrusive rocks are characterized by small crystals which are not visible to the naked eye, intrusive 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.

Phaneritic : The magma is crystallized slowly below the Earth's surface owing to which large crystals―visible to the naked eye, are formed. Examples - Granite, dorite, etc.

Aphanitic: The magma cools rapidly as its comes in contact with the atmosphere, thus giving very little time for the formation of crystals. Examples - Basalt, andesite, etc.

Porphyritic : The conditions change rapidly in course of magma cooling. For instance, magma starts cooling beneath the surface of the Earth, but is suddenly thrown out as a result of eruption. You can see both large and small crystals in these rocks. Examples - Porphyritic basalt, porphyritic granite, etc.

Glassy : An igneous rock gets this texture when the magma cools down so fast that there is absolutely no time for the formation of crystals. Examples - Obsidian rock, pumice, etc.

Pyroclastic or Fragmental : When volcanic eruption blasts lava into the air, it results in its fragmentation. Rocks which are formed as a result of cooling of this fragmented magma sport the pyroclastic texture. Examples - Welded tuff

Pegmatitic : This texture can be attributed to its formation process, wherein some minerals become very large in course of cooling of magma. Examples - Granite pegmatites, diorite pegmatites, etc.

Though the mineral composition of igneous rocks can help in their identification, there are certain limitations to this. For instance, in case of extrusive rocks wherein the crystals are very small, it is difficult to identify the mineral composition of the rock.

In such cases, their texture can be of significant help in identification and classification of the said rock type.