The Textures of Igneous Rocks

Igneous texture is a characteristic of igneous rocks that describes the size, shape, and arrangement of the crystals in the rock. Igneous texture is a crucial factor in identifying and classifying igneous rocks. The texture of an igneous rock is determined by the rate at which the rock cools. 

The slower the cooling rate, the larger the crystals will grow, and the more visible they will be to the naked eye. Conversely, the faster the cooling rate, the smaller the crystals will be, and the rock may even have a glassy texture.

Igneous Rock Textures

There are six main types of igneous textures; Phaneritic, Aphanitic, Porphyritic, Glassy, Pyroclastic and Vesicular.

There are six main types of igneous textures; Phaneritic, Aphanitic, Porphyritic, Glassy, Pyroclastic and Vesicular.

Phaneritic Texture

Phaneritic texture is a characteristic of igneous rocks that have visible crystals that can be seen with the naked eye. These crystals are typically interlocking, which means that they are tightly packed together and have irregular shapes. Phaneritic texture is formed when magma cools slowly underground. This slow cooling allows the minerals in the magma to have time to grow into large crystals. The size of the crystals in a phaneritic rock can vary from a few millimeters to several centimeters. 

Rocks with a phaneritic texture (phaner mean visible) are typically intrusive igneous rocks, which means that they formed from magma that cooled underground. Examples of intrusive igneous rocks with a phaneritic texture include granite, diorite, and gabbro.

Aphanitic Texture

Aphanitic texture is a characteristic of igneous rocks that have very small crystals that cannot be seen with the naked eye or even with a hand lens. These crystals are typically less than 0.5 millimeters in size. Aphanitic texture is formed when magma cools very quickly, either at or near the Earth's surface. This rapid cooling prevents the minerals in the magma from having time to grow into large crystals.

Rocks with an aphanitic texture (a mean not, phaner mean visible) are typically extrusive igneous rocks, which means that they formed from magma that erupted onto the surface of the Earth. Examples of extrusive igneous rocks with an aphanitic texture include basalt, andesite and rhyolite.

Glassy Texture

Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. 

Glassy texture is most common in volcanic rocks, which form when magma erupts onto the surface of the Earth. The rapid cooling of the magma prevents the atoms in the rock from arranging themselves into a crystalline structure, resulting in the formation of glass. The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice.

Porphyritic Texture

Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals. Examples of porphyritic igneous rocks include andesite and porphyry. 

Vesicular Texture

Vesicular texture is a volcanic rock texture characterized by small cavities or holes called vesicles. These holes are formed as dissolved gases in the magma escape as the pressure decreases during extrusion. Vesicular texture is most common in extrusive rocks, such as scoria and pumice. Vesicles can range in size from less than 1 millimeter to several centimeters. They can be spherical, elongated, or irregular in shape. In some cases, the vesicles may be filled with secondary minerals, such as calcite or quartz. Vesicular texture is a common feature of many types of volcanic rocks, including basalt, scoria, and pumice.

The Texture of Igneous Rocks

Pyroclastic Texture

Pyroclastic texture is characterized by fragments of igneous rock of various sizes and shapes. Pyroclastic (pyro meaan igneous, clastic mean fragment) textures occur when explosive eruptions blast the lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash, lapilli and volcanic bombs. Examples of pyroclastic igneous rocks include tuff and breccia. 

Pegmatitic Texture

Pegmatitic texture is characterized by extremely large crystals, some of which can be several feet in diameter. Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive (the size ranges from a few centimetres to several metres). This texture is common in intrusive igneous rocks that form from the crystallization of highly volatile magmas.  This is typical of pegmatites.

Other Igneous Rocks Texture

In addition to the main types of igneous rock textures, there are a number of other textures that can be found in igneous rocks. Some of these other textures include: 

Poikilitic texture: Poikilitic rocks are characterized by large mineral grains that enclose smaller mineral grains. 

Ophitic texture: Ophitic rocks are characterized by lath-shaped plagioclase feldspar crystals that are enclosed by larger pyroxene crystals.

Subophitic texture: Subophitic rocks are characterized by lath-shaped plagioclase feldspar crystals that are partly enclosed by larger pyroxene crystals.

Equigranular texture: Equigranular rocks have mineral grains that are all approximately the same size and shape.

Spinifex texture: Spinifex rocks are characterized by needle-shaped crystals that are arranged in radiating bundles.

The type of igneous rock texture that a rock has can be used to determine a number of things about the rock, including its cooling history, composition, and origin. For example, phaneritic rocks typically form from magmas that cool slowly beneath the Earth's surface, while aphanitic rocks typically form from magmas that cool quickly at the Earth's surface. Porphyritic rocks can form from either intrusive or extrusive magmas, and glassy rocks are typically formed from extrusive magmas that cool very quickly. Pyroclastic rocks are formed from the ejection of volcanic material, and pegmatitic rocks are formed from the crystallization of highly volatile magmas.