Igneous rocks are formed from solidification of molten material called magma. They constitute one of the three main rock types, the other two being sedimentary and metamorphic rocks. The classification of igneous rocks is primarily based on their texture, mineral composition, and mode of occurrence. In this article, we will explore the various types of igneous rocks and their classification.
Introduction
Igneous rocks are classified into intrusive and extrusive rocks based on the location of their formation. Intrusive rocks form when magma solidifies below the earth's surface, while extrusive rocks form when magma solidifies above the earth's surface. The texture of igneous rocks is determined by the rate of cooling of magma. Slow cooling results in large crystals, while rapid cooling leads to small crystals or even glassy textures. Texture plays a crucial role in determining the strength, durability, and appearance of igneous rocks.
Classification of Igneous Rocks
The classification of igneous rocks is based on several factors, including texture, mineral composition, and mode of occurrence. Let's take a closer look at each of these factors.
Texture
Texture refers to the size, shape, and arrangement of mineral grains in an igneous rock. There are three primary types of textures: phaneritic, aphanitic, and glassy. Phaneritic texture is characterized by visible crystals that are typically greater than 1 millimeter in size. Aphanitic texture has tiny crystals that are usually less than 1 millimeter in size. Glassy texture is devoid of any crystal structure, and the rock looks like glass.
Mineral Composition
The mineral composition of igneous rocks is another essential factor in their classification. The primary minerals found in igneous rocks include quartz, feldspar, mica, olivine, and pyroxene. Depending on the relative abundance of these minerals, igneous rocks are classified as felsic, intermediate, mafic, or ultramafic.
Felsic rocks contain more than 65% silica and are rich in potassium feldspar and quartz. They are light-colored rocks with a low density and are typically found in continental crusts.
Intermediate rocks have a silica content between 55% and 65% and contain plagioclase and amphibole. They are darker in color and denser than felsic rocks.
Mafic rocks have a silica content between 45% and 55% and are rich in magnesium and iron. They are dark-colored rocks and are typically found in oceanic crusts.
Ultramafic rocks have less than 45% silica and are primarily composed of olivine and pyroxene. They are greenish in color and are usually associated with mantle rocks.
Mode of Occurrence
The mode of occurrence refers to the location where igneous rocks are formed. There are two main types: intrusive and extrusive.
Intrusive rocks form when magma solidifies below the earth's surface. They are also known as plutonic rocks and can be further classified into several subtypes based on their texture and mineral composition. Some examples of plutonic rocks include granite, diorite, and gabbro.
Extrusive rocks form when magma solidifies above the earth's surface. They are also known as volcanic rocks and can be further classified based on their texture and mineral composition. Examples of volcanic rocks include basalt, andesite, and rhyolite.
Igneous rocks are an essential component of the earth's crust, and their classification helps us understand their properties and formation. Texture, mineral composition, and mode of occurrence are the primary factors used in the classification of igneous rocks. By gaining insight into each of these factors, we can learn more about the diverse world of magmatic rocks.