What are Igneous Rocks?

What are Igneous Rocks?

Igneous rocks are formed from the solidification of molten material, such as magma or lava, that has cooled and hardened. They are one of the three main types of rocks, alongside sedimentary and metamorphic rocks. Igneous rocks can be classified into two categories: intrusive and extrusive. Intrusive igneous rocks form beneath the Earth’s surface, and their crystals are generally large because they have had more time to cool and solidify. Extrusive igneous rocks form at the Earth’s surface or in shallow water and are characterized by small crystals or glassy textures. Examples of igneous rocks include granite, basalt, pumice, and obsidian.

When magma cools and solidifies beneath the Earth’s surface, it forms intrusive or plutonic igneous rocks. Examples of these rocks include granite, diorite, and gabbro. Intrusive igneous rocks typically have large crystals because they cool slowly over a long period of time, allowing the crystals to grow.

When lava cools and solidifies on the Earth’s surface, it forms extrusive or volcanic igneous rocks. Examples of these rocks include basalt, andesite, and rhyolite. Extrusive igneous rocks typically have smaller crystals or a glassy texture because they cool quickly, not allowing enough time for large crystals to form.

Igneous rocks can have different textures depending on the cooling speed, mineral content, and formation environment. They can also have a variety of colours, from dark to light and dull to bright. Igneous rocks are important because they provide clues about the geologic history of the Earth and can be used in construction, art, and many other applications.

Types of Igneous Rocks

There are two types of igneous rocks; intrusive and extrusive igneous rocks.

01. Intrusive Igneous Rocks

Intrusive igneous rocks, also known as plutonic rocks, are formed when magma cools and solidifies beneath the Earth’s surface. As the magma cools, it solidifies slowly, giving the crystals time to grow and develop. This results in the formation of coarse-grained rocks, with visible crystals that can be seen with the naked eye. Examples of intrusive igneous rocks include granite, gabbro, diorite, and peridotite.

Intrusive igneous rocks can be classified into two main types: acidic and basic. Acidic intrusive rocks are high in silica and aluminium, and typically form in the continental crust. Granite is a common example of acidic intrusive rock. Basic intrusive rocks, on the other hand, are low in silica and aluminium and are typically found in the oceanic crust. Gabbro is a common example of basic intrusive rock.

Intrusive igneous rocks can have a significant impact on the surrounding environment. For example, the slow cooling of granite can result in the development of large mineral deposits, including gold, silver, and copper. In addition, the heat from the magma can alter the surrounding rocks, creating new minerals and changing their physical properties.

Overall, intrusive igneous rocks play an important role in the geology of the Earth and have a wide range of applications in industries such as construction, mining, and geothermal energy.

02. Extrusive Igneous Rocks

Extrusive igneous rocks are formed from volcanic eruptions that occur on or near the Earth’s surface. These rocks are also known as volcanic rocks and have a fine-grained texture due to the rapid cooling of lava or magma. The cooling process is so fast that there is little time for mineral crystals to form, resulting in the formation of small crystals or no crystals at all. Extrusive rocks are usually composed of feldspar, pyroxene, and olivine.

Common examples of extrusive igneous rocks are basalt, andesite, rhyolite, and obsidian. Basalt is the most common extrusive rock on the Earth’s surface, forming from the solidification of lava flows. Andesite is similar to basalt, but with a higher silica content, making it more viscous. Rhyolite is a light-coloured rock that forms from highly explosive volcanic eruptions. Obsidian is a black, glassy rock that forms when lava cools rapidly, preventing crystal formation.

Extrusive igneous rocks are important to scientists because they provide clues to the geological history of an area. They can be used to determine the type of volcanic eruption that occurred, the age of the rock, and the conditions that existed during its formation. They can also be used as building materials, as they are durable and resistant to weathering.

Examples of Intrusive Igneous Rocks

As the types of igneous rocks are classified into two categories; extrusive and intrusive igneous rocks, but first we will discuss the different varieties of intrusive igneous rocks. According to the research, 95% of the total rocks are igneous. Most of the igneous rocks are underground. While the percentage of extrusive igneous rocks in exposed rocks is 25%, and the remaining 75% of the exposed rocks consist of sedimentary rocks.

(1). Diorite

Among the types of igneous rocks, diorite is the most common. Diorite is a Plutonic igneous rock, which is composed of partially acidic and partially basic magmatic material. Principal minerals found in diorite are plagioclase felspar (Oligoclase to Andesine). Hornblende is the chief dark mineral of diorite. Usually, biotite is also present. Pyroxenes are very rare minerals of diorite. A sufficient amount of mafic minerals is also found in these rocks, which is why they look dark.

Textures of Diorite: Diorite has equigranular coarse to medium-grained textures.

Occurrence of Diorite: Diorites occur as marginal facies of granite. They also occur as stocks and bosses.

(2). Gabbro

Gabbro is a black-coloured basic plutonic rock, which occurs in the form of intrusive igneous bodies.

Mineral Composition of Gabbro: Plagioclase felspar and mafic minerals are present in almost equal amounts. Essential minerals are Calcic-plagioclase (labradorite to anorthite), pyroxene (augite), and magnetite. Olivine is also present in most gabbros. Accessory minerals are biotite, hornblende and ilmenite.

The texture of Grabbro: Gabbro is generally found in coarse to medium-sized equigranular textures.

Varieties of Gabbro: There are two known varieties of Gabbro;

• Norite: Norite is a type of Gabbro, which contains enstatite and hypersthene (orthorhombic pyroxene) along with plagioclases.
• Anorthosite: This type of rock is composed almost entirely of plagioclase.

(3). Syenite

A coarse-grained rock of a light colour, immediately composed of acidic as well as basic material. Essential minerals of Syenite are K-felspar and oligoclase. The average syenite contains from 80-85% felspar. Cheif accessory minerals are hornblende, biotie and pyroxene.

Textures of Syenite: Generally syenites are found in equigranular coarse-grained textures. Some varieties may show porphyritic texture.

Varieties of Syenite: There are two known varieties of Syenite;

• Monzonite: A syenite with alkali felspar and plagioclase (andesine) in almost equal amounts is called monzonite. It usually contains a greater amount of dark minerals.
• Nepheline Syenite: A syenite with alkali felspar and more than 5% nepheline is called nepheline-syenite.

(4). Granite

Granite is a white-coloured plutonic acidic rock, which commonly occurs as major intrusive bodies, such as batholiths and stocks. Many granites are considered to be the result of crystallization from melts at relatively low temperatures.

Granites are named according to the main accessory mineral, such as biotite-granite, hornblende-granite, etc. There is a complete series of grading from granite to granodiorite. “Granodiorites” are those in which plagioclases exceed K-felspar. In these rocks percentage of dark minerals also increases.

The texture of granite varies from very fine to coarse crystals. The equigranular texture is common. Some varieties may show porphyritic texture.

(5). Peridotite

Nature of Peridotite: Plutonic, ultrabasic, and dark-coloured rock.

Mineral Composition: Peridotite is composed almost entirely of ferromagnesian minerals. The mafic minerals are chiefly pyroxenes and olivine in varying proportions. Felspars are negligible. Accessory minerals are hornblende, biotite, and spinel. Magnetite, chromite, ilmenite, and garner are frequently associated with peridotites.

Textures of Peridotite: Peridotite is usually found in equigranular coarse grains.

Varieties of Peridotite: There are three basic varieties of Peridotite rocks.

• Dunite: The rock composed almost wholly of Olivine si called dunite.
• Pyroxenite: The rock composed essentially of pyroxene is called pyroxenite.
• Kimberlite: A variety of altered peridotite in which diamonds are found, is called kimberlite.

(6). Diabase (Dolerite)

Diabase is an altered form of dolerite that has dull green colour. Dolerite is a dark-coloured, equigranular fine-grained rock, which is mainly composed of calcic-plagioclase and augite. Augite forms nearly 50% of the rock. Dolerite occurs mainly as dykes and sills.

(7). Pegmatite

An extremely coarse-grained rock commonly associated with granites, which has a composition that of granite. Most pegmatites contain the common minerals found in granites, but they are of extremely large size. These minerals are quartz, felspar, and mica. Crystals of the minerals measuring 25 cm across are common. Tourmaline, beryl, topaz, apatite, monazite, and fluorite may be found associated with pegmatite.

The texture of pegmatite: They are of extremely coarse-grained and irregular texture. The constituent minerals are 3 cm or more in size.

Varieties of Pegmatite: A peculiar textural variety known as “graphic granite” is commonly found in pegmatites. This rock contains crystals showing the inter-growth of quartz and felspar.

Occurrence of Pegmatite: Pegmatites are closely related genetically to large masses of plutonic rocks. They may be found as veins or dykes traversing the plutonic igneous rock. More commonly they extend out from it into the surrounding country rocks. The coarse grains of pegmatites are largely the result of the presence of volatiles during crystallization rather than the result of slow cooling.

Examples of Extrusive Igneous Rocks

These types of igneous rocks are different from intrusive igneous rocks, because, they do not lie under the earth’s surface, but rather they are extruded during the volcanic eruption. That is extrusive igneous rocks are also known as volcanic rocks.

(1). Andesite

Andesite is an extrusive volcanic rock, which is immediately acidic and basic. The rock is mainly composed of plagioclase (oligoclase or andesine). Hornblende, biotite, augite, or hypersthene may be present frequently as phenocrysts. K-felspar and quartz are absent or present in amounts of less than 10%. The rock is fine-grained with a porphyritic texture. Felspars and mafic minerals occur as phenocrysts.

Andesites are named according to the prominent ferromagnesian mineral present, such as hornblende-andesite or biotite-andesite. As andesite are extrusive igneous rocks, therefore they are found in volcanic lava flows.

(2). Basalt

Basalt is a volcanic rock equivalent to gabbro and composed of basic to ultra-basic dark coloured material. The essential minerals are augite, calcic-plagioclase, and iron oxide. Usually, olivine is also present in Basalt. Labradorite felspar is the chief constituent of the groundmass where more calcic-plagioclase (bytownite or anorthite) may occur as phenocrysts. Augite is frequently found both as phenocrysts and in the groundmass, but olivine as a rule occurs only as phenocrysts.

The texture of Basalt: Fine-grained to glassy. The porphyritic texture is common. Abundant gas cavities may occur near the top of basalt flows to make the rock vesicular.

Varieties of Basalt: There are five main varieties of Basalt.

• Olivine-Basalt: A very charming type of igneous rock, which is formed when basalt contains a notable amount of Olivine.
• Quartz-Basalt: When a sufficient quantity of quartz is present in basalt rock.
• Trachylyte: The basaltic glass is called Trachylyte.
• Ankaramite: Ultrabasic basalt is a type of igneous rock that is rich in augite called ankaramite.
• Oceanite: Ultrabasic basic basalt rich in olivine is called oceanite.

(3). Dacite

Dacite is a common volcanic rock, which is formed by the rapid solidification of lava that is silica-rich and has a poor quantity of alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.

Dacite is relatively common, occurring in many tectonic settings. It is associated with andesite and rhyolite as part of the sub-alkaline tholeiitic and calc-alkaline magma series.

Composition of Dacite: This type of igneous rock (Dacite) consists mostly of plagioclase feldspar and quartz with biotite, hornblende, and pyroxene (augite or enstatite). The quartz appears as rounded, corroded phenocrysts, or as an element of the groundmass. The plagioclase in dacite ranges from oligoclase to andesine and labradorite. Sanidine occurs, although in small proportions, in some dacites, and when abundant gives rise to rocks that form transitions to the rhyolites.

The texture of Dacite: In hand specimens, many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars and black crystals of biotite and hornblende. Other dacites, especially pyroxene-bearing dacites, are darker coloured.

In thin sections, dacites may have an aphanitic to porphyritic texture. Porphyritic dacites contain blocky highly zoned plagioclase phenocrysts and/or rounded corroded quartz phenocrysts. Subhedral hornblende and elongated biotite grains are present. Sanidine phenocrysts and augite (or enstatite) are found in some samples. The groundmass of these rocks is often aphanitic microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites, it is largely vitreous, while in others it is felsitic or cryptocrystalline.

(4). Obsidian

Obsidian is a deep black or blackish green coloured, naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Obsidian is produced from felsic lava, rich in lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.

(5). Pumice

Pumice is also called pumicite, when it is in powdered or dust form. It is a volcanic glass (rock), which has a highly rough porous/vascular texture. Due to the abundance of pores and vessels in pumice, it is extraordinarily light in weight. Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases (including water and CO₂) that are dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO₂ that appear when a carbonated drink is opened). The simultaneous cooling and depressurization freeze the bubbles in a matrix. Eruptions under water are rapidly cooled and the large volume of pumice created can be a shipping hazard for cargo ships.

Pumice is a very lightweight, porous, and abrasive material and it has been used for centuries in the construction and beauty industry as well as in early medicine. It is also used as an abrasive, especially in polishes, pencil erasers, and the production of stone-washed jeans. Pumice was also used in the early book-making industry to prepare parchment paper and leather bindings. There is high demand for pumice, particularly for water filtration, chemical spill containment, cement manufacturing, horticulture, and increasingly for the pet industry. The mining of pumice in environmentally sensitive areas has been under more scrutiny after such an operation was stopped in the U.S. state of Oregon, at Rock Mesa in the southern part of the Three Sisters Wilderness.

(6). Rhyolite

Rhyolite is an extrusive igneous rock, formed from magma rich in silica that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of mafic minerals, and it is typically very fine-grained (aphanitic) or glassy.

(7). Scoria

Among the types of igneous rocks, the importance of pumice can not be neglected. Like pumice scoria is also a highly porous/vascular rock, light is in weight, and can be used as an abrasive material. The difference is pumice is whitish, while scoria is deep black in colour, and pumice has a specific gravity of less than 1 (lighter than water), while scoria has specific gravity larger than 1, which does not float on the surface of the water.

Chemical Composition of Scoria: Chemically scoria is composed of volcanic glass with a few zeolites (e.g. clinoptilolite). Most scoria is composed of glassy fragments and may contain phenocrysts.

(8). Tuff

Another important rock included in the types of igneous rocks is Tuff. Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous.

Chemical Composition of Tuff: A rock is considered as Tuff when it is composed of 75% of ash. An Ash tuff consists of feldspars and quartz and a small amount of chalcedony, calcite, dolomite, epidote, and basalt fragments. Green lapilli tuff consists of feldspar, quartz, and muscovite and a small amount of calcite, chalcedony, sericite, chlorite, quartzite, and basalt fragments.