There are three types of rock: igneous, sedimentary, and metamorphic.
In this article, we drilled down deep to dig out all of the information you need to learn about the most common type of rock, igneous rock.
Read on for more info on what they are, how they develop, and how to recognize them — to the very heart of the subject.
Characteristics of Igneous Rocks (Let’s Learn More)
How do igneous rocks form?
The creation of igneous rock is a natural process that occurs as part of the Earth’s geological cycle.
During this cycle, plate tectonics, weathering, and other processes convert the Earth’s rocks from one kind to another on a continuous basis.
Igneous rocks are formed as a result of the cooling of molten magma or lava that has formed near, at, or below the Earth’s surface.
Intrusive vs Extrusive Igneous Rocks
There are two basic types of igneous rocks: intrusive and extrusive.
Intrusive igneous rocks are the most common kind of igneous rock.
Intrusive igneous rocks are created when magma cools beneath the surface of the Earth.
Extrusive igneous rocks are created when lava cools near the Earth’s surface and extrudes outward.
Intrusive igneous rocks cool very slowly when they are deep under the earth’s surface or when magma is rising to the surface of the earth’s atmosphere.
Intrusive igneous rocks cool at a very slow rate and feature big crystals, which result in a coarse-grained rock when they are exposed to air.
When it comes to coarse-grained rocks, phaneritic rocks are those that contain individual crystals that are reasonably uniform in size and large enough in size to allow scientists to distinguish between the numerous mineral grains that make up the rock.
Faster cooling rates result in smaller individual crystals in the rock as a result of the rock cooling faster.
Extrusive igneous rocks are characterized by tiny grains that result in a fine-grained rock as a result of the rapid cooling of the rock.
When extrusive igneous rocks reach the earth’s surface, they cool rapidly because they are typically exposed to the elements through volcanoes or fissures.
Igneous Rock Texture
The texture of a rock is determined by the size and arrangement of mineral crystals, which are often referred to as “grains.”
Inferring information about the environmental setting in which distinct igneous rocks are generated is accomplished by geologists through the use of mineral and textural classifications.
When it comes to igneous rocks, texture is a phrase that describes the size, shape, and arrangement of interconnecting crystalline mineral grains in the rock.
The size of crystal grains in an igneous rock is influenced by two important factors:
1) The speed at which molten rock cools; it can be slow or fast.
2) The quantity of dissolved gases or fluids present in the magma.
The grain size of igneous material varies significantly.
Similar to the sedimentary scale, grain-sized classes are divided into smaller groups of sizes with a wider variety of sizes than the sedimentary scale.
When it comes to igneous rocks, phenocrysts are grains that are significantly bigger than the other grains that comprise the remainder of the rock.
Igneous Rock Crystallization
The mineral makeup of the crystallized rocks is determined by the chemical content of the magma as it cools during the cooling process.
In total, 98 percent of all magma is composed primarily of silicate (SiO2) ions bonded with alkali metals such as aluminum, calcium, sodium, potassium, magnesium, and iron (Fe).
Magma may also include trace amounts of other elements such as titanium (Ti), manganese (Mn), gold (Au), silver (Ag), and uranium (U), as well as trace amounts of other elements (U).
After crystallization, the minerals combine to form two major groups of silicate minerals.
The dark-colored ferromagnesian silicates, which crystallize at high temperatures, and the light-colored nonferromagnesian silicates, which crystallize at lower temperatures, are the two major groups of silicate minerals.
Igneous Rock Composition
Igneous rocks are classified into three major groups based on the percentage of felsic (light-colored) minerals in comparison to mafic (dark-colored) minerals in their composition: granitic, basaltic, and andesitic.
Granitic rocks are referred to as felsic rocks by geologists because of the high concentration of feldspar and silica in them (fel for feldspar and si for silica).
Minerals with bright colors, such as feldspar and silica, are found in greater abundance in granitic rocks than minerals with dark colors.
Minerals found in granitic rocks include the minerals quartz, feldspar, biotite, and amphibole, among others.
Granitic rocks account for approximately 70% of the Earth’s crustal composition.
Basaltic rocks are composed mostly of darker silicate minerals and calcium-rich plagioclase feldspar, with just a trace amount of quartz.
The term “mafic” refers to basaltic rocks that have a high concentration of ferrromagnesian minerals, as opposed to other types of rocks (ma for magnesium and f for ferrum).
Basaltic rocks are black in color and tend to be denser than granitic rocks in terms of density.
Andesitic rocks are intermediate in composition between granites and basalts, including around 25% dark silicate minerals (amphibole, pyroxene, and biotite mica), with the remaining 75% consisting primarily of plagioclase feldspar as the dominant mineral.
Types of Igneous Rocks
In its felsic intrusive igneous state, granite may be distinguished by its texture, which can be either phaneritic or porphyritic.
Granite cools at a very slow rate, resulting in the formation of enormous masses of rock known as plutons or batholiths.
Granite is typically composed of 20 to 50% quartz, 30-60% feldspar.
You might see another 5-10% of darker minerals such as biotite.
The remaining 5-10% is often a mix of other minerals.
The quartz grains are typically spherical in form and range in color from white to grey in hue.
The feldspar grains are predominantly potassium and sodium-rich, with individual rectangular-shaped grains, and they are found in large quantities.
Depending on the chemical makeup of the feldspar, the hue might range from white to grey to reddish in appearance.
The remaining darker minerals, which are often composed of muscovite, biotite, and amphibole, are typically black in color.
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Rhyolite is a felsic, extrusive igneous rock that typically has an aphanitic structure with glassy fragments and phenocrysts, depending on the pace at which the rock was formed and cooled.
Rapid cooling results in the formation of glassy fragments, whereas slower cooling results in the formation of phenocrysts.
Rhyolite is a kind of rock that forms swiftly from lava flows on the Earth’s surface.
Rhyolite is composed primarily of light-colored quartz and feldspar minerals, which give the rock a pink or grey appearance.
In the igneous rock world, pumice is a felsic extrusive igneous rock with a glassy, vesicular structure that is created as a result of fast cooling and a high concentration of gas.
Pumice develops in a similar manner to obsidian, and the two may often be found together in the same area of the world.
Pumice is so light as a result of the existence of many gas bubble pockets that it frequently floats when submerged in liquid.
See also: Can Pumice Scratch Glass?
Unlike other minerals and rocks, obsidian is amorphous, meaning it lacks structure, and as a result, when broken, it generates a conchoidal fracture.
Obsidian is formed when silica-rich lava cools rapidly, resulting in the fast cooling of obsidian-forming lava.
It is the presence of numerous metallic ions in obsidian that gives it its overall black color, despite the fact that thin pieces of obsidian seem translucent.
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A large gabbro intrusion is commonly the source of economically important nickel, chromium, and platinum.
Gabbro is a mafic, coarse-grained igneous rock with an aphaneritic structure.
It is comprised mostly of pyroxene, with calcium-rich plagioclase feldspar and tiny quantities of olivine and amphibole.
Basalt is a mafic, extrusive, fine-grained dark green to black volcanic rock with a porphyritic texture that ranges from dark green to black in color.
A basalt rock’s primary constituents are pyroxene and calcium-rich plagioclase, with trace quantities of olivine and amphibole thrown in for good measure.
Diabase is a mafic, intrusive igneous rock that is medium to fine-grained in texture.
The diabase is predominantly composed of iron-rich pyroxene and plagioclase labradorite, with minor amounts of other minerals.
It is often quite black in color, although it may also be speckled with lighter hues in certain cases.
In the geologic record, dike dikes are tabular diabase intrusions that fill fissures under the Earth’s surface.
When volcanic eruptions occur, andesite is formed as an intermediate, extrusive igneous rock with a mostly fine-grained porphyritic texture.
Andesite is characterized by the presence of phenocrysts, which are often large-grained feldspar or amphibole minerals.
The mineral makeup of diorite is similar to that of granite, with a major difference.
Diorite is generally found to contain more of the darker mafic minerals.
Diorite is made up of primarily quartz, along with sodium-rich plagioclase, and amphibole or biotite.
Diorite is an intermediate intrusive igneous rock with a predominantly coarse-grained phaneritic texture.
In geology, tuff is an extrusive, pyroclastic rock made of a composite of small ash pieces expelled during a volcanic eruption.
Tuff is a kind of volcanic ash.
Tuff may be used as a description for other rocks based on the relative concentration of rock to ash ratio, for example, rhyolite tuff.
Tuff may also be used as a descriptor alongside other rocks depending on the relative concentration of rock to ash ratio, for example, rhyolite tuff.
On the surface, igneous rocks may seem simple and common, but there is much to learn beneath the surface of the subject.
Today we unearthed enough information about igneous rocks to give you a solid foundation to build upon with further research and learning.
You should now understand the formation process, textures, compositions, and types of igneous rocks.
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