"Obsidian 1" by James St. John is licensed under CC BY 2.0
Definition of Igneous Rock
Igneous rock or igneous rock (from Latin : ignis , "fire") is a type of rock formed from cooled and hardened magma , with or without a crystallization process , either below the surface as intrusive (plutonic) rock or above the surface as rock. extrusive (volcanic). This magma can come from semi-molten rock or rock that is already present, either in the mantle or crust. Generally, the melting process occurs by one of the following processes: temperature rise , pressure drop, or a change in composition. More than 700 types of igneous rock have been described, most of which form beneath the surface of the Earth's crust .
According to scholars such as Turner and Verhoogen (1960), F. F Groun (1947), Takeda (1970), magma is defined as a naturally occurring viscous, incandescent, viscous silicate liquid, with a high temperature between 1,500–2.5000C and mobile (can move). and is found in the lower crust of the earth. In the magma there are several soluble, volatile materials (water, CO2, chlorine, fluorine, iron, sulfur, etc.) found in igneous rocks.
When magma experiences a decrease in temperature due to travel to the earth's surface, minerals will form. This event is known as the scattering event. Based on the crystallization of silicate minerals (magma), by NL. Bowen compiled a series known as Bowen's Reaction Series.
Texture of Igneous Rock
Texture is defined as a condition or a tight relationship between minerals as part of a rock and between minerals with a glass mass that forms the foundation mass of the rock.
The texture of igneous rocks is generally determined by three important things, namely:
Crystallinity
Crystallinity is the degree of crystallization of an igneous rock at the time the rock was formed. Crystallinity in its function is used to indicate how much is in the form of crystals and which is not in the form of crystals, but it can also reflect the speed of magma freezing. If magma continues to freeze slowly then the crystals are coarse. Meanwhile, if the freezing continues quickly then the crystals will be smooth, but if the cooling continues very quickly then the crystals are amorphous.
In its formation, there are three classes of degrees of crystallization, namely:
- Holocrystalline, i.e. igneous rock where everything is composed of crystals. The holocrystalline texture is characteristic of plutonic rocks, which are microcrystalline rocks that have frozen near the surface.
- Hypocrystalline, ie when some rocks consist of glass mass and some consist of crystal mass.
- Holohyaline, i.e. igneous rock which is all composed of glass mass. Most holohyaline textures are formed as lava (obsidian), dike and sill, or as facies smaller than rock bodies.
Granularity
Granularity is defined as the grain size (size) in igneous rocks. There are two types of grain size textures known, namely:
- Phaneric/phanerocrystalline
Most of the crystals of this group can be distinguished from each other megascopically with the naked eye. The crystals of this faneric type can be divided into:
- Fine, if the grain diameter is less than 1 mm.
- Medium (medium), if the grain diameter is between 1-5 mm.
- Coarse, if the grain diameter is between 5-30 mm.
- Very coarse, if the grain diameter is more than 30 mm.
Afanitik
Most of the crystals of this group are indistinguishable with the naked eye, so a microscope is needed. Rocks with an aphanitic texture can be composed of crystal, glass or both. In microscopic analysis can be distinguished:
- Microcrystalline, if the minerals in igneous rocks can be secured with the help of a microscope with a grain size of about 0.1 – 0.01 mm.
- Cryptocrystalline, when the minerals in the igneous rock are too small to be secured even with the aid of a microscope. The grain size ranges from 0.01 to 0.002 mm.
- Amorphous / glassy / hyaline, if the igneous rock is composed of glass.
Crystal Form
Crystal form is the nature of a crystal in rock, so it is not a rock property as a whole. From a two-dimensional view, there are three forms of crystals, namely:
- Euhedral, if the boundary of the mineral is the original form of the crystal member.
- Subhedral, when some of the crystal boundaries are no longer visible.
- Anhedral, if the mineral no longer consists of original crystals.
From a three-dimensional perspective, four crystal forms are known, namely:
- Equidimensional, if the three-dimensional crystal form is the same length.
- Tabular, if the two-dimensional crystal form is longer than one of the other dimensions.
- Prismatic, if the crystal form in one dimension is longer than the other two dimensions.
- Irregular, if the crystal form is chaotic.
Relationship Between Crystals
The relationship between crystals or also called the relationship is defined as the relationship between crystals / minerals with one another in a rock. Broadly speaking, relations can be divided into two,
Equigranular
Equigranular is, if the relative size of the crystals that make up the rock are of the same size. Based on the ideality of the crystals, therefore equigranular is divided into three, namely:
- Panidiomorphic granular, ie when most of the minerals consist of euhedral minerals.
- Hipidiomorphic granular, ie when most of the minerals consist of subhedral minerals.
- Allotriomorphic granular, ie when most of the minerals consist of anhedral minerals.
Inequigranular
Inequigranular is if the size of the crystal grains as rock-forming is very different. The large mineral is called the phenocryst and the other is called the ground mass or matrix which can be mineral or glass.
Structure of Igneous Rock
Structure is the macro appearance of rocks which includes the location of a clear / general layer of rock layers. Most igneous rock structures can only be seen and observed in the field, for example:
- Pillow lava or pillow lava, which is a very typical structure of underwater volcanic rock, forms a pillow-like structure.
- Joint structure, is a structure characterized by the presence of joints that are arranged regularly perpendicular to the direction of the current. While the structure that can be seen and observed in rock samples (hand speciment samples), namely:
- Massive, that is, if it does not show the presence of currents, traces of gas (does not indicate the presence of holes) and does not indicate the presence of other fragments embedded in the igneous rock body.
- Vesicular, which is a structure with holes caused by the release of gas at the time of magma freezing. The holes show a regular direction.
- Scoria, which is the same structure as the vesicular structure but the holes are large and show a chaotic direction.
- Amygdaloidal, namely the structure where the gas holes have been filled by secondary minerals, mostly silicate or carbonate minerals.
- Xenoliths, namely structures that show the presence of other rock fragments / fragments that enter the intrusive rock.
- Most igneous rocks are without structure (massive), while the structures that are present in igneous rocks are made by joints or fractures and magma freezing, for example: columnar joints and sheeting joints.
Mineral Composition of Igneous Rock
To determine the mineral composition of igneous rock, it is enough to use the color index of the crystalline rock. On the basis of the color of minerals as a constituent of igneous rocks can be grouped into two, namely:
- Felsic minerals, which are light colored minerals, mainly consist of quartz, feldspar, feldspathoid and muscovite minerals.
- Mafic minerals are dark colored minerals, especially biotite, pyroxene, amphibole and olivine.
Igneous Rock Classification
Igneous rocks can be classified according to the manner in which they occur, the content of SiO2, and the color index. Thus, different rock names can be determined, even though they are in the same rock type, according to their classification basis.
Classification by way of occurrence
According to Rosenbusch (1877-1976) igneous rocks are divided into:
Effusive rock, for igneous rock that forms on the surface.
Dike rock, for igneous rock that forms near the surface.
Deep seated rock, for igneous rock deep in the earth. By WT Huang (1962), this rock type is called plutonic, while effusive rock is called volcanic rock.
Classification based on SiO2
According to (CL Hugnes, 1962), namely:
Acid igneous rock, if the SiO2 content is more than 66%. For example, rhyolite.
Intermediate igneous rock, if the SiO2 content is between 52% - 66%. For example, dasit.
Alkaline igneous rock, if the SiO2 content is between 45% - 52%. For example, andesite.
Ultra-alkaline igneous rock, when the SiO2 content is less than 45%. For example, basalt.
Classification by color index
According to (SJ Shand, 1943), namely:
Leucoctaris rock, if it contains less than 30% mafic minerals.
Mesococtic rock, if it contains 30% - 60% mafic minerals.
Melanocratic rock, if it contains more than 60% mafic minerals.
Meanwhile, according to SJ Ellis (1948) also divides igneous rocks based on their color index as follows:
Holofelsic, for igneous rocks with a color index of less than 10%.
Felsic, for igneous rocks with a color index of 10% to 40%.
Mafelsic, for igneous rocks with a color index of 40% to 70%.
Mafic, for igneous rocks with a color index of more than 70%.
Types of igneous rock
Igneous rocks can be divided into 3 categories:
- Deep igneous rocks, for example: granite, diorite, and gabbro
- Igneous rock alley / middle, for example: porphyry granite
- External igneous rocks, for example: andesite, obsidian, and basalt
