Definition of Igneous Rocks: Texture, Structure, Mineral Composition, and Classification

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Definition of Igneous Rocks

Igneous rock or igneous rock (from Latin: ignis , “fire”) is a type of rock that is formed from magma that cools and solidifies, with or without a crystallization process, either below the surface as intrusive rock (plutonic) or above the surface as rock. extrusive (volcanic). This magma can come from semi-molten rock or pre-existing rock, either in the mantle or the earth’s crust.

Generally, melting occurs by one of the following processes: increasing temperature, decreasing pressure, or changing composition. More than 700 types of igneous rocks have been described, most of which have formed below the surface of the Earth’s crust.

According to experts such as Turner and Verhoogen (1960), F. F Groun (1947), Takeda (1970), magma is defined as a thick, incandescent silicate liquid formed naturally, with high temperatures between 1,500–2,500 °C and is mobile (can move ) and is found in the lower crust of the earth.

In the magma there are several dissolved materials that are volatile (water, carbon dioxide, chlorine, fluorine, iron, sulfur, etc.) which are the causes of magma mobility, and non-volatile (non-gas) which are common mineral forming elements. found in igneous rocks.

When magma experiences a decrease in temperature on its way up to the earth’s surface, minerals will form. This event is known as the crystallization event. Based on the crystallization of silicate minerals (magma), by NL. Bowen arranged a series known as the Bowen reaction series. In identifying igneous rocks, it is very necessary to know the characteristics of igneous rocks which include the physical properties and mineral composition of igneous rocks.

Geological Significance

Igneous and metamorphic rocks make up approximately 90–95% by volume of the upper crust or as deep as 15 kilometers. Igneous rocks are important geologically because:

  • The minerals and global chemistry provide information about the composition of the mantle, from which the igneous rock was extracted, as well as the temperatures and pressures that allowed this extraction to occur, and/or the origin of the thawed rock.
  • Absolute ages can be obtained by various types of radiometric dating and thus can be compared with adjacent geological strata, so that the time sequence of occurrence can be determined.
  • Their features are characteristic of particular tectonic environments, thus enabling tectonic reconstruction (see plate tectonics).
  • In certain situations, igneous rocks are where ore deposits exist: For example, tungsten, tin, and uranium are usually associated with granite and diorite, whereas chromium and platinum ores are usually associated with gabbro.

Morphology and Setting

Formation of igneous rock.

In terms of its formation, igneous rocks are divided into three: intrusive (plutonic), extrusive (volcanic), and hypabysal.

1. Intrusive

Granite, one of the intrusive igneous rocks (Petromine Laboratory, Padjadjaran University).

Intrusive igneous rock is igneous rock that solidifies and petrifies beneath the surface or in the Earth’s crust, surrounded by the original rock (commonly called country rock ). Magma cools slowly, and as a result, this igneous rock is coarse-grained.

The mineral grains in these rocks can be easily identified with the naked eye. Intrusive rocks can also be classified according to the shape and size of the body of the intrusion and its relationship to the other formations it intrudes into. Typical intrusive formations are batholith, stock, laccolith, sill and dike. When magma solidifies in the Earth’s crust, it slowly cools to form coarse-textured rock, such as granite, gabbro, or diorite.

The core vents of major ridges consist of intrusive igneous rock, usually granite. When exposed by erosion, the cores ( called batholiths) can occupy large areas of the Earth’s surface. Coarse-grained intrusive igneous rock that forms at depth in the crust known as abyssal; an intrusive igneous rock that forms near the surface of F 121 18 088 which is called hypabysal.

2. Extrusive

Extrusive igneous rock, also known as volcanic rock, forms on the surface of the crust as a result of partial melting of rock in the mantle and crust. Extrusive igneous rock cools and hardens more quickly than intrusive igneous rock. They are formed by the cooling of molten magma at the Earth’s surface.

Magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Because this type of rock is smooth, crystalline and fine grained. Basalt is a common extrusive igneous rock and forms lava flows , sheeting lava and lava plateaus . Several types of basalt help form the old polygonal columns. The Giant’s Causeway in Antrim, Northern Ireland is one such example.

Molten rock, with or without suspended crystals and gas bubbles, is called magma. Magma rises up because it has a lower density than the rock they created. When magma reaches the surface from under water or air, it is called lava. Volcanic eruptions into the air are called subaerial, while those that occur under the sea are called submarine. Black smokers and mid-ocean ridges are examples of underwater volcanic activity.

Basalt, a type of extrusive igneous rock with columnar joint texture, in Scotland.

The volume of extrusive rock erupted annually by volcanoes varies according to the plate tectonic setting. Extrusive rock is produced in the following proportions:

  • Divergent limit: 73%.
  • Convergent limit (subduction zone): 15%.
  • Hotspots: 12%.

Magma that erupts from a volcano behaves according to its viscosity, which is determined by its temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, when cooled, caramel-like. Long, thin basalt flows with a pahoehoe surface are very common in this type of magma.

Intermediate magma compositions, such as andesite, tend to form a conical flue composed of a mixture of ash, tuff and lava, and may have a viscosity similar to that of cold, thick molasses or even rubber when erupted. Felsic magmas, such as rhyolite, usually erupt at low temperatures and are up to 10,000 times more viscous than basalt. Volcanoes with rhyolitic magma generally erupt explosively, and rhyolitic lava flows are usually limited in extent and have steep slopes, because the magma is so viscous.

Felsic and intermediate magma eruptions are often destructive, with the explosion driven by the ejection of dissolved gases—usually water vapor, as well as carbon dioxide. Pyroclastic materials that erupt explosively are called tephra and include tuff, agglomerate and ignimbrite. Fine volcanic ash also erupts and forms tuff ash deposits which can often cover large areas.

Because lava cools and crystallizes quickly, this rock is fine grained. If cooling is so rapid that it prevents the formation of even small crystals after extrusion, the resulting rock may be mostly glassy (such as obsidian rock). If the cooling of the lava occurs more slowly, the rock will be coarser.

Because the minerals are mostly refined, it is much more difficult to differentiate between different types of extrusive igneous rock than it is between different types of intrusive igneous rock. Generally, the fine mineral constituents of extrusive igneous rocks can only be determined by examination of thin sections of the rock under a polarizing microscope, so only an approximate classification can be made in the field.

3. Hipabisal

Hipabisal igneous rocks form at depths between plutonic and volcanic rocks. These rocks are formed due to cooling and freezing resulting from the rise of magma below the earth’s surface. Hipabisal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccolites, lopolites or pakoliths.

 

Igneous Rock Texture

Texture is defined as the state or close relationship between the minerals as part of the rock and between the minerals and the glass mass that forms the bedrock of the rock.

Texture in igneous rocks is generally determined by four important things, namely:

1. Crystallinity

Crystallinity is the degree of crystallization of an igneous rock at the time it was formed. Crystallinity in its function is used to show 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 the magma freezes slowly, the crystals are coarse, whereas if the freezing takes place quickly, the crystals will be smooth, but if the cooling takes place very quickly, the crystals will be amorphous.

In its formation, three classes of degrees of crystallization are known, namely:

  • Holocrystalline, namely igneous rocks where everything is composed of crystals. Holocrystalline texture is characteristic of plutonic rocks, i.e. microcrystalline that has congealed near the surface.
  • Hypocrystalline, that is, if some of the rock consists of glass mass and some of it consists of crystal mass.
  • Holohyalin, namely igneous rock which is all composed of glass mass. Holohyaline textures are mostly formed as lava (obsidian), dikes and sills, or as smaller facies of rock bodies.

2. Granularity

Granularity is defined as the grain size (size) of igneous rock. In general, there are two groups of grain size textures, namely:

a. Phaneric/Phanerocrystalline

The size of the crystals of this group can be distinguished from one another megascopically with the naked eye. Crystals of this faneric type can be divided into:

  • Smooth (fine), if the size of the grain diameter is less than 1 mm.
  • Medium (medium), if the size of the grain diameter is between 1–5 mm.
  • Coarse (coarse), if the size of the grain diameter is between 5–30 mm.
  • Very coarse (very coarse), if the grain size is more than 30 mm.

b. Afanitic

The size of the crystals from this group cannot be distinguished by the naked eye, so a microscope is needed. Rocks with an aphanitic texture may be composed of crystals, glass, or both. In microscopic analysis can be distinguished:

  • Microcrystalline, if the minerals in igneous rock can be observed with the help of a microscope with a grain size of around 0.1 – 0.01 mm.
  • Cryptocrystalline, when the minerals in igneous rock are too small to be observed even with the aid of a microscope. The grain size ranges from 0.01 – 0.002 mm.
  • Amorphous/glassy/hyaline, if the igneous rock is composed of glass.

3. Crystal Form

Crystal form is the nature of a crystal in a rock, so it’s not the nature of the rock as a whole. Judging from a two-dimensional view, three crystal forms are known, namely:

  • Euhedral, if the boundary of the mineral is the original shape of the crystal field.
  • Subhedral, when some of the crystal boundaries are no longer visible.
  • Anhedral, if the mineral no longer has the original crystal field.

From a three-dimensional perspective, there are four crystal forms, namely:

  • Equidimensional, if the three dimensions of the crystal form are the same length.
  • Tabular, if the two-dimensional crystal form is longer than one other dimension.
  • Prismitic, if one dimension of crystal form is longer than the other two dimensions.
  • Irregular, if the crystal shape is not regular.

4. Relations Between Crystals

The relationship between crystals (relationships) is defined as the relationship between crystals/minerals with one another in a rock. Broadly speaking, relations can be divided into two:

a. Equigranular

That is, if the relative size of the crystals that form rocks are the same size. Based on the ideality of the crystals, equigranular is divided into three, namely:

  • Panidiomorphic granular, that is, when most of the minerals consist of euhedral minerals.
  • Granular hyperidiomorphic, that is, when most of the minerals consist of subhedral minerals.
  • Granular allotriomorphic, that is, when most of the minerals consist of anhedral minerals.

b. Inequigranular

That is, if the size of the crystal grains as rock formers are not as large. The larger minerals are called phenocrysts and the smaller ones are called ground masses which may be crystals or glass. Inequigranular is divided into three, namely:

  • Faneroporphyritic, ie when the crystals that make up the ground mass can be seen clearly with the eye or a loupe.
  • Porphyroafanitic, ie when the crystals that make up the ground mass cannot be seen with the eye or with a loupe.

Igneous Rock Structure

Structure is the macro appearance of the rock which includes the clear/general position of the rock layers. Most of the igneous rock structures can only be seen in the field, for example:

  • Pillow lava or pillow lava, which is the most 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 flow. While the structure that can be seen in rock samples (hand speciment samples), namely: massive, that is, if it does not show any flow properties, traces of gas (does not show any holes) and does not show any other fragments embedded in the body of igneous rock ; vesicular, which is a structure with holes caused by the release of gas during the freezing of magma, the holes show a regular direction; scoria, which is the same structure as the vesicular structure, but the holes are large and show an irregular direction; amygdaloidal, namely structures where the gas holes have been filled with secondary minerals, usually silicate or carbonate minerals; xenolite,

In general, igneous rocks are structureless (massive), while structures in igneous rocks are formed by joints or fractures and magma freezing, for example: columnar joints and sheeting joints. ).

 

Igneous Rock Mineral Composition

To determine the mineral composition of igneous rocks, it is enough to use the color index of crystalline rocks. On the basis of color, minerals as constituents of igneous rocks can be grouped into two, namely:

  • Felsic minerals, which are light-colored minerals, mainly consisting of the minerals quartz, feldspar, feldspathoid and muscovite.
  • Mafic minerals, namely minerals that are dark in color, especially biotite, pyroxene, amphibole and olivine.

Classification of Igneous Rocks

Igneous rocks can be classified based on how they occur, SiO2 content, and color index. In this way, different rock names can be determined even if they are in the same rock type, according to the basis of their classification.

1. Classification Based on How It Occurs

According to Rosenbusch (1877–1976) igneous rocks are divided into:

  • Effusive rock, for igneous rocks that form on the surface.
  • Dike rock, for igneous rocks that form near the surface.
  • Deep seated rock, for igneous rocks deep within the earth. By WT Huang (1962), this type of rock is called plutonic, while effusive rocks are called volcanic rocks.

2. Classification Based on SiO2 Content

According to CL Hughes (1962), namely:

  • Acidic igneous rock, if the SiO2 content is more than 66%. Examples are rhyolite, granite and dacite.
  • Intermediate igneous rock, if the SiO2 content is between 52%–66%. Examples are andesite and diorite.
  • Alkaline igneous rock, if the SiO2 content is between 45%–52%. Examples are basalt and gabbro.
  • Ultra alkaline igneous rock, when the SiO2 content is less than 45%. Examples are peridotite, dunit, and komatite.

3. Classification Based on Color Index

According to SJ Shand (1943), namely:

  • Leukocratic rock, if it contains less than 30% mafic minerals.
  • Mesocratic rock, if it contains 30%–60% mafic minerals.
  • Melanocratic rocks, if they contain more than 60% mafic minerals.

Meanwhile, according to SJ Ellis (1948) also divides igneous rocks based on their color index as follows:

  • Holofelsik, for igneous rocks with a color index of less than 10%.
  • Felsic, for igneous rocks with a color index of 10% to 40%.
  • Mafelsik, for igneous rocks with a color index of 40% to 70%.
  • Mafic, for igneous rocks with a color index of more than 70%.