The igneous rocks are those that are formed by the cooling of magma inside the Earth or by emerging as volcanic lava. They are rocks with a variable proportion of crystallized matter and vitrified matter (non-crystalline amorphous solids), pH from acidic to basic and colors from light to very dark tones.
Igneous rocks form at points on Earth where the earth's crust is destroyed or new crust emerges. This is in the subduction zones (where the old ocean floor sinks under the continents), or in the mid-ocean ridges..
These areas under the earth reach temperatures above 1,000 ºC that melt the rocks and minerals, becoming part of the magma. As it rises to the surface, the magma cools and igneous or magmatic rocks are formed.
Igneous rocks are composed of 59% feldspars, 17% amphiboles and pyroxenes, 12% quartz, 4% micas and 8% other minerals. There are some richer in silica and with little iron and magnesium (siliceous), and others with more iron and magnesium than silica (ferromagnesian).
Its texture is variable, being defined by the proportion between glass and glass, the size and shape of its particles and their arrangement with each other. These rocks can be intrusive if they form when magma cools below the surface and extrusive if they originate from lava..
Igneous rocks make up about 95% of the rocks in the earth's crust, but they are less visible than sedimentary rocks. Among them are basalt, granite, obsidian and pumice, in addition to about 700 more described types.
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The general characteristics of igneous rocks are given by their origin, being the product of the solidification of magma. These are the only rocks that come from a solidified liquid material.
The type of igneous rock is defined by the composition of the magma, as well as how and where it solidifies, with more than 700 different types being known. When iron and magnesium predominate in the composition of magma, mafic rocks are produced and if silica oxide does, felsic rocks are obtained..
Similarly, the proportion of silica oxide determines the pH of the igneous rock and if it is higher than 65% the rock will be acidic. While if it is between 45% and 65%, neutral rocks are obtained and below 45% they are basic..
In addition, the magma cooling process affects the resulting rock, because under the crust the cooling is slower, generating greater crystallization. If magma is exposed to air and water as it rises as lava, it cools faster, vitrification occurs and vitreous rocks (volcanic glass) can form.
Igneous rocks are formed from magma, which is a liquid made up of molten rock, suspended crystals, and gases. This magma is found in the earth's mantle and is recycled in the process of renewal of the earth's crust in continental drift..
Magma rises from the deeper layers of the crust and solidifies, crystallizes, forming igneous rocks under the crust. These undergo a slow cooling process that determines the type of crystallization called fractional.
Therefore, in each cooling stage (depending on the temperature) some minerals crystallize and then others. Thus, igneous rocks with large crystals and with a lower proportion of glass originate..
Magma can sometimes rise violently to the surface through volcanic eruptions in the form of lava, cooling more rapidly. For example, igneous rocks called Pele's hair are formed when the wind carries fragments of molten lava in suspension..
The sudden cooling of basalt magma drops or lava flows that flow into the sea can also occur. These igneous rocks have smaller crystals and a higher proportion of glass.
The Earth has a solid iron core surrounded by a molten phase and over this a mantle that has a first layer that goes from liquid to semi-solid and a solid upper layer (the crust). This crust fractures into plates, which are displaced by the movement generated by thermal convection below it..
Magma rises and outcrops in the mid-ocean ridges, which are volcanic ridges on the seafloor. There the crust is thinner and magma emerges forming new ocean floor, which pushes the old one and when it collides with the continental plates it sinks, melting again.
In this process, the rocks and minerals melt, forming part of the magma, which will reappear in the continental ridges and volcanic areas. It is at these points where igneous rocks are formed as the magma cools.
The magma that gives rise to igneous rocks includes a liquid phase formed by molten silicates, a solid of crystals of these silicates in suspension and a third gas phase. The latter includes water vapor (HtwoO), carbon dioxide (COtwo) and sulfur dioxide (SOtwo).
The main chemical elements present are silicon dioxide (SiOtwo), aluminum oxide (AltwoOR3) and ferric oxide (FetwoOR3). Like ferrous oxide (FeO), magnesium oxide (MgO), calcium oxide (CaO), sodium oxide (NatwoO) and potassium oxide (KtwoOR).
In general, the resulting rocks show a composition of 59% feldspars, 17% amphiboles and pyroxenes, 12% quartz, 4% micas and 8% other minerals. Feldspars include calcium (such as anorthite), sodium (such as albite), olivines, clinopyroxenes, orthopyroxenes, hoblende and biotite.
Also, on its way up to the surface, the magma drags and includes fragments of the rocks through which it passes. These inclusions can be very varied and are called xenoliths..
The texture or structure of an igneous rock refers to the way in which the crystals and amorphous materials that make up the rock are arranged. This includes the ratio of glass to crystal present (crystallinity), as well as the size and shape of the crystals..
Another aspect is the structural relationship between said crystals and other materials, that is, how they are arranged with respect to each other..
In igneous rocks the crystallinity ranges from 100% crystallized (crystal domain) to 100% vitreous (glass domain). For example, the holocrystalline granite from Ross of Mull in Scotland (Great Britain), composed of 100% crystals.
In contrast, the Dacite rock from Chemnitz (Germany) is called hypocrystalline, that is, it is mostly glass with crystal inclusions. While the so-called Pele hairs of the Erta Alé volcano (Ethiopia) are strands of basalt glass.
At this point it corresponds to the description regarding the size of the particles that make up the rock, its shape and colors. For this, thin sections of the igneous rock are made that are seen with polarized light in a stereomicroscope..
In these studies, various phenomena can be found that alter the microscopic appearance of the rock, such as when two liquids that are unable to mix with each other combine in its formation. This creates small glass globules within larger glass fragments..
To define size there are both qualitative and quantitative criteria. With the qualitative method, we speak of phanerocrystalline igneous rocks when all their crystals are visible to the naked eye..
While the aphanitic are those rocks where almost all of their crystals can not be seen with the naked eye. These rocks differ in microlithic (the crystals can be seen with a microscope) and cryptocrystalline where the crystals are not appreciated even under a microscope..
For more accurate descriptions, use is made of quantitative methods, in which the crystals are measured. According to this characteristic, they are separated into thick (greater than 5 mm), medium (between 1 and 5 mm) and fine (less than 1 mm).
One of the characteristics used is the shape of the glass faces and others to its three-dimensional shape. Among the former, there is talk of ideomorphic or automorphic crystals when they present defined faces..
While allomorphs or xenomorphs do not have straight faces anywhere, and subidiomorphs are intermediate (some straight faces). On the other hand, the three-dimensional shape is also described, finding polyhedral, spherical, laminar, prismatic or acicular crystals (like needles).
To define this relationship, the aforementioned characteristics are integrated with the description of the arrangement of crystals, glasses and other particles present. Thus, the intrusive igneous rocks have granitic, porphyroid, apolytic and pegmatitic structures and the extrusive ones are microcrystalline and porphyry..
The granitic rocks have more or less uniform crystals of intermediate size (less than 2 cm) and the porphyroids are similar, but with crystal inclusions greater than 2 cm. The apolytics show veins of microcrystals and the pegmatites are formed by crystals larger than 2 cm.
In the case of extrusive igneous rocks, some are made up of microscopic (microcrystalline) crystals. While others consist of a matrix of microscopic crystals with some larger crystals (porphyritic).
Igneous rocks can be classified by their origin or by their composition, in the first case we speak of intrusive and extrusive igneous rocks. While the composition classifies them as siliceous if they have little iron and magnesium, being rich in silica.
Ferromagnesians have a high content of iron and magnesium compared to silica. In addition, they are differentiated by their color, where the siliceas are light and the ferromagnesian ones are dark..
These igneous rocks originate from magma found in the Earth's mantle by undergoing a slow cooling. This enables the formation of large crystals, which is why they show a phanerocrystalline texture, that is, it is perceived with the naked eye..
Extrusive igneous rocks originate from lava expelled by volcanic eruptions. In this case, aphanitic textures predominate, with smaller crystals, due to low crystallization due to rapid cooling..
In turn, this type of igneous rock is subdivided into lava or effusive and pyroclastic or explosive. In the first case, the lava goes from the liquid phase to the solid phase, while in the pyroclastic ones there is a gaseous phase involved..
If the cooling of the lava is too fast, for example when it comes into contact with water, the rock becomes vitrified. Examples of this are obsidian and pumice..
It is an intrusive or plutonic igneous rock, with a high percentage of crystallization and a proportion of quartz of 20 to 60% and more than 50% of alkaline feldspars. These rocks come from the solidification of saturated magma, that is, with a high content of silica, solidified at great depths.
Granite represents the most abundant type of igneous rock on the continental surface and is characterized by light tones, with colors such as gray, bluish-black, pinkish-violet, greenish or yellow.
It is a rock of great resistance and hardness, susceptible to polishing and has been used for various purposes such as construction of monuments, buildings, reservoirs and in kitchen furniture..
It is a barely crystallized, translucent, dark and acidic extrusive or volcanic igneous rock, considered a volcanic glass. This rock is formed from silicon dioxide-rich lava that cools very quickly at the edges of the volcanic flow and is black or dark brown in color..
Since prehistoric times, humans have used obsidian to make objects such as plates, knives, mirrors, and arrowheads. Today it is used as a precious stone, in the manufacture of knives, scalpel blades and ornaments.
It is another extrusive or volcanic igneous rock, in this case formed from lava projected into the air in the eruption. This causes a violent loss of gases that gives it a porous structure, resulting in a low-density rock..
This rock is white to gray without crystallization (it is mainly a type of glass) composed mainly of potassium feldspar, quartz and minerals of the plagioclase type. It has a density that allows it to float in water and is used as an abrasive, such as to remove hardness on the soles of the feet..
Among the samples of lunar rocks brought by the Apollo 17 mission, number 74220 from the Taurus Littrow valley, corresponds to a basaltic igneous rock. Under the microscope of polarized light, spherical fragments of orange-brown glass and other partially crystallized and almost black particles are observed..
Basalt is a dark-colored extrusive igneous rock, which originates from the rapid cooling of lava rich in iron and magnesium (mafic lava). This type of very fine-grained rock makes up about 90% of the entire volcanic rock mass on Earth and the Moon..
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