Volcanic rocks (often abbreviated to volcanoes in a scientific context) are rocks formed from magmas that erupt from volcanoes. In other words, it is different from other igneous rocks by being a volcanic origin. Like all types of rocks, the concept of volcanic rock is artificial, and in nature the class of volcanic rock becomes hypabyssal and metamorphic rock and is an important element of some sediments and sedimentary rocks. For this reason, in geology, volcanoes and shallow hypabisic rocks are not always treated as distinct. In the geological context of the Precambrian shield, the term "volcano" is often applied to highly metavolcanic rocks.
Volcanic rock is one of the most common rock types on the surface of the earth, especially in the oceans. On land, they are very common on plate boundaries and in the flood basalt province. It is estimated that volcanic rocks cover about 8% of Earth's current surface area.
Video Volcanic rock
Characteristics
Settings and size
- Lava
- Tephra
- Volcanic Bomb
- Lapilli
- Volcanic ash
Texture
Volcanic rocks are usually fine grained or aphanitic to glass texture. They often contain clusters of rocks and other phenocrysts. Phenocrysts are crystals that are larger than the matrix and can be identified with the naked eye. Porphyry porphyry is an example with large diamond-shaped phenocrysts embedded in a very fine grained matrix.
Volcanic rocks often have vesicular texture caused by voids left behind by volatiles trapped in liquid lava. Pumice stone is a very vesicular stone that is produced in an explosive volcano eruption.
Chemistry
Most modern petrologists classify igneous rocks, including volcanic rocks, with their chemistry when dealing with their origin. The fact that different mineralogies and textures can be developed from the same early magma has caused petrologists to rely heavily on chemistry to see the origin of volcanic rocks.
Volcanic rock chemistry depends on two things: the primary composition of the primary magma and the subsequent differentiation. Differentiation of most volcanic rocks tends to increase the content of silica (SiO 2 ), especially by fractionation of crystals.
The earliest compositions of most volcanic rocks are basaltic, although small differences in the initial compositions can yield multiple series of differentiation. The most common of these series are tholeiitic, calc-alkaline, and alkaline.
Mineralogy
Most volcanic rocks have a number of common minerals. Differentiation of volcanic rocks tends to increase the content of silica (SiO 2 ) mainly by fractionation of crystallization. Thus, more evolved volcanic rocks tend to be richer in minerals with higher amounts of silica such as phyllo and tektosilicates including feldspar, quartz polymorph and muscovite. Although still dominated by silicates, more primitive volcanic rocks have a collection of minerals with little silica, such as olivine and piroxene. Bowen's reaction series correctly predicts the order of the most common mineral formation in volcanic rock.
Sometimes, magma can take crystals that crystallize from other magma; these crystals are called xenocrysts. The diamonds found in the kimberlite are rare but are the famous xenocrysts; kimberlite does not create diamonds, but picks them up and moves them to the surface of the Earth.
Maps Volcanic rock
Naming
Volcanic rocks are named according to their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has a silica content similar to granite, while basalt is the same composer as gabbro. Medium volcanic rocks include andesite, dacite, trachyte, and latit.
Pyroclastic rocks are a product of explosive volcanism. They are often felsic (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as ash, bomb and tephra, and other volcanic ejecta. Examples of pyroclastic rocks are tuff and ignimbrite.
Shallow intrusion, which has a structure similar to volcanic rather than plutonic stone, is also considered volcanic, shading becomes subvolcanic.
The terms lava rock and lava rock are mostly used by marketers rather than geologists, who are likely to say "volcanic rock" (because lava is liquid liquid and rock solid). "Lava stones" can describe anything from pumice silicic to basalt mafik streams dense, and sometimes used to describe rocks that never lava, but look as if they are (like limestone sediment with hole dissolution). To convey anything about the physical or chemical properties of the rock, more specific terms should be used; Good suppliers will know what kind of volcanic rock they are selling.
Volcanic rock composition
Sub-families of rocks formed from volcanic lava are called frozen volcanic rocks (to distinguish them from igneous rocks formed from the magma below the surface, called frozen plutonic rocks).
The different volcanic lava, when cooled and hardened, are very different in their appearance and composition. If the rhyolite lava flow cools rapidly, it can quickly freeze into a black glass substance called obsidian. When filled with gas bubbles, the same lava can form a pumice stone that appears. Allowed to cool slowly, it forms a uniformly colored solid rock called rhyolite.
The lava, having cooled rapidly in contact with air or water, is mostly fine crystalline or has at least a fine grained ground that represents a portion of the lumpy semi-crystalline lava flow which is still liquid at the time of the eruption. At this moment they are exposed only to atmospheric pressure, and other vapors and other gases, contained in large quantities, are free to escape; Many important modifications arise from this, most striking is the frequent presence of many vapor cavities (vesicular structures) often drawn into elongated forms which are then filled with minerals by infiltration (amygdaloidal structures).
When crystallization takes place while the mass still creeps forward beneath the Earth's surface, the last formed minerals (in the mass-soil) are usually arranged in the winding lines below the line following the direction of motion (fluxion or fluid structure) - and the initial mineral the larger one that was previously crystallized might show the same settings. Most lava falls well below its original temperature before being emitted. In their behavior, they present an analogy close to the hot solution of the salt in water, which, when approaching the saturated temperature, first stores large pieces of large, well-formed crystals (labile stages) and then deposits a cloud smaller than a less than perfect crystal. particles (metastable stage).
In igneous rocks, first-generation crystals generally form before lava appears to the surface, that is, during the climb from the depths down to the volcanic crater. It has often been verified by the observation that the newly emitted lavas contain large crystals that are shared in the liquid, liquid mass. Large, well-formed crystals (phenocrysts) are said to be porphyritic; crystals smaller than the matrix or surrounding soil mass belonging to the post-effusion stage. More rarely lava actually blends at the time of expenditure; they can then become cold to form a non-porphyritic, smooth, or if chilly crystal stone, most of them are not crystals or glasses (vitreous stones such as obsidian, tachylyte, pitchstone).
A common feature of glass stones is the existence of spherulites, consisting of fine divergent fibers radiating from the center; they consist of feldspar crystals that are not perfect, mixed with quartz or tridymite; Similar bodies are often produced artificially in glasses that are left cold slowly. Rarely these spherulites are hollow or consist of concentric shells with intermediate space (lithophysae). The perlitic structure, also common in glass, consists of the existence of concentric spherical cracks due to contraction in cooling.
Phenocryst or porphyritic minerals are not only larger than the soil mass; because the matrix is ​​still liquid when they are formed they are free to take the form of perfect crystals, without interference by adjacent crystalline pressure. They seem to have grown rapidly, as they are often filled with crystal-like material such as glass or smooth enclosures of the soil mass. Microscopic examination of phenochrics often reveals that they have a complex history. Very often they show different layers of composition, represented by color variations or other optical properties; thus augit can be green in the center surrounded by various colors of brown; or they may be green pale green centered and dark with strong pleochroism (aegirine) on the edges.
In central feldspars are usually more rich in calcium than the surrounding layer, and successive zones can often be recorded, each less calcic than those in it. Quartz phenocrysts (and other minerals), rather than sharp, perfectly crystalline faces, may show a rusty, round surface, with dulled spots and projections such as irregular tongues of the matrix into the crystal substance. It is clear that after the crystallizing minerals are partially soluble or corroded in some periods before the matrix is ​​compacted.
Phenocrysts of biotite and hornblende are very common in some lavas; they are surrounded by black magnetite fringes mixed with pale green augits. The hornblende or biotite substance has been shown to be unstable at certain stages of consolidation, and has been replaced by the paramorph of augite and magnetite, which can partially or completely replace the original crystal but still retain its characteristics.
See also
- Intrusion
References
Source of the article : Wikipedia
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