Volcanic rock is an igneous rock of volcanic origin. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Plate tectonics and hotspots Divergent plate boundaries At the
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Texture
Volcanic rocks are usually fine-grained or aphanitic to glassy in texture. They often contain clasts of other rocks and phenocrysts. This article concerns the geologic term for other uses see Xenolith (disambiguation A xenolith ( Greek: 'foreign rock' is a rock In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere A phenocryst is a relatively large and usually conspicuous Crystal distinctly larger than the grains of the rock groundmass of a Porphyritic Igneous Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating The matrix or groundmass of rock is the fine-grained mass of material in which larger grains or Crystals are embedded Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix. Porphyry is a variety of Igneous rock consisting of large-grained Crystals such as Feldspar or Quartz, dispersed in a fine-grained In Geometry, a rhombus (from Ancient Greek ῥόμβος - rrhombos “rhombus spinning top” (plural rhombi or rhombuses A phenocryst is a relatively large and usually conspicuous Crystal distinctly larger than the grains of the rock groundmass of a Porphyritic Igneous
Volcanic rocks often have a vesicular texture, which is the result voids left by volatiles escaping from the molten lava. Vesicular texture is a Volcanic rock texture characterised by or containing many vesicles In Planetary science, volatiles, are that group of elements and compounds with low boiling points (see volatile) that are associated with a planet's or moon's Lava is molten rock expelled by a Volcano during an eruption When first expelled from a volcanic vent it is a Liquid at Temperatures Pumice is a rock, which is an example of explosive volcanic eruption. Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass Pyroclastic with very thin translucent An explosive eruption is a Volcanic term to describe a violent explosive type of Eruption. It is so vesicular that it floats in water. Water is a common Chemical substance that is essential for the survival of all known forms of Life.
Naming
Volcanic rocks are named according to both their chemical composition and texture. Use in chemistry In Chemistry, the empirical formula of a Chemical compound is a simple expression of the relative number of each type of Atom Basalt is a very common volcanic rock with low silica content. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Rhyolite is a volcanic rock with high silica content. This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Gabbro (ˈɡæbrəʊ is a dark coarse-grained intrusive Igneous rock chemically equivalent to Basalt. Intermediate volcanic rocks include andesite, dacite, trachyte and latite. For the extinct cephalopod genus see Andesites. Andesite (ˈændəsaɪt is an igneous, Volcanic rock, of intermediate Dacite ( deɪsaɪt) is an igneous, volcanic rock with a high iron content Trachyte is an Igneous, Volcanic rock with an Aphanitic to Porphyritic texture Latite is an igneous, volcanic ( extrusive) rock, with Aphanitic -aphyric to aphyric- Porphyritic texture
Pyroclastic rocks are the product of explosive volcanism. Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire and κλαστός, meaning broken are Clastic rocks They are often felsic (high in silica). Felsic is a term used in Geology to refer to Silicate minerals, Magma, and rocks which are enriched in the lighter elements such as Silicon Pyroclastic rocks are often the result of volcanic debris, such as ash, bombs and tephra, and other volcanic ejecta. Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size Plate tectonics and hotspots Divergent plate boundaries At the Examples of pyroclastic rocks are tuff and ignimbrite. Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption Ignimbrite is a Volcanic Pyroclastic rock often of Dacitic or Rhyolitic composition
Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks are also considered to be volcanic. In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth.
Composition of volcanic rocks
The sub-family of rocks which form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks which form from magma, below the surface of the earth, called igneous plutonic rocks). Volcanic rock is an Igneous rock of volcanic origin Texture Volcanic rocks are usually fine-grained or Aphanitic to glassy in In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth.
The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade. Obsidian is a naturally occurring Glass formed as an extrusive Igneous rock. When filled with bubbles of gas, the same lava may form the spongy mineral pumice. Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass Pyroclastic with very thin translucent Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite. This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade.
The lavas, having cooled rapidly in contact with the air or water, are mostly finely crystalline or have at least fine-grained ground-mass representing that part of the viscous semi-crystalline lava flow which was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the steam and other gases, which they contained in great quantity were free to escape; many important modifications arise from this, the most striking being the frequent presence of numerous steam cavities (vesicular structure) often drawn out to elongated shapes subsequently filled up with minerals by infiltration (amygdaloidal structure). Vesicular texture is a Volcanic rock texture characterised by or containing many vesicles Amygdules form when the vesicular cavities (created by expanding gas bubbles in Volcanic Lava) are filled with a secondary Mineral such as As crystallization was going on while the mass was still creeping forward under the surface of the Earth, the latest formed minerals (in the ground-mass) are commonly arranged in subparallel winding lines following the direction of movement (fluxion or fluidal structure), and the larger early minerals which had previously crystallized may show the same arrangement. The matrix or groundmass of rock is the fine-grained mass of material in which larger grains or Crystals are embedded Most lavas have fallen considerably below their original temperatures before they are emitted. In their behavior they present a close analogy to hot solutions of salts in water, which, when they approach the saturation temperature, first deposit a crop of large, well-formed crystals (labile stage) and subsequently precipitate clouds of smaller less perfect crystalline particles (metastable stage). In igneous rocks the first generation of crystals generally forms before the lava has emerged to the surface, that is to say, during the ascent from the subterranean depths to the crater of the volcano. It has frequently been verified by observation that freshly emitted lavas contain large crystals borne along in a molten, liquid mass. The large, well-formed, early crystals (phenocrysts) are said to be porphyritic; the smaller crystals of the surrounding matrix or ground-mass belong to the post-effusion stage. A phenocryst is a relatively large and usually conspicuous Crystal distinctly larger than the grains of the rock groundmass of a Porphyritic Igneous Porphyry is a variety of Igneous rock consisting of large-grained Crystals such as Feldspar or Quartz, dispersed in a fine-grained More rarely lavas are completely fused at the moment of ejection; they may then cool to form a non-porphyritic, finely crystalline rock, or if more rapidly chilled may in large part be non-crystalline or glassy (vitreous rocks such as obsidian, tachylyte, pitchstone). Obsidian is a naturally occurring Glass formed as an extrusive Igneous rock. Tachylite (also spelled tachylyte is a Vitreous form of Basaltic Volcanic glass. Pitchstone is a dull black glassy volcanic rock formed when Lava cools swiftly A common feature of glassy rocks is the presence of rounded bodies (spherulites), consisting of fine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixed with quartz or tridymite; similar bodies are often produced artificially in glasses which are allowed to cool slowly. Spherulites, in Petrology, are small rounded bodies that commonly occur in Vitreous Igneous rocks They are often visible in specimens of Obsidian Tridymite is a high-temperature polymorph of Quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal Triclinic crystals or scales Rarely these spherulites are hollow or consist of concentric shells with spaces between (lithophysae). A lithophysa (plural lithophysae) is a small cavity found in Felsic Volcanic rocks believed to be caused by expanding gases in Tuffs Perlitic structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling. Perlite is an Amorphous Volcanic Glass that has a relatively high Water content typically formed by the hydration of Obsidian
The phenocrysts or porphyritic minerals are not only larger than those of the ground-mass; as the matrix was still liquid when they formed they were free to take perfect crystalline shapes, without interference by the pressure of adjacent crystals. They seem to have grown rapidly, as they are often filled with enclosures of glassy or finely crystalline material like that of the ground-mass . Microscopic examination of the phenocrysts often reveals that they have had a complex history. Very frequently they show layers of different composition, indicated by variations in color or other optical properties; thus augite may be green in the center surrounded by various shades of brown; or they may be pale green centrally and darker green with strong pleochoism (aegirine) at the periphery. Aegirine is an inosilicate member of the clino Pyroxene group In the feldspars the center is usually richer in calcium than the surrounding layers, and successive zones may often be noted, each less calsic than those which lie within it. Phenocrysts of quartz (and of other minerals), instead of sharp, perfect crystalline faces, may show rounded corroded surfaces, with the points blunted and irregular tongue-like projections of the matrix into the substance of the crystal. It is clear that after the mineral had crystallized it was partly again dissolved or corroded at some period before the matrix solidified. Corroded phenocrysts of biotite and hornblende are very common in some lavas; they are surrounded by black rims of magnetite mixed with pale green augite. The hornblende or biotite substance has proved unstable at a certain stage of consolidation and has been replaced by a paramorph of augite and magnetite which may be partially or completely substituted for the original crystal but still retains its characteristic outlines. [1]
See also
References
- ^ This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Petrology", a publication now in the public domain. In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. The Encyclopædia Britannica Eleventh Edition (1910–1911 is a 29-volume reference work that marked the beginning of the Encyclopædia Britannica The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone
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