Volcanic rock

Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Volcanic rocks are among the most common rock types on Earth's surface, particularly in the oceans. On land, they are very common at plate boundaries and in flood basalt provinces. It has been estimated that volcanic rocks cover about 8% of the Earth's current land surface.[1]

Ignimbrite
Ignimbrite is a deposit of a pyroclastic flow.

Characteristics

Setting and size

Classification of Volcaniclastic rocks and sediments[2][3]
Pyroclastic deposit
Clast size in mm Pyroclast Primarily unconsolidated: tephra Primarily consolidated: pyroclastic rock
> 64 mm Bomb, block Agglomerate, bed of blocks or bomb, block tephra Agglomerate, pyroclastic breccia
64 to 2 mm Lapillus Layer, bed of lapilli or lapilli tephra Lapilli tuff
2 to 1/16 mm Coarse ash grain Coarse ash Coarse (ash tuff)
< 1/16 mm Fine ash grain (dust grain) Fine ash (dust) Fine (ash) tuff (dust tuff)

Texture

LvMS-Lvm
Photomicrograph of a volcanic lithic fragment (sand grain); upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.

Volcanic rocks are usually fine-grained or aphanitic to glass in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix.

Volcanic rocks often have a vesicular texture caused by voids left by volatiles trapped in the molten lava. Pumice is a highly vesicular rock produced in explosive volcanic eruptions.

Chemistry

Most modern petrologists classify igneous rocks, including volcanic rocks, by their chemistry when dealing with their origin. The fact that different mineralogies and textures may be developed from the same initial magmas has led petrologists to rely heavily on chemistry to look at a volcanic rock's origin.

The chemistry of volcanic rocks is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most volcanic rocks tends to increase the silica (SiO2) content, mainly by crystal fractionation.

The initial composition of most volcanic rocks is basaltic, albeit small differences in initial compositions may result in multiple differentiation series. The most common of these series are tholeiitic, calc-alkaline, and alkaline.

Mineralogy

Most volcanic rocks share a number of common minerals. Differentiation of volcanic rocks tends to increase the silica (SiO2) content mainly by fractional crystallization. Thus, more evolved volcanic rocks tend to be richer in minerals with a higher amount of silica such as phyllo and tectosilicates including the feldspars, quartz polymorphs and muscovite. While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as olivine and the pyroxenes. Bowen's reaction series correctly predicts the order of formation of the most common minerals in volcanic rocks.

Occasionally, a magma may pick up crystals that crystallized from another magma; these crystals are called xenocrysts. Diamonds found in kimberlites are rare but well-known xenocrysts; the kimberlites do not create the diamonds, but pick them up and transport them to the surface of the Earth.

Naming

Classification extrusive rocks EN
IUGS classification of aphanitic volcanic rocks according to their relative alkali (Na2O + K2O) and silica (SiO2) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot. Original source: *Le Maitre, R.W. (ed.); 1989: A classification of igneous rocks and glossary of terms, Blackwell Science, Oxford.
LvMS-Lvv
An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope
Olivine basalt2
Vesicular olivine basalt from La Palma (green phenocrysts are olivine).
Pumice on 20 dollars
A 15-centimeter (5.9 in) piece of pumice supported by a rolled U.S. $20 bill demonstrates its very low density.

Volcanic rocks are named according to both 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 silica content similar to that of granite while basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, trachyte, and latite.

Pyroclastic rocks are the product of explosive volcanism. They are often felsic (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as ash, bombs and tephra, and other volcanic ejecta. Examples of pyroclastic rocks are tuff and ignimbrite.

Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks, are also considered to be volcanic, shading into subvolcanic.

The terms lava stone and lava rock are more used by marketers than geologists, who would likely say "volcanic rock" (because lava is a molten liquid and rock is solid). "Lava stone" may describe anything from a friable silicic pumice to solid mafic flow basalt, and is sometimes used to describe rocks that were never lava, but look as if they were (such as sedimentary limestone with dissolution pitting). To convey anything about the physical or chemical properties of the rock, a more specific term should be used; a good supplier will know what sort of volcanic rock they are selling.[4]

Composition of volcanic rocks

Latit - Boxberg, Hocheifel
A German example of latite, a type of volcanic rock

The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks that form from magma below the surface, called igneous plutonic rocks).

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. When filled with bubbles of gas, the same lava may form the spongy appearing pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite.

Rhyolite 2013
A sample of rhyolite
Scoria AmsterdamIsland 2 edit
Basaltic scoria from Amsterdam Island in the Indian Ocean

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 that 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).[5][6][7][8]

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 that follow the direction of movement (fluxion or fluidal structure)—and larger early minerals that previously crystallized may show the same arrangement. Most lavas fall considerably below their original temperatures before 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).[5]

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. 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).[5]

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 that are allowed to cool slowly. Rarely these spherulites are hollow or consist of concentric shells with spaces between (lithophysae). Perlitic structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling.[5]

Volcanic rock
Volcanic rocks, Porto Moniz, Madeira

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 pleochroism (aegirine) at the periphery.[5]

In the feldspars the center is usually richer in calcium than the surrounding layers, and successive zones may often be noted, each less calcic than those 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.[5]

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 partially or completely substitute for the original crystal but still retains its characteristic outlines.[5]

See also

References

  1. ^ Wilkinson, Bruce H.; McElroy, Brandon J.; Kesler, Stephen E.; Peters, Shanan E.; Rothman, Edward D. (2008). "Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies". Geological Society of America Bulletin. 121 (5–6): 760–779. Bibcode:2009GSAB..121..760W. doi:10.1130/B26457.1. Archived from the original on 2016-05-31.
  2. ^ LE BAS, M. J.; STRECKEISEN, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. doi:10.1144/gsjgs.148.5.0825.
  3. ^ "Rock Classification Scheme - Vol 1 - Igneous". British Geological Survey: Rock Classification Scheme. NERC. 1: 1–52. 1999. Archived from the original on 2016-11-24.
  4. ^ "Archived copy". Archived from the original on 2017-08-16. Retrieved 2017-09-09.CS1 maint: archived copy as title (link)
  5. ^ a b c d e f g  One or more of the preceding sentences incorporates text from a publication now in the public domainFlett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 21 (11th ed.). Cambridge University Press. p. 327.
  6. ^ Pinkerton, H.; Bagdassarov, N. (2004). "Transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials". Journal of Volcanology and Geothermal Research. 132 (2–3): 115–136. Bibcode:2004JVGR..132..115B. doi:10.1016/s0377-0273(03)00341-x.
  7. ^ "Lava Stones". Archived from the original on October 27, 2016. Retrieved October 27, 2016.
  8. ^ "Rheological properties of basaltic lavas at sub-liquidus temperatures: laboratory and field measurements on lavas from Mount Etna". cat.inist.fr. Archived from the original on November 2, 2015. Retrieved October 27, 2016.
Andesite

For the extinct cephalopod genus, see Andesites.

Andesite ( or ) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts they are a major component of the Martian crust. The name andesite is derived from the Andes mountain range.

Basaltic andesite

Basaltic andesite is a volcanic rock containing about 55% silica. It is distinct from basalt and andesite in having a different percentage of silica content. Minerals in basaltic andesite include olivine, augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America and the Andes of South America.

Benmoreite

Benmoreite is a silica-undersaturated volcanic rock of intermediate composition. It is a sodium-rich variety of trachyandesite and belongs to the alkalic suite of igneous rocks.

Dacite

Dacite ( ) is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains (now modern Romania and Moldova) where the rock was first described.

Extrusive rock

Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. This is as opposed to intrusive rock formation, in which magma does not reach the surface.The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals. Sometimes, a residual portion of the matrix fails to crystallize at all, instead becoming a natural glass or obsidian.

If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt. Examples of extrusive rocks include basalt, rhyolite, andesite, obsidian and pumice, scoria, and feldspar.

Felsite

Felsite is a very fine-grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light-colored rock that typically requires petrographic examination or chemical analysis for more precise definition. Color is generally white through light gray, or red to tan and may include any color except dark gray, green or black (the colors of trap rock). The mass of the rock consists of a fine-grained matrix of felsic materials, particularly quartz, sodium and potassium feldspar, and may be termed a quartz felsite or quartz porphyry if the quartz phenocrysts are present. This rock is typically of extrusive origin, formed by compaction of fine volcanic ash, and may be found in association with obsidian and rhyolite. In some cases, it is sufficiently fine-grained for use in making stone tools. Its fine texture and felsic components allow for good knapped pieces, much like working chert, producing conchoidal fracture.

Dendritic manganese oxides such as pyrolusite and/or iron oxides such as limonite may precipitate along rock crevices, giving some rock chunk surfaces multicolored or arborescent patterned textures.

Lava, California

Lava is a former settlement in El Dorado County, California. It was located near Volcanoville.A post office operated at Lava from 1880 to 1881. The town was named for volcanic rock deposits in the nearby valley.

Lava cave

A lava cave is any cave formed in volcanic rock, though it typically means caves formed by volcanic processes, which are more properly termed volcanic caves. Sea caves, and other sorts of erosional and crevice caves, may be formed in volcanic rocks, but through non-volcanic processes and usually long after the volcanic rock was emplaced.

List of rock types

The following is a list of rock types recognized by geologists. There is no agreed number of specific types of rocks. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe a rock type. Additionally, different classification systems exist for each major type of rock. There are three major types of rock: igneous rock, metamorphic rock, and sedimentary rock.

Pillow lava

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They form the upper part of Layer 2 of normal oceanic crust.

Powder Mountain (British Columbia)

This article is for the mountain in British Columbia, Canada. For the ski resort in Utah see Powder Mountain.Powder Mountain, 2,347 m (7,700 ft), is a volcanic summit in the Powder Mountain Icefield in the Pacific Ranges of the Coast Mountains in southwestern British Columbia, Canada.

Much of the volcanic history of Powder Mountain remains hidden apart from a few scraps of volcanic rock extending from under its current ice cap.

Pumice

Pumice ( ), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough textured volcanic glass, which may or may not contain crystals. It is typically light colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles, thicker vesicle walls and being dark colored and denser.Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases (including water and CO2) that are dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling and depressurization freezes the bubbles in a matrix. Eruptions under water are rapidly cooled and the large volume of pumice created can be a shipping hazard for cargo ships.

Pyroclastic rock

Pyroclastic rocks or pyroclastics (derived from the Greek: πῦρ, meaning fire; and κλαστός, meaning broken) are sedimentary clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.

Pyroclastic rocks may be a range of clast sizes, from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli, and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas-and-ash mix of a pyroclastic flow event.

Rhyodacite

Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium-rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate-colored matrix.

Rhyodacite is a high silica rock containing 20% to 60% quartz with the remaining constituents being mostly feldspar. The feldspar is a mix of alkaline feldspar and plagioclase, with plagioclase forming 35% to 65% of the mix.

Rhyodacite often exists as explosive pyroclastic volcanic deposits.

Rhyodacite lava flows occur, for example, in northwestern Ferry County (Washington), and at An Sgùrr on the island of Eigg in Scotland.

Rhyolite

Rhyolite ( RY-ə-lyte, RY-oh-) is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2 – see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 – see the QAPF diagram). Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.

Scoria

Scoria is a highly vesicular, dark colored volcanic rock that may or may not contain crystals (phenocrysts). It is typically dark in color (generally dark brown, black or purplish red), and basaltic or andesitic in composition. Scoria is relatively low in density as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity greater than 1, and sinks in water. The holes or vesicles form when gases that were dissolved in the magma come out of solution as it erupts, creating bubbles in the molten rock, some of which are frozen in place as the rock cools and solidifies. Scoria may form as part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks and bombs), for instance in Strombolian eruptions that form steep-sided scoria cones. Chemical analysis of scoria found in Yemen showed that it was mainly composed of volcanic glass with a few zeolites (e.g. clinoptilolite). Most scoria is composed of glassy fragments, and may contain phenocrysts. The word scoria comes from the Greek σκωρία, skōria, rust. A colloquial term for scoria is cinder.

Trachybasalt

Trachybasalt is a volcanic rock with a composition between trachyte and basalt. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and likely very small amounts of leucite or analcime. Trachybasalt is a basalt with high alkali content (5 to 7% Na2O + K2O, see TAS diagram).

Vesicular texture

Vesicular texture is a volcanic rock texture characterized by a rock being pitted with many cavities (known as vesicles) at its surface and inside.

This texture is common in aphanitic, or glassy, igneous rocks that have come to the surface of the earth, a process known as extrusion. As magma rises to the surface the pressure on it decreases. When this happens gasses dissolved in the magma are able to come out of solution, forming gas bubbles (the cavities) inside it. When the magma finally reaches the surface as lava and cools, the rock solidifies around the gas bubbles and traps them inside, preserving them as holes filled with gas called vesicles.

A related texture is amygdaloidal in which the volcanic rock, usually basalt or andesite, has cavities, or vesicles, that are filled with secondary minerals, such as zeolites, calcite, quartz, or chalcedony. Individual cavity fillings are termed amygdules (American usage) or amygdales (British usage). Sometimes these can be sources of semi-precious stones such as diamonds.

Rock types that display a vesicular texture include pumice and scoria.

Volcanic glass

Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Volcanic glass can refer to the interstitial, or matrix, material in an aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to obsidian, a rhyolitic glass with high silica (SiO2) content.

Other types of volcanic glass include:

Pumice, which is considered a glass because it has no crystal structure.

Apache tears, a kind of nodular obsidian.

Tachylite (also spelled tachylyte), a basaltic glass with relatively low silica content.

Sideromelane, a less common form tachylyte.

Palagonite, a basaltic glass with relatively low silica content.

Hyaloclastite, a hydrated tuff-like breccia of sideromelane and palagonite.

Pele's hair, threads or fibers of volcanic glass, usually basaltic.

Pele's tears, tear-like drops of volcanic glass, usually basaltic.

Limu o Pele (Pele's seaweed), thin sheets and flakes of brownish-green to near-clear volcanic glass, usually basaltic.

Common igneous rocks classified by silicon dioxide content
Types
Volcanic rocks
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