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Volcanic rock (often shortened to volcanics in scientific contexts) is a
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
formed from
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
erupted from a
volcano A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet a ...

volcano
. In other words, it differs from other
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock i ...
by being of
volcanic A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...

volcanic
origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into
hypabyssalA subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at medium to shallow depths (<2 km) within the crust
and
metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism upright=1.35, Schematic representation of a metamorphic reaction. Abbreviat ...

metamorphic rock
s and constitute an important element of some
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
s and
sedimentary rock Sedimentary rocks are types of rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compoun ...

sedimentary rock
s. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologi ...

Precambrian
shield A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from melee weapon, close-ranged weaponry or projectiles such as arrows, ...
geology, the term "volcanic" is often applied to what are strictly
metavolcanic rock In geology, metavolcanic rock is a type of metamorphic rock. Such a rock was first produced by a volcano, either as lava or tephra. Then, the rock was buried underneath subsequent rock and was subjected to high pressures and temperatures, causing t ...
s. Volcanic rocks and sediment that form from
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

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.


Characteristics


Setting and size

*
Lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

Lava
*
Tephra Tephra is fragmental material produced by a volcanic eruption Several types of volcanic eruptions—during which lava Lava is magma once it has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural sa ...

Tephra
*
Volcanic bomb A volcanic bomb or lava bomb is a mass of molten rock (tephra Tephra is fragmental material produced by a volcanic eruption Several types of volcanic eruptions—during which lava Lava is magma once it has been expelled from ...
*
Lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcano, volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli ...
*
Volcanic ash Volcanic ash consists of fragments of rock, mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure ...


Texture

Volcanic rocks are usually fine-grained or
aphanitic is aphanitic. Image:LvMS-Lvv.jpg, An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope Aphanite, or aphanitic as an adjective (from the Greek language, Greek αφανης, "invisible"), is a name ...
to glass in texture. They often contain
clasts Clastic rocks are composed of fragments, or clasts, of pre-existing minerals In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with t ...
of other
rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proc ...
and
phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses * ...
s. Phenocrysts are
crystal A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformatio ...

crystal
s that are larger than the
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
and are identifiable with the unaided
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light L ...

eye
.
Rhomb porphyry porphyry from Colorado; scale bar in lower left is Porphyry ( ) is a textural term for an igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Ind ...
is an example with large
rhomb In plane Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandria ) , name = Alexandria ( or ; ar, الإسكندرية ; arz, اسكندرية ; Coptic language, Coptic: ...

rhomb
shaped
phenocrysts 300px, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
embedded in a very fine grained matrix. Volcanic rocks often have a
vesicular texture left, A volcanic sand grain with many vesicles, viewed with a petrographic microscope. Scale box in millimeters. Vesicular texture is a volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth Earth is ...
caused by voids left by
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily Volatility (chemistry), vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as Refractory (planetary scien ...
trapped in the molten
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
.
Pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a formed from erupted from a . In other words, it differs from other by being of origin. ...

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
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

magma
s has led petrologists to rely heavily on chemistry to look at a volcanic rock's origin. The chemical classification of igneous rocks is based first on the total content of
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
and alkali metals (
sodium Sodium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

sodium
and
potassium Potassium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, b ...

potassium
) expressed as weight fraction of silica and alkali oxides ( K2O plus Na2O). These place the rock in one of the fields of the TAS diagram.
Ultramafic rock Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch ...
and
carbonatite Carbonatite lava at Ol Doinyo Lengai volcano, Tanzania Carbonatite () is a type of Intrusive rock, intrusive or extrusive igneous Rock (geology), rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbon ...

carbonatite
s have their own specialized classification, but these rarely occur as volcanic rocks. Some fields of the TAS diagram are further subdivided by the ratio of potassium oxide to sodium oxide. Additional classifications may be made on the basis of other components, such as aluminum or iron content. Volcanic rocks are also broadly divided into subalkaline, alkaline, and peralkaline volcanic rocks. Subalkaline rocks are defined as rocks in which SiO2 < -3.3539 × 10−4 × A6 + 1.2030 × 10−2 × A5 - 1.5188 × 10−1 × A4 + 8.6096 × 10−1 × A3 - 2.1111 × A2 + 3.9492 × A + 39.0 where both silica and total alkali oxide content (A) are expressed as
molar fraction In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
. Because the TAS diagram uses weight fraction and the boundary between alkaline and subalkaline rock is defined in terms of molar fraction, the position of this curve on the TAS diagram is only approximate. Peralkaline volcanic rocks are defined as rocks having Na2O + K2O > Al2O3, so that some of the alkali oxides must be present as
aegirine Aegirine is a member of the clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY(Si,Al)2O6, w ...

aegirine
or sodic
amphibole Amphibole () is a group of Silicate minerals, inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedron, tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their s ...

amphibole
rather than
feldspar Feldspars are a group of rock-forming aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity ...
. The chemistry of volcanic rocks is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most magmas tends to increase the silica ( SiO2) content, mainly by crystal fractionation. The initial composition of most magmas is
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
ic, albeit small differences in initial compositions may result in multiple differentiation series. The most common of these series are the
tholeiitic The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the Calc-alkaline magma series, calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the e ...
,
calc-alkaline The calc-alkaline magma series is one of two main subdivisions of the Subalkaline volcanic rock, subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the ...
, and
alkaline In chemistry, an alkali (; from ar, القلوي ''al-qaly'' "ashes of the saltwort") is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
.


Mineralogy

Most volcanic rocks share a number of common
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s. 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 Filo or phyllo is a very thin unleavened dough Freshly mixed dough in the bowl of a stand mixer Dough is a thick, malleable, sometimes elastic paste made from grain A grain is a small, hard, dry seed, with or without an attached husk, ...
and
tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs ...
s including the feldspars,
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
polymorphs and
muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'' ...

muscovite
. While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as
olivine The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

olivine
and the
pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming Silicate minerals#Inosilicates, inosilicate minerals found in many Igneous rock, igneous and metamorphic rock, metamorphic rock (geology), rocks. Pyroxenes have t ...
s.
Bowen's reaction series Within the field of geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of whic ...

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
xenocryst Sierra_Nevada_(U.S.).html"_;"title="Gabbroic_xenolith_in_granite_in_Rock_Creek_Canyon,_eastern_Sierra_Nevada_(U.S.)">Sierra_Nevada,_California A_xenolith_("foreign_rock")_is_a_rock_(geology).html" ;"title="Sierra Nevada (U.S.)">Sierra Nevada, Cal ...
s.
Diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At Standard conditions for temperature and pressure, room temperature and pressure, another solid form of ...

Diamond
s found in
kimberlite Cross-section of kimberlite from South Africa. The kimberlite matrix is made up of clay minerals and carbonates, presented in blue, purple and buff colours. Kimberlite is an igneous rock, which sometimes contains diamonds. It is named after the ...

kimberlite
s 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

Volcanic rocks are named according to both their
chemical composition {{Unreferenced, date=December 2017 Chemical composition refers to identity and number of the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an el ...
and texture.
Basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

Basalt
is a very common volcanic rock with low
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

silica
content.
Rhyolite Rhyolite ( ) is the most silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and ...

Rhyolite
is a volcanic rock with high silica content. Rhyolite has silica content similar to that of
granite Granite () is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cool ...

granite
while basalt is compositionally equal to
gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusion, intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a crystallinity, holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coa ...

gabbro
. Intermediate volcanic rocks include
andesite Andesite ( or ) is an extrusive volcanic rock of intermediate composition. In a general sense, it is the intermediate type between basalt and rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of so ...

andesite
,
dacite Dacite () is a formed by rapid solidification of that is high in and low in s. It has a fine-grained () to texture and is intermediate in composition between and . It is composed predominantly of and . Dacite is relatively common, occurri ...
,
trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanite, aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica ...
, and
latite of thin section vein in mica Mica is a group of sheet silicate minerals, not to be confused with Micah. Mica or MICA may also refer to: Companies * Mica DIY, a UK Retailer Co-operative, Symbol group Acronyms * Mahone Islands Conservati ...
.
Pyroclastic rock Image:Pyroclastic Flow St. Helens.jpg, 300px, United States Geological Survey, USGS scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St. Helens File:Volcanic Stone 3D.ogg, 3D-Rendering of the above image stack, in par ...
s are the product of explosive volcanism. They are often
felsic In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...
(high in silica). Pyroclastic rocks are often the result of volcanic debris, such as
ash Ash or ashes are the solid remnants of fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Fire is hot because th ...
,
bomb A bomb is an explosive weapon that uses the Exothermic process, exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy. Detonations inflict damage principally through ground- and atmosphere-tr ...
s and
tephra Tephra is fragmental material produced by a volcanic eruption Several types of volcanic eruptions—during which lava Lava is magma once it has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural sa ...

tephra
, and other
volcanic A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...

volcanic
ejecta Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma ...
. Examples of pyroclastic rocks are
tuff Tuff is a type of Rock (geology), rock made of volcanic ash ejected from a Volcano, vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is consider ...

tuff
and
ignimbrite Ignimbrite is a variety of hardened tuff. Ignimbrites are igneous rocks made up of crystal and rock fragments in a glass-shard Matrix (geology), groundmass, albeit the original Texture (geology), texture of the groundmass might be obliterated du ...

ignimbrite
. Shallow s, which possess structure similar to volcanic rather than
pluton In , an igneous intrusion (or intrusive body or simply intrusion) is a body of that forms by crystallization of slowly cooling below the surface of the . Intrusions have a wide variety of forms and compositions, illustrated by examples like ...

pluton
ic 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 Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
is a molten liquid and
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
is solid). "Lava stone" may describe anything from a friable
silicicSilicic is an adjective to describe magma 300px, Lava flow on Hawaii (island), Hawaii. Lava is the extrusive equivalent of magma. Magma (from Ancient Greek μάγμα (''mágma'') meaning "thick unguent") is the molten or semi-molten natural mater ...
pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a formed from erupted from a . In other words, it differs from other by being of origin. ...

pumice
to solid
mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composit ...
flow basalt, and is sometimes used to describe rocks that were never
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
, but look as if they were (such as
sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentatio ...

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.


Composition of volcanic rocks

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 Rhyolite ( ) is the most silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and ...

rhyolite
lava-stream cools quickly, it can quickly freeze into a black glassy substance called
obsidian Obsidian (; ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Obsidian is produced from felsic lava, rich in the lighter element ...

obsidian
. When filled with bubbles of gas, the same lava may form the spongy appearing
pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a formed from erupted from a . In other words, it differs from other by being of origin. ...

pumice
. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite. 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). 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). 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 (
phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses * ...
s) are said to be
porphyritic porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada (U.S.), Sierra Nevada, Rock Creek Canyon, California. Porphyritic is an adjective used in geology, specifical ...
; 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 Tachylite (also spelled tachylyte) is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily Melting, fusible. The col ...
,
pitchstone The pitchstonen ridge An Sgùrr (Eigg), An Sgùrr in Eigg, Scotland Pitchstone is a dull black glassy volcanic rock formed when felsic lava or magma cools quickly. It is similar to obsidian but is defined by the International Union of Geological Sc ...
). A common feature of glassy rocks is the presence of rounded bodies (
spherulites , Arizona. Image:Rhyolite pmg ss 2006.jpg, Thin section, Photomicrograph of rhyolite showing spherulitic texture (brown, between grey to white crystals). In petrology, spherulites () are small, rounded bodies that commonly occur in Glass, vitreou ...
), consisting of fine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixed with quartz or
tridymite Tridymite is a high-temperature polymorph of silica Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds t ...

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). structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling. 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 Pleochroism (from Greek wikt:πλείων#Ancient Greek, πλέων, ''pléōn'', "more" and wikt:χρώμα#Ancient Greek, χρῶμα, ''khrôma'', "color") is an optical phenomenon in which a substance has different colors when observed at d ...
(aegirine) at the periphery. 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. Corroded phenocrysts of
biotite Biotite is a common group of Silicate minerals#Phyllosilicates, phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid solution, solid-solution series between the iron-endmember annite, and ...

biotite
and
hornblende Hornblende is a complex inosilicate series of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

hornblende
are very common in some lavas; they are surrounded by black rims of
magnetite Magnetite is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure for ...

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.


Mechanical behaviour of volcanic rocks

The mechanical behaviour of volcanic rocks is complicated by their complex microstructure. For example, attributes such as the partitioning of the void space (pores and microcracks), pore and crystal size and shape, and hydrothermal alteration can all vary widely in volcanic rocks and can all influence the resultant mechanical behaviour (e.g., Young’s modulus, compressive and tensile strength, and the pressure at which they transition from brittle to ductile behaviour). As for other crustal rocks, volcanic rocks are brittle and ductile at low and high effective confining pressures, respectively. Brittle behaviour is manifest as faults and fractures, and ductile behaviour can either be distributed (cataclastic pore collapse) or localised (compaction bands). Understanding the mechanical behaviour of volcanic rocks can help us better understand volcanic hazards, such as flank collapse.


See also

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Extrusive rock A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extrusive rock refers to the mode of igneous volcanic r ...
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Intrusive rock Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, dike (geology), dikes, Sill (geology), sills, laccoliths, and volcanic necks.I ...


References

{{Authority control Igneous petrology Volcanology