Volcanic rock

thumb|right|[[Ignimbrite, a volcanic rock deposited by pyroclastic flows]]

Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. 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 "pyroclastics," and these are also 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

Volcanic rocks are classified based on their formation environment and particle size. They can originate from lava flows or be ejected explosively as fragmented material known as tephra.

• Lava – When molten rock erupts and solidifies on the Earth's surface, it forms coherent volcanic rocks such as basalt, andesite, and rhyolite. The size and structure of lava formations vary, with common types including pahoehoe (smooth, ropy lava) and ʻaʻā (rough, jagged lava).
• Tephra – Fragmented volcanic material ejected during eruptions, which varies in size and composition. Tephra includes:
• Volcanic bomb – Large, semi-molten fragments ejected from a volcano that solidify before reaching the ground. They often acquire aerodynamic shapes due to their flight through the air.
• Lapilli – Rock fragments between 2 and 64 mm in diameter, formed from lava droplets or broken volcanic material. Lapilli can accumulate to form volcanic breccia or tuff.
• Volcanic ash – Fine particles (<2 mm) of pulverized rock, minerals, and glass created during explosive eruptions. Ash can travel long distances, affecting air quality and climate.

The size and setting of volcanic rock influence its distribution, physical properties, and impact on the environment.<br />

{| class="wikitable"

|+Classification of Volcaniclastic rocks and sediments

|-

|

|

! colspan="2" |Pyroclastic deposit

|-

!Clast size in mm

!Pyroclast

!Primarily unconsolidated: tephra

!Primarily consolidated: pyroclastic rock

|-

|> 64&nbsp;mm

|Bomb, block

|Agglomerate, bed of blocks or bomb, block tephra

|Agglomerate, pyroclastic breccia

|-

|64 to 2&nbsp;mm

|Lapillus

|Layer, bed of lapilli or lapilli tephra

|Lapilli tuff

|-

|2 to 1/16&nbsp;mm

|Coarse ash grain

|Coarse ash

|Coarse (ash tuff)

|-

| class="nowrap"|< 1/16 mm

|Fine ash grain (dust grain)

|Fine ash (dust)

|Fine (ash) tuff (dust tuff)

Type 3-Class Rogue

|}

Texture

thumb|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.

thumb|upright=1.3|[[IUGS 'total alkali silicates' 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:

• (ed.); 1989: A classification of igneous rocks and glossary of terms, Blackwell Science, Oxford. ]]

The chemical classification of igneous rocks using the TAS classification is based first on the total content of silicon and alkali metals (sodium and 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 and carbonatites 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. 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 in sodic pyroxenes such as aegirine or sodic amphibole in addition to in feldspar.

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). 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