Volcanic rock

Source: Wikipedia, the free encyclopedia.
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.[1]

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

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

phenocrysts embedded in a very fine grained matrix.[4]

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.[citation needed]

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.[citation needed]

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.

The chemical classification of igneous rocks 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.[5][6][7][8]

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 as aegirine or sodic amphibole rather than feldspar.[9][8]

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 (

Mineralogy

Most volcanic rocks share a number of common

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.[citation needed
]

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.[citation needed
]

Naming

An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope
phenocrysts are olivine
).
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

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.[citation needed
]

volcanic ejecta. Examples of pyroclastic rocks are tuff and ignimbrite.[citation needed
]

Shallow

]

The terms lava stone and lava rock are more used by marketers than geologists, who would likely say "volcanic rock" (because

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.[10]

Composition of volcanic rocks

ʻAʻā next to pāhoehoe lava at the Craters of the Moon National Monument and Preserve, Idaho, United States.
A German example of latite, a type of volcanic rock

The sub-family of rocks that form from volcanic lava are 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.[citation needed]

A sample of rhyolite
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).[11][12][13][14]

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).[11]

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 (

tachylyte, pitchstone).[11]

A common feature of glassy rocks is the presence of rounded bodies (

lithophysae). Perlitic structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling.[11]

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.[11]

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.[11]

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.[11]

Mechanical behaviour of volcanic rocks

The mechanical behaviour of volcanic rocks is complicated by their complex microstructure.[15][16] 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[15]). 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).[15] Understanding the mechanical behaviour of volcanic rocks can help us better understand volcanic hazards, such as flank collapse.[citation needed]

See also

References

  1. .
  2. .
  3. ^ "Rock Classification Scheme - Vol 1 - Igneous". British Geological Survey: Rock Classification Scheme. 1. NERC: 1–52. 1999. Archived from the original on 24 November 2016.
  4. S2CID 225300187
    .
  5. .
  6. ^ "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
  7. ^ "Classification of igneous rocks". Archived from the original on 30 September 2011.
  8. ^ .
  9. ^ .
  10. ^ a b "What is Lava Rock". reddome.com. Red Dome Lava Rock. Archived from the original on 10 September 2017. Retrieved 9 Sep 2017.
  11. ^ 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. Vol. 21 (11th ed.). Cambridge University Press. p. 327.
  12. .
  13. ^ a b "Der online Shop für Lavasteine". lavasteine24.de (in German). Archived from the original on 27 October 2016. Retrieved 27 Oct 2016.
  14. .
  15. ^ .
  16. .