Volcanic rock
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
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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
Volcanic rocks are usually fine-grained or
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]
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
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
Occasionally, a magma may pick up crystals that crystallized from another magma; these crystals are called
Naming
Volcanic rocks are named according to both their
Shallow
The terms lava stone and lava rock are more used by marketers than geologists, who would likely say "volcanic rock" (because
Composition of volcanic rocks
The sub-family of rocks that form from volcanic lava are called
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]
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 (
A common feature of glassy rocks is the presence of rounded bodies (
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
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- ^ "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.
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- ^ "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
- ^ "Classification of igneous rocks". Archived from the original on 30 September 2011.
- ^ ISBN 9780521880060.
- ^ doi:10.1139/e71-055.
- ^ a b "What is Lava Rock". reddome.com. Red Dome Lava Rock. Archived from the original on 10 September 2017. Retrieved 9 Sep 2017.
- ^ a b c d e f g public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. p. 327. One or more of the preceding sentences incorporates text from a publication now in the
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- ^ a b "Der online Shop für Lavasteine". lavasteine24.de (in German). Archived from the original on 27 October 2016. Retrieved 27 Oct 2016.
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