Volcanic Rock


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volcanic rock

[väl′kan·ik ′räk]
(geology)
Finely crystalline or glassy igneous rock resulting from volcanic activity at or near the surface of the earth. Also known as extrusive rock.

Volcanic Rock

 

vulcanites, rock formed as a result of volcanic eruptions. Depending on the type of explosion (a lava flow or explosive eruption), two types of rock form—effusive and volcanogenic-detrital (pyroclastic). The latter type is divided into friable (volcanic ash, sand, bombs, and so on), compacted, and cemented rock (tuff, tuff breccia, and others). In addition, there are intermediate types of volcanic rocks—tuff lavas, which occur as a result of eruptions of gas-rich foaming lava flows, and ignimbrites, which occur as a result of violent eruptions when pieces of lava are carried into the air and fall on to the surface, forming masses of melted matter that occasionally occupy wide areas measuring hundreds and thousands of square kilometers.

The viscosity of the lava and the terrain of the volcanic area determine the body forms of effusive rock. Sheets and flows are formed from low-viscosity basaltic lavas. Cupolas and needles occur during eruptions of viscous lava (dacites and liparites). Dikes and necks are caused by the filling of cracks and underlying channels by lava.

Volcanic rock is distinguished by its chemical composition and structural and textural peculiarities and by the degree of disintegration of the rock substance. According to chemical composition, effusive volcanic rock is divided into alkaline-earth and alkaline rock and, in addition, into basic (not saturated with silicic acid), medium (saturated with silicic acid), and acidic (supersaturated with silicic acid) rock. The degree of crystallization of the lava, as well as its structures and textures, depends on its viscosity. The internal parts of effusive bodies are usually crystallized, whereas the external parts are slaglike, porous, and glassy. Porphyritic, microlitic, semi vitreous, and vitreous structures and fluidal, striped, massive, and porous textures are characteristic of effusive rock.

Profoundly changed, usually older effusive rock is called a paleotype, and unchanged rock is called a cenotype. The most widely distributed cenotypic rocks are basalts, andesites, trachytes, and liparites; their paleotypic analogues are called diabases, basaltic and andesitic porphyrites, and trachytic and liparitic porphyry.

Volcanogenic sedimentary rock with pyroclastic rocks (tuff and breccia), also belongs to the group of detrital volcanic rocks. Many volcanic rocks (for example, basalt, tuff, and pumice) are widely used in industry.

REFERENCES

Zavaritskii, A. N. Izverzhennye gornye porody. Moscow, 1955.
Maleev, E. F. Vulkanoklasticheskie gornye porody. Moscow, 1963.
Koptev-Dvornikov, V. S., E. B. Iakovleva, and M. A. Petrova. Vulkanogennye porody i melody ikh izucheniia. Moscow, 1967.

V. I. VLODAVETS

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