Igneous Rocks


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Igneous Rocks

 

(or magmatic rocks), rocks formed from molten magma as it solidifies and crystallizes. Based on solidification conditions, two principal types of igneous rocks are distinguished: effusive (volcanic, extrusive), which have solidified on the surface of the earth after magma has poured forth in the form of lava during volcanic eruptions, and intrusive (abyssal), which have solidified in a stratum of the earth’s crust between other rocks. Owing to their rapid solidification, effusive rocks are usually fine-grained and consist partially (sometimes completely) of glass. Larger crystals called phenocrysts are often found in them, giving rise to what is known as “porphyritic texture.” Intrusive rocks, which solidify slowly deep within the crust, have a holo-crystalline and more coarse-grained structure.

Igneous rocks are normally composed of silicates. Silica (SiO2) is their chief component and depending on silica content igneous rocks are divided into ultrabasic (less than 40 percent SiO2), basic (40-56 percent), intermediate (56-65 percent), acidic (65-75 percent), and ultra-acidic (more than 75 percent). Igneous rocks that do not contain silicates (for example, carbonatites) are very rare. The mineral composition of the different groups of igneous rocks changes correspondingly. Ultrabasic rocks (pyroxenites, dunites, olivines) are composed entirely of olivines and pyroxenes; basic rocks (gabbro, basalts) contain calcic plagioclase in addition to olivines and pyroxenes. In acidic rocks (granites, liparites, dacites) the content of ferromagnesian and calcic silicates decreases and alkali feldspars and quartz appear. The intermediate rocks include chiefly feldspars with small amounts of ferromagnesian minerals (diorites, andesites).

Rocks of the normal series and the alkalic series (alkali granites, nepheline syenites, phonolites) are distinguished on the basis of the alkali content of each group of igneous rocks. The alkali silicates (aegirines, alkali amphiboles, and feldspathoids) are placed in the alkalic series.

Various minerals are also associated with different types of igneous rocks. For example, tin, tungsten, and gold are associated with acidic igneous rocks; titanomagnetite and copper with basic igneous rocks; chromium, platinum, nickel, and other minerals with ultrabasic igneous rocks; and titanium, phosphorus, apatites, zirconium, rare earths, and other minerals with alkali igneous rocks.

Igneous rocks can be used as building materials (Artik tuffs and labradoritites, for example), abrasive materials (pumice), and thermal insulation materials (pumice, perlite). In addition, they can serve as raw material for the extraction of valuable components (for example, aluminum from nepheline syenites) and as foundations for hydraulic and other structures.

REFERENCE

Zavaritskii, A. N. Izverzhennye gornye porody. Moscow, 1955.

V. P. PETROV and T. I. FROLOVA

References in periodicals archive ?
Igneous rocks are mainly found in the northwestern, western, and southwestern U.S.
Frost and Frost (2008) devised a means of classifying alkaline igneous rocks and comparing their geochemistry with "normal" granitic rocks based a plot of alkalinity index (AI) versus feldspathoid silica saturation index (FSSI) (refer to Fig.
The most extensive lava on the left abutment of the dam site and downstream of the dam is seen as a monolithic igneous intrusion but in other areas of the layers of limestone and radiolarian and sometimes grains of igneous rock is found in the other layers.
One of these is a disordered and poorly sorted boulder conglomerate formed by very coarse clasts (up to 30-50 cm) of reddish or pinkish acid-intermediate igneous rocks, and smaller dark-coloured clasts of basic igneous rocks (Fig.
Igneous Rocks. Density of igneous rocks depends almost exclusively on the mineralogical and chemical composition of these rocks, it increases with the increasing content of dark minerals [19].
The association head explained that oil and natural gas exploitation in Turkey has been inadequate and that if these fields are researched more there is a potential of finding additional natural gas in between igneous rocks as well as shale gas.
In three showcases, specimens of Sedimentary Rocks, Metamorphic Rocks and Igneous Rocks have been displayed which provide information about their chemistry, industrial uses with products prepared from these rocks.
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For example, "Igneous rocks form from cooled magma and are found near dormant volcanoes," s ays Casey Long, a 12-year-old rockhound from Modesto, California.