Igneous rock constitutes one of the three main groups in standard rock classification, along with sedimentary rock and metamorphic rock. Igneous rocks form from the cooling and crystallization of molten or partially molten material called magma.
According to some theories, the entire outer part of the Earth may have once been molten, and extensive igneous activity and volcanism followed during early stages of the planet's development . Differences in the crystallization rates and the times of formation for minerals of different compositions would have led to differentiation of rock types. Lighter elements, such as silicon, aluminum, sodium, and potassium, would have concentrated in the upper parts of the crust while heavier elements, such as iron, magnesium, and others, would have tended to sink. Such long-term igneous differentiation processes may have been one major factor contributing to the formation of the Earth's chemically and mineralogically layered structure.
The wide variety of presumably igneous rocks has long puzzled petrologists. Many workers once thought that most igneous rocks, including even granite and rhyolite, originated from chemical evolution of a single--probably basaltic--parent magma. Now the variety can apparently be better explained by the partial melting of many different source materials. Contamination of these melts by assimilation of surrounding, nonhomogeneous materials would have extended the variety. The great variety of igneous rocks may also be linked to differentiation processes occurring during sedimentary action at the Earth's surface, followed by subsequent burial and partial melting.
Key variables in subdividing igneous rocks include mode of occurrence; grain size and texture; and mineralogy and chemistry. Rocks thought to be igneous may be subdivided into volcanic, subvolcanic (hypabyssal), plutonic, and pyroclastic types. Direct evidence of igneous origin is available only for volcanic and pyroclastic rocks, which form directly from the cooling of a viscous fluid (lava) or other materials that flow or erupt from volcanic vents and fissures. Geologists have inferred an igneous origin for other old rocks that have textures, chemical compositions, and mineral components identical to those of present-day volcanic rocks.
Volcanic rocks tend to be fine-grained (less than 1 mm/0.04 in), and their matrix usually contains glass as well as tiny crystals. Wholly glassy varieties are called obsidian. In porphyritic varieties (porphyry), larger crystals are embedded in a fine-grained groundmass. This texture probably results from extrusion of lava containing crystals already grown and developed under conditions of slow cooling below the Earth's surface; subsequent rapid cooling leads to crystallization of the finer-grained matrix.
The most common volcanic rocks are basalt, andesite, and rhyolite. Basalt, thought to form during partial melting of rocks in the Earth's mantle, is the most abundant volcanic rock. Andesite may form from partial melting of crustal rocks deep in fold belts. Rhyolite, composed largely of alkali feldspar and free silica (silica minerals) such as quartz, forms lava flows and tuffs. Other, less common, low-silica volcanic rocks contain neither feldspar nor quartz, but are rich in feldspathoids such as leucite or nepheline (feldspathoid). Carbonatite, an unusual carbonate rock showing igneous characteristics, was discounted as actually being igneous until sodium carbonate lavas were observed erupting from African volcanoes.
Volcanic rocks occur in many parts of the world. A great chain of volcanoes known as the ring of fire encircles the Pacific Ocean. The Mid-Atlantic Ridge is a great suboceanic chain of volcanoes, as are the East-Pacific and Indian Ocean Ridges (mid-oceanic ridge). The east african rift system is largely filled with volcanic material, as is the extension of this trough northward through the Rhine Valley and the Oslo Fjord. Volcanoes rim the northern edge of the Mediterranean and cover central France. Great sheetlike flows of lava cover the Columbia River Plateau in the northwestern part of the United States and the deccan plateau in India. Such active volcanic regions appear mostly along the borders of continental plates (plate tectonics), with the more basaltic materials rising to the surface at divergent plate boundaries, and the more andesitic and rhyolitic materials appearing along convergent plate boundaries.
Subvolcanic rocks are fine-grained rocks occurring in dikes or sills and connected directly with volcanic rocks. The discordant character of dikes and other types of forceful intrusions supports the theory that they were formed from a fluid. Some such rocks are medium-grained (1 to 5 mm/0.04 to 0.2 in), but their patterns of grain intergrowth and their compositions strongly resemble those of surface volcanics. Many subvolcanic rocks are porphyritic.
Plutonic rocks are coarse-grained (greater than 5 mm/0.2 in) or medium-grained and commonly occur in large ovoid, circular, or tabular masses. Most common among them are rocks of the granite, granodiorite, and gabbro clans. The margins of the mineral grains in these rocks are tightly interlocking. Some plutonic rocks may be porphyritic. Most plutonic rocks, with the exception of some types thought to form by accumulation in layered complexes, have chemical and mineralogical equivalents among the volcanic and subvolcanic rocks.
Erosion has made it possible to trace direct gradations from some plutonic bodies to subvolcanic and volcanic structures; such bodies must be igneous. When intrusive plutonic rock seems to have cut across or broken through adjacent rocks, it is usually thought to be igneous. When rocks surrounding a mass of plutonic rocks show evidence of having been "baked" (contact metamorphism), they are considered to be of igneous origin. Many plutonic bodies appear to have formed deep within the Earth's crust and to have cooled and crystallized at depth, in environments near their melting point. Their surface exposure apparently results from uplift and erosion. Such bodies show ambiguous relationships to the rocks (usually metamorphic) surrounding them; they grade into them imperceptibly and seem to be interlayered with them, their edges being parallel to them, and showing no appearance of having either shoved aside or intruded into the surrounding rocks.
Some of these plutonic bodies may be igneous, having formed by partial melting of rock masses at depth, followed by recrystallization without displacement or movement. Others may have formed by metamorphism or transformation of preexisting rocks, without any melting. The deeper the locus of formation of plutonic bodies the more ambiguous is their origin. In deep zones of the Earth's crust, igneous and metamorphic processes converge. These processes may produce similar-looking rocks, indistinguishable by present criteria. The processes themselves may indeed merge to form conditions unknown at the surface. Experimental work involving high pressures and temperatures has thrown new light on these processes and on how rock materials may behave in deep zones of the Earth's crust.
Some plutonic rocks, the so-called layered igneous rocks, also show features common in sedimentary rocks. They are inferred to be igneous because of their mineral composition and texture, as seen in the intergrowth of grains. Prominent layering in these large bodies is thought to occur because of the faster settling of larger or denser, early-formed crystals and the slower settling--or even floating--of smaller or less dense crystals. The process is similar to the settling of grains of sand in water, except that it happens at temperatures exceeding 1,000 deg C and at locations deep within the Earth.
Plutonic igneous rocks may also be subdivided in terms of their relationship to structural events. Some bodies were emplaced before significant folding or mountain-building events, others during folding and mountain building. The origin of bodies that formed near or at the culmination of major structural episodes is perplexing. Plutonic rocks that were emplaced after the conclusion of structural events and clearly cutting across the main fold structures are probably igneous.
Granitic rocks, including quartz monzonite and granodiorite, are the most abundant plutonic rocks. They may form from the partial melting of deeply buried sedimentary rocks during periods of mountain building. Rocks such as syenite are rich in potassium feldspar and contain no quartz.
Plutonic rocks of probable igneous origin form the cores of many of the world's great mountain ranges: the Rockies, Appalachians, Sierra Nevada, Alps, and Himalayas. In addition, the Precambrian shields--which form the central portions of all of the continents and underlie the world's sedimentary basins--contain plutonic rocks, most of which may have been formed by the melting or partial melting of other crustal rocks, followed by subsequent slow cooling and crystallization or by more active flow, injection, and intrusion.
Igneous and sedimentary processes converge in the formation of pyroclastic rocks. Tuffs, breccias, ashflows, and other fragmental rocks have many of the same characteristics as sedimentary rocks. Ash, volcanic bombs, and other clearly igneous fragmental material are ejected from volcanoes as red-hot, molten debris. When such material falls to the ground, however, it forms deposits similar in structure to air-laid sedimentary rocks, such as wind deposits in deserts. When volcanic debris falls into water, it settles to the bottom and forms layers even more similar to sedimentary materials. Mineral composition may be the only feature by which pyroclastic rocks can be distinguished from more ordinary sedimentary rocks.
Some pyroclastic eruptions have caused great catastrophes, such as the eruptions of Vesuvius, which buried Pompeii and Herculaneum, and of Mount Pelee, which killed 35,000 people in 1902 and 1932. The great sheets of rhyolitic ash that cover wide areas in Nevada and other western states must be of similar origin.
Basic vs. Acidic
Igneous rocks are often divided into basic and acidic types. These terms relate to percentage of silica rather than to hydrogen-ion content (pH). Some petrologists prefer the terms mafic and felsic. Ultrabasic (ultramafic) rocks, containing less than 45 percent silica, include plutonic rocks such as dunite and peridotite, which often contain olivine and pyroxene and are free of quartz and feldspars. Some contain feldspathoids. Basic rocks contain 45 to 62 percent silica and include gabbroic (plutonic) rocks and basaltic (volcanic) rocks. Plagioclase feldspar, olivine, and pyroxene are the key minerals in these rocks, and quartz is usually absent. Intermediate rocks (52 to 66 percent silica) include diorite (plutonic) and andesite (volcanic). These rocks contain hornblende (see amphibole), pyroxene, and plagioclase feldspar. Quartz, potassium feldspar, and biotite (mica) may occur. Acidic (felsic) rocks, containing more than 66 percent silica, include the granites (plutonic) and rhyolites (volcanic). Quartz and potassium feldspar are important mineral components, along with muscovite (mica), biotite, and hornblende.
Rocks that are known or inferred to be of igneous origin are the sources of many economically important materials. Volcanic rocks are sources of sulfur and mercury. Volcanic ash-flow deposits are sources of much of the world's copper supply--specifically the porphyry copper deposits in Chile, Nevada, and Utah. The chromium-rich mineral chromite is common in layered igneous complexes such as the Stillwater Complex of Montana. Low-silica complexes of probable igneous origin are sources of much asbestos, as at Thetford Mines, Quebec. Precious-metal deposits of gold, silver, and platinum are related to late stages in the formation of igneous granitic rocks. The Great Dike of Zimbabwe and the Bushveld Complex of southern Africa contain rich deposits of copper, gold, and other valuable minerals. Lead-zinc ore bodies result from late-stage concentration of these metals in cooling igneous complexes.
Many high-grade iron-ore deposits (magnetite) are probably igneous, as are great nickel deposits, such as those at Sudbury, Ontario. Titanium ores occur in plagioclase-rich gabbro called anorthosite. Many anorthosites are thought to be igneous, although the absence of volcanic equivalents leads to a possibility of metamorphic origin also. Anorthosite is a low-grade source of aluminum; no economically feasible way, however, has been found to process it. Many volcanic rocks are used in various types of construction, especially as a crushed material for road building. Plutonic rocks, lavas, and tuffs are used in house construction and as decorative stone. Volcanic ash and flows (when weathered) furnish fertile soils. The cooling of igneous rocks at depth also provides sources of geothermal energy.
William D. Romey