IGNEOUS ROCK

any of various crystalline or glassy rocks formed by the cooling and solidification of molten earth material. Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary. Igneous rocks occur as intrusions or extrusions. Plutonic intrusive rocks congeal far below the Earth's surface and occur as batholiths, bosses, laccoliths, and veins. They include granite, syenite, diorite, gabbro, and peridotite. The hypabyssal intrusive rocks (i.e., those formed at a moderate distance below the surface) occur as dikes, sills, and other similar small bodies and are represented by porphyry, porphyrite, diabase, and lamprophyre. The extrusive igneous rocks are volcanic and found typically as lava flows. They include obsidian, perlite, rhyolite, trachyte, andesite, basalt, and tephrite. Igneous rocks may also occur as pyroclastic ejecta, which are the fragmented products of explosive volcanic eruptions. Examples of pyroclastic rocks are agglomerate and certain types of breccia and tuff. Although the different types of igneous rocks vary widely in composition, the vast majority of them consist of fewer than a dozen groups of minerals. These are quartz, feldspars, pyroxenes, amphiboles, micas, olivines, nephelinite, leucite, and apatite. Igneous rocks also show considerable variation in texture and structure. For example, plutonic intrusive rocks cool slowly, and, as a result, their crystals are coarse, well-shaped, and approximately of equal size. Because crystallization is complete, there is no glassy matter present. Miarolitic cavities (irregular openings) commonly develop in plutonic rocks during the late stages of crystallization; they often are filled with vapour deposits. Extrusive igneous rocks, having cooled rapidly in contact with air, are as a rule finely crystalline or at least have a fine-grained groundmass representing that part of the viscous semicrystalline lava flow that was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the gases that they contained were free to escape. Various important modifications arise from this, the most striking being the presence of numerous steam cavities often drawn out to elongated shapes and subsequently filled up with minerals by infiltration. Because crystallization was going on while the lava was creeping forward over the surface, the latest formed minerals are commonly arranged in subparallel winding lines that follow the direction of movement. In some cases, the lava cooled at an extremely rapid rate, forming noncrystalline rock (i.e., glassy rock such as obsidian and tachylyte). A common feature of glassy rock is the presence of rounded bodies (spherulites) consisting of fine divergent fibres of imperfect feldspar crystals radiating from the centre. any of various crystalline or glassy rocks formed by the cooling and solidification of molten earth material. Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary. Igneous rocks are formed from the solidification of magma, which is a hot (600 to 1,300 C, or 1,100 to 2,400 F) molten or partially molten rock material. The Earth is composed predominantly of a large mass of igneous rock with a very thin veneer of weathered materialnamely, sedimentary rock. Whereas sedimentary rocks are produced by processes operating mainly at the Earth's surface by the disintegration of mostly older igneous rocks, igneousand metamorphicrocks are formed by internal processes that cannot be directly observed and that necessitate the use of physical-chemical arguments to deduce their origins. Because of the high temperatures within the Earth, the principles of chemical equilibrium are applicable to the study of igneous and metamorphic rocks, with the latter being restricted to those rocks formed without the direct involvement of magma. Magma is thought to be generated within the plastic asthenosphere (the layer of partially molten rock underlying the Earth's crust) at a depth below about 60 kilometres (40 miles). Because magma is less dense than the surrounding solid rocks, it rises toward the surface. It may settle within the crust or erupt at the surface from a volcano as a lava flow. Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in physical and chemical conditions prevalent in the two environments. Within the Earth's deep crust the temperatures and pressures are much higher than at its surface; consequently, the hot magma cools slowly and crystallizes completely, leaving no trace of the liquid magma. The slow cooling promotes the growth of minerals large enough to be identified visually without the aid of a microscope (called phaneritic, from the Greek phaneros, meaning visible). On the other hand, magma erupted at the surface is chilled so quickly that the individual minerals have little or no chance to grow. As a result, the rock is either composed of minerals that can be seen only with the aid of a microscope (called aphanitic, from the Greek aphanes, meaning invisible) or contains no minerals at all (in the latter case, the rock is composed of glass, which is a highly viscous liquid). This results in two groups: (1) plutonic intrusive igneous rocks that solidified deep within the crust and (2) volcanic, or extrusive, igneous rocks formed at the Earth's surface. Some intrusive rocks, known as subvolcanic, were not formed at great depth but were instead injected near the surface where lower temperatures result in a more rapid cooling process; these tend to be aphanitic and are referred to as hypabyssal intrusive rocks. The deep-seated plutonic rocks can be exposed at the surface for study only after a long period of denudation or by some tectonic forces that push the crust upward or by a combination of the two conditions. (Denudation is the wearing away of the terrestrial surface by processes including weathering and erosion.) Generally, the intrusive rocks have cross-cutting contacts with the country rocks that they have invaded, and in many cases the country rocks show evidence of having been baked and thermally metamorphosed at these contacts. The exposed intrusive rocks are found in a variety of sizes, from small veinlike injections to massive dome-shaped batholiths, which extend for more than 100 square kilometres (40 square miles) and make up the cores of the great mountain ranges. Extrusive rocks occur in two forms: (1) as lava flows that flood the land surface much like a river and (2) as fragmented pieces of magma of various sizes (pyroclastic materials), which often are blown through the atmosphere and blanket the Earth's surface upon settling. The coarser pyroclastic materials accumulate around the erupting volcano, but the finest pyroclasts can be found as thin layers located hundreds of kilometres from the opening. Most lava flows do not travel far from the volcano, but some low-viscosity flows that erupted from long fissures have accumulated in thick (hundreds of metres) sequences, forming the great plateaus of the world (e.g., the Columbia River plateau of Washington and Oregon and the Deccan Plateau in India). Both intrusive and extrusive magmas have played a vital role in the spreading of the ocean basin, in the formation of the oceanic crust, and in the formation of the continental margins. Igneous processes have been active since the formation of the Earth some 4.6 billion years ago. Their emanations have provided the water for the oceans, the gases for the primordial oxygen-free atmosphere, and many valuable mineral deposits. Additional reading Paul C. Hess, Origins of Igneous Rocks (1989); Anthony R. Philpotts, Principles of Igneous and Metamorphic Petrology (1990); Daniel S. Barker, Igneous Rocks (1983); Gordon A. MacDonald, Volcanoes (1972); Alexander R. McBirney, Igneous Petrology (1984); Howel Williams and Alexander R. McBirney, Volcanology (1979). Albert M. Kudo