ORIGIN OF IGNEOUS ROCKS The origin of magma is not known from direct observations, but during the last few decades, our understanding of the chemistry and physics of liquid rock material has increased greatly. This knowledge is based on observations of synthetic magmas made in the laboratory, on the observation of volcanic products, and on studies of Earth’s geophysical properties. We know from seismic evidence that Earth is essentially solid to a depth of 2900 km, and that only its outer core is liquid. The liquid core, however, is unlikely the source of most magma. The core’s density ranges from 9 to 10 gin/cm3, a density much greater than that of any magma or igneous rock in the crust. (Basalt, one of the densest igneous rocks, has a density of only 3 gm/cm3.) Magma must therefore originate from the localized melting of solid rock in the upper mantle and lower crust, at depths ranging from only 50 to 200 km below the surface. To understand the origin of magma, remember that a rock does not have a specific melting point. Each mineral within the rock begins to melt at a different temperature. As shown in the diagram in Figure 3.1, muscovite, K-feldspar, and quartz not only crystallize in sequence at temperatures ranging from a little over 800 to 600 °C, but they begin to melt in this temperature range as well. Amphibole melts at a higher temperature, followed by the melting of pyroxene and olivine. Sodium-rich plagioclase begins to melt at about 600 °C, whereas calcium-rich plagioclase may not melt until it reaches a temperature of 1200 °C. In reality, melting and crystallization are complex physical and chemical processes. It is, however, well established that some minerals melt at lower temperatures than others. This fact is important in understanding the general way in which a magma originates. In addition to temperature, remember that other important factors—such as pressure, amount of water present, and overall composition of the rock— influence melting. For example, melting can be initiated by either an increase in temperature or a decrease in pressure and is enhanced by the amount of water present in the pore spaces of the rock. The diagrams in Figure 3.14 summarize our present understanding of how and where a magma originates. Global studies of the distribution of flood basalts, volcanoes, batholiths, and associated mountain belts indicate that basaltic magma is generated at divergent plate margins and that granitic-andesitic magma is generated in sub-duction zones. In both areas, variations in temperature and pressure occur as a result of plate movement, and magma is produced by the partial melting of the lower crust or upper mantle. Generation of Basaltic Magma Basaltic magma is believed to be generated at divergent plate margins by partial melting of the asthenosphere (Figure 3.14A). The asthenosphere, as well as the lower mantle, is almost certainly composed of peridotite, a rock made up largely of the minerals olivine, pyroxene, and minor amounts of plagioclase. The balance between temperature and pressure in the asthenosphere is just about right for peridotite to begin to melt. Below the asthenosphere, the pressure is too great for melting to occur, and at more shallow depths above the asthenosphere, the temperature is too low. The general process of how basaltic magma originates follows: As material in the asthenosphere moves slowly upward and outward, the lithosphere splits and moves apart. This reduces pressure above a section of the asthenosphere, and melting begins. Plagioclase melts first, followed by pyroxene, and ultimately by the melting of olivine. If only part of the mantle rock melts, much of the olivine remains solid, so the resulting magma is richer in those elements that compose plagioclase and pyroxene. In fact, that is the composition of basaltic magma. Scientists therefore hypothesize that basaltic magma originates from partial melting of the asthenosphere at rift zones, where pressure is reduced as a result of the splitting and spreading apart of lithospheric plates. A decrease in pressure thus plays an important role in the generation of basaltic magma. The basaltic magma, being less dense than the surrounding peridotite, rises along the spreading center and is extruded as basalt flows in the rift zone. 1 Generation of Granitic Magma In a subduction zone, the basaltic oceanic crust and water-saturated oceanic sediments descend into the mantle. This mixture of basalt and sediment is heated by the friction between colliding lithospheric plates and by the higher temperatures at depth as the mixture is submerged (Figure 3.14B). As the basalt and oceanic sediments are heated, the silica-rich minerals begin to melt first, some at temperatures as low as 500 °C. Partial melting of the basaltic crust thus produces a magma richer in silica than the magma produced at divergent plate boundaries. (See Figure 3.1.) This silica-rich magma rises upward in the orogenic belt to produce granitic intrusions and andesitic-rhyolitic flows. The extreme pressure in the roots of a deformed mountain belt, at converging plate margins, would also increase the temperature enough to begin the melting of some of the minerals in metamorphic rocks. The liquid would rise, collect in larger bodies, and produce chambers of granitic magma. Generation of Magma in Mantle Plumes The origin of magma in ocean islands in the middle of the plates—far from plate boundaries—has long been a mystery. The details of the composition of these basaltic lavas suggest to many geologists that they are generated by partial melting of rising plumes of solid mantle (Figure 4.23). Presumably, they rise because they are more buoyant and warmer than the rest of the mantle. As the material in the plume nears the sur face, the plume partially melts to produce basalt that rises to the surface. The basaltic magma may erupt to form chains of shield volcanoes, such as the Hawaiian Islands, or to form large provinces covered by flood basalts. Thus the magma-generating process is very similar to the process that generates oceanic ridges, but the composition and the shape of the zone of magma generation and eruption are significantly different. 2 3 IGNEOUS ROCK BODIES Igneous rock bodies form below and on the Earth’s surface and are produced in a variety of shapes and sizes (see the diagram above). 1. Batholiths. Large, massive intrusions covering an area of more than 50 miles. 2. Caldera. A large, circular depression caused by explosion and/or collapse of a volcano. 3. Cinder cone. A cone-shaped hill composed of loose volcanic fragments erupted from a central vent. 4. Dike. A tabular intrusive rock body that cuts across the strata of the surrounding rock. 8. Plateau basalt. Extensive layers of basalt, which after uplift erodes into plateaus. 5- Fissure eruption. Extrusion of lava along a system of fractures or fissures. 9. Sill. A tabular body of igneous rock injected between layers of the enclosing rock. 6. Inverted valley. A valley that has been filled with lava and has subsequently been eroded into an elongated ridge. 10. Stock. A roughly circular intrusion, usually less than 50 miles square in surface exposure. 7. Laccolith. An intrusive body that has a flat floor but arches up the strata into which it was ejected to form a blister-like structure. 4 11. Volcanic neck. An igneous rock body that originally formed the vent or neck of a volcano and has subsequently been exposed by erosion. IGNEOUS ROCKS OBJECTIVE To recognize the major types of igneous rocks and to understand the genetic significance of their texture and composition. MAIN CONCEPT Igneous rocks can be classified on the basis of composition and texture. The composition of a rock provides information about the magma from which it formed, and about the tectonic setting in which it originated. The texture of a rock gives important insight into the cooling history of the magma. SUPPORTING IDEAS 1. Igneous rocks are composed of silicate minerals, the most important of which are (a) plagioclase, (b) Kfeldspar, (c) quartz, (d) mica, (e) amphibole, (f) pyroxene, and (g) olivine. 2. The major textures in igneous rocks are (a) phaneritic, (b) porphyritic-phaneritic, (c) aphanitic, (d) porphyritic-aphanitic, (e) glassy, and (f) pyroclastic. 3. Silicic magmas originate at subduction zones by partial melting of the oceanic crust. 4. Mafic magmas originate by partial melting of the upper mantle at divergent plate margins. COMPOSITION OF IGNEOUS ROCKS Approximately 99% of the total bulk of most igneous rocks is made up of only eight elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. Most of these elements occur in the crystal structures of the rock-forming silicate minerals feldspars, olivines, pyroxenes, amphiboles, quartz, and mica. These six minerals constitute over 95% of the volume of all common igneous rocks and are therefore of paramount importance in studying their classification and origin. Magmas Magmas rich in silica and aluminum are referred to as silicic; they tend to produce more quartz, potassium feldspar, and sodium plagioclase and generally form light-colored rocks. Magmas rich in iron, magnesium, and calcium are referred to as mafic; they produce greater quantities of olivine, pyroxene, amphibole, and calcium plagioclase. The resulting rocks are dark colored because of an abundance of the dark ferromagnesian minerals. (There are, however, many exceptions to such generalizations, and the mineral composition of igneous rocks can only be roughly approximated by an observation of color.) Crystallization Crystallization of minerals from a magma occurs between 600 and 1200 °C. Those minerals with the highest freezing point crystallize first and thus develop well-formed crystal faces. Minerals that crystallize at lower temperatures are forced to grow in the spaces between the earlier-formed crystals and are commonly irregular in shape with few well-developed crystal faces. From laboratory studies of artificially produced magmas, and from petrographic studies of igneous rocks, a general order of crystallization has been established. This sequence is summarized in Figure 3.1 and is fundamental to the study of igneous rocks. It is apparent from Figure 3.1 that in a mafic magma, olivine and Ca-plagioclase are the first minerals to form, followed by pyroxenes, amphiboles, and Na-plagioclase. Such a magma crystallizes between 900 and 1200 °C and produces rocks of the gabbro-basalt family. In magmas rich in silica and aluminum, biotite and quartz form first, followed by K-feldspar and muscovite. Rocks of the granite-rhyolite family develop from such a magma at temperatures below 900 °C. TEXTURE OF IGNEOUS ROCKS Texture refers to the size, shape, and boundary relationships of adjacent minerals in a rock mass. In igneous rocks, texture develops primarily in response to the magma’s composition and rate of cooling. Magmas located 1 deep in Earth’s crust cool slowly. Individual crystals grow to a more or less uniform size and may be more than an inch in diameter. In contrast, a lava extruded at Earth’s surface cools rapidly, so the mineral crystals have only a short time in which to grow. The crystals from such a magma are typically so small that they cannot be seen without the aid of a microscope; the resultant rock appears massive and structureless. Regardless of crystal size, the texture of most igneous rocks is distinguished by a network of interlocking crystals. Igneous rock textures are divided into the following types: (1) phaneritic, (2) porphyritic-phaneritic, (3) aphanitic, (4) porphyritic-aphanitic, (5) glassy, and (6) pyroclastic (Figures 3.2-3.7). Phaneritic Texture In phaneritic textures, individual crystals are large enough to be visible to the naked eye (Figure 3-2A and B). The grains in each specimen are approximately equal in size and form an interlocking mosaic. The size of crystals in a phaneritic texture can range from those barely visible, to crystals of more than an inch in length. Phaneritic texture develops from magmas that cool slowly and commonly develops in intrusive igneous bodies such as batholiths or stocks. Very coarse phaneritic rocks, in which the crystals are several feet long, almost invariably are found in large veins. Porphyritic-Phaneritic Texture A porphyritic-phaneritic texture is characterized by two distinct crystal sizes, both of which can be seen with the naked eye. The smaller crystals constitute a matrix, or groundmass, that surrounds the larger crystals, or phenocrysts (Figure 3-3). Aphanitic Texture In aphanitic texture, individual crystals are so small they cannot be detected without a microscope (Figure 3.4). Rocks with this texture therefore appear to be massive and structureless. When a thin section of an aphanitic rock is viewed under a microscope, however, its crystalline structure is readily apparent. The rock is seen to be composed of numerous small crystals and, usually, some glass. Other examples of aphanitic texture viewed under a microscope are shown in the photomicrographs in Figures 3.10A and 3. HA. 2 Porphyritic-Aphanitic Texture A porphyritic-aphanitic texture is defined as the texture of a rock with an aphanitic matrix in which embedded phenocrysts make up more than 10% of the total rock volume (Figure 3.5A and B). The phenocrysts are visible to the unaided eye. When phenocrysts are abundant, the rock at first glance may appear to be phaneritic. Careful study of the area between the phenocrysts will indicate whether the matrix is aphanitic or phaneritic. Glassy Texture Glassy texture is similar to that of ordinary glass. It may occur in massive units (Figure 3-6A) or in a thread-like mesh resembling spun glass (Figure 3-6B). Crystals cannot be discerned in a glassy texture, even when the specimen is viewed under high magnification. Pyroclastic Texture Pyroclastic textures consist of broken, angular fragments of rock material (Figure 3.7A and B). In pyroclastic rocks, the fragmental material is composed of pumice, glass, droplets of lava, and broken crystals. Some sorting and stratification are generally present. Material finer than 4 mm is known as tuff. Material larger than 4 mm is referred to as volcanic breccia. If the fragments are exceptionally hot when deposited, they may fuse or weld together to form a dense mass. 3 PORPHYRITIC-PHANERITIC TEXTURES 4 PORPHYRITIC-APHANITIC TEXTURES 5 GLASSY AND PYROCLASTIC TEXTURES 6 CLASSIFICATION AND IDENTIFICATION OF IGNEOUS ROCKS CLASSIFICATION The most useful and significant classification system for igneous rocks is based on two criteria: composition and texture. These criteria are important, not only in describing the rock so that it can be distinguished from other rock types, but also in drawing important implications about the rock’s origin. A chart (Figure 3-8) in which variations in composition are shown horizont

Do you have a similar assignment and would want someone to complete it for you? Click on the ORDER NOW option to get instant services at essayloop.com. We assure you of a well written and plagiarism free papers delivered within your specified deadline.