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سنگهای آذرین - Igneous rocks

Igneous rocks

  Rocks that are formed from melted rock, usually by cooling. The melted rock is called magma when it is below the surface, and lava when it flows out onto the surface. When the magma or lava cools or moves into areas of lower pressure, crystals or glass or both form. Thus, igneous rocks are composed of crystals, glass, or combinations of these materials. Magmas sometimes erupt explosively, creating ash that is composed of broken crystals, glass, and rock materials called pyroclastic material. Rocks formed at or very near the surface, including pyroclastic rocks, are called volcanic rocks, whereas those formed from magma at depth are called plutonic rocks. In the past, some rocks that formed below the surface but near it were called hypabyssal rocks.  See also: Lava; Magma

Like all rocks, igneous rocks are classified on the basis of their composition and texture. Both mineral and chemical compositions are used for classification, but the rocks are first subdivided by texture into three categories. (1) Volcanic rocks are dominated by interlocking grains too small to see with the unaided eye or a simple lens. (2) Plutonic rocks are composed almost entirely of interlocking grains big enough to see. (3) Pyroclastic rocks are composed of fragments (clasts) of glass, minerals, and preexisting rocks.  See also: Petrology

Composition

The range of igneous rock chemistries is relatively large. The common igneous rocks consist of silica (SiO2), 40 to 77%; alumina (Al2O3), <1% to nearly 20%; magnesia (MgO), <1 to 50%; total iron oxide, <1% to more than 10%; and lime (CaO), soda (Na2O), and potash (K2O), <1% to 9% (see table).

The minerals in common igneous rocks include quartz, alkali and plagioclase feldspars, micas (muscovite, biotite, and phlogopite), amphiboles (such as hornblende and riebeckite), pyroxenes (including augite and the orthopyroxenes), and olivines. In addition, minerals occurring in small amounts include apatite, zircon, epidote, titanite, magnetite, and ilmenite.  See also: Amphibole; Feldspar; Mica; Olivine; Pyroxene; Quartz

For purposes of description, igneous minerals are divided into three groups. Essential minerals are those on which the rock name is based. Characterizing accessory minerals are nonessential minerals that occur in amounts of 5% or more. Minor accessory minerals occur in amounts of less than 5%.  See also: Mineral

 Classification

 Traditionally, classification of all igneous rocks was based on minerals and textures. In volcanic rocks, however, the minerals are often so small that identification is difficult or impossible, and glass contains no minerals. Thus, in recent years volcanic rock classification has been based largely on rock chemistry, except for classifications used in field studies.

Volcanic flow and pyroclastic rocks

 The International Union of Geological Sciences (IUGS) volcanic rock classification defines the common volcanic rock types on the basis of silica (SiO2) content versus alkalis (Na2O + K2O). The most common types of volcanic rocks have less than 7% total alkalis and include basalt (45–52% SiO2), basaltic andesite (52–57% SiO2), andesite (57–63% SiO2), dacite (63% to 69–77% SiO2), and rhyolite (>69% SiO2).  See also: Andesite; Basalt; Dacite; Rhyolite

Pyroclastic volcanic rocks are classified on the basis of grain size, in addition to the compositional name. If the dominant grain size is less than 2 mm, the volcanic material is called ash and the rock composed of these grains is called tuff. If the rock contains a significant percentage of grains between 2 and 64 mm, it is called a lapilli tuff. Rocks dominated by fragments larger than 64 mm are called volcanic breccias. Thus, a complete pyroclastic rock name might be lapilli tuff rhyolite.  See also: Breccia; Pyroclastic rocks; Tuff

Some volcanic rocks are composed entirely or almost entirely of glass. If the glass largely lacks vesicles (gas bubble holes), the rock is called obsidian (or rhyolite obsidian). If the glassy rock contains abundant, small, aligned, tubular holes, it is called pumice. Larger vesicles, characteristic of the less siliceous rocks, require the modifier “vesicular,” as in vesicular basalt. Rocks dominated by vesicles are called (basalt) scoria.  See also: Obsidian; Pumice

 Plutonic rock

 Classification of plutonic rocks is based primarily on mineralogy. The IUGS classification is the most widely used plutonic rock classification. In this classification, which features two triangles joined at their bases, the essential minerals are quartz, alkali feldspar, plagioclase, and feldspathoid minerals (foids). All other minerals are ignored for classification purposes, except in classification of low-silica rocks, for which separate sets of triangles are used.  See also: Feldspathoid

In order to classify a rock, the essential mineral percentages are observed and totaled, and the individual percentages are divided by the total to obtain a percentage of the three essential minerals. The single point on the triangle represented by the three mineral percentages determines the name for the rock. Among the most common plutonic rocks are granite, composed of quartz, alkali feldspar, and some plagioclase feldspar; and gabbro, composed of augite and plagioclase (Fig. 1). In the United States, some geologists use alternative classifications that subdivide the “granite” field of the IUGS classification.  See also: Gabbro; Granite

 

 Fig. 1  Some plutonic rock names and their volcanic equivalents.

 

 

 For low-silica rocks—the gabbros and ultrabasic rocks—separate triangles are used for classification (Fig. 2). The essential minerals for these classifications are plagioclase, olivines, pyroxenes, and hornblende.  See also: Hornblende

 

 

Fig. 2  IUGS classifications of gabbroic rocks (triangle AQP) and ultramafic rocks (triangle AFP). (From A. Streckeisen, To each plutonic rock its proper name, Earth Sci. Rev., 12:1–33, 1976)

 

 

 Plutonic rocks are also classified by some geologists on the basis of their chemistry. Thus, there are peraluminous, metaluminous, and peralkaline rocks, which have chemistries high, moderate, and low in alumina, respectively. And there are S-, I-, and A-type granitoid rocks formed from melts of sedimentary, igneous, and relatively high-alkali materials.

 Textures and structures

The texture of a rock includes the size, shape, arrangement, and grain boundary features. Many geologists also include abundant holes and the relationship between glass and crystals in volcanic rocks as part of the texture. In general, a feature is textural if there are many of them in a hand-sized sample. Structures are generally larger features. There may be a few or only one in a hand specimen, or they may be much larger than a hand specimen.  See also: Petrography

All of the plutonic rocks are crystalline in texture. This means that the grain boundaries in the rock are interlocking (Fig. 3a). Rocks with grains large enough to see are said to have phaneritic texture. If the grain length exceeds 3 cm in length, the rock has pegmatitic texture. Volcanic rocks include those with crystalline textures too small to see without magnification (aphanitic textures), those with aphanitic crystals surrounding larger visible crystals (porphyritic textures) [Fig. 3b] and those composed of fragments (clasts) formed by explosive eruption of magmas (pyroclastic textures) [Fig. 3c]. In the last, clasts may include fragments of glass, fragments of crystals, or fragments of earlier-formed rocks.

 

 Fig. 3  Photomicrographs of common igneous rock textures. (a) Hypidiomorphic-granular texture. (b) Porphyritic texture. (c) Pyroclastic texture. Width of field of view is 3.2 mm (0.125 in.).

 

 

 

 

Grain size, shape, and arrangement

 

Grain (crystal) sizes in igneous rocks vary widely. The smallest crystals are less than 0.01 mm, whereas the largest crystals are meters long. These largest crystals form in pegmatitic rocks, especially those rich in SiO2. Fine-grained rocks have grains less than 1 mm in length. In medium-grained rocks, the average grains are 1 to 5 mm in length. Coarse-grained rocks have average grain lengths of 5 mm to 3 cm. In pegmatitic rocks, the dominant grain size exceeds 3 cm. In any given rock, there may be a range of crystal sizes for each mineral, called a crystal size distribution (CSD), which reflects the crystallization history.  See also: Pegmatite

The crystalline grains of igneous rocks have shapes that vary and depend on a variety of factors controlling the growth history of the crystal. In addition, the crystal form of each mineral (the natural shape of the particular mineral) is a dominant influence on the shape. Mineral grains that have flat surfaces (crystal faces) on all sides, which formed as the crystal grew, are said to be euhedral. Crystals partially bounded by crystal faces are subhedral, and those not bounded by crystal faces are anhedral.

Early or rapid crystal growth from a melt yields partial, incomplete, or skeletal crystals. In volcanic glass, tiny beginning crystals of this type are called crystallites and microlites. In plutonic rocks, they are called skeletal or snowflake grains.  See also: Volcanic glass

Where growth of crystals is prevented by explosive eruption of magma, glass fragments erupt into the atmosphere. Tuffs form from accumulations of these variably sized, typically curved pieces of glass, mixed with fragments of crystals and rocks (Fig. 3 c).

Minerals may form in random orientations, or they may have specific arrangements. In volcanic rocks in which elongate crystals are aligned (usually because of flow of the magma), the rock has a trachytic texture, but similar textures in plutonic rocks are called trachytoidal textures. Randomly oriented grains that are subhedral, on average, yield a hypidiomorphic-granular texture in plutonic rocks (Fig. 3 a). This is the typical texture of the granites. The similar texture at a smaller scale in volcanic rocks is called intergranular texture.

Many specialized textures exist and have been named based on the kinds, shapes, or arrangement of grains. Porphyritic texture consists of two distinct sizes of grains, with substantially larger grains (phenocrysts) enclosed by a matrix of smaller grains (Fig. 3b). Porphyritic textures can be either volcanic or plutonic. Ophitic texture consists of large grains of pyroxene enclosing smaller, rectangular grains of plagioclase. Graphic texture, a texture that occurs within some pegmatitic rocks, consists of V- and L-shaped grains of quartz enclosed in a large alkali feldspar grain. The quartz in graphic texture is usually part of a single but incomplete skeletal grain. Corona textures are those in which a rim of one mineral surrounds a core of another mineral.  See also: Phenocryst; Porphyry

 

Mineral textures

 Individual minerals within igneous rocks may also have special textures. Commonly in igneous rocks, larger grains of one mineral, such as alkali feldspar, will enclose many smaller grains of other minerals to form a poikilitic texture (Fig. 4 a). Ophitic and graphic textures are poikilitic textures that occur throughout and characterize particular rock types.

 

Fig. 4  Photomicrographs of some mineral textures in igneous rocks. (a) Poikilitic texture in alkali feldspar. (b) Perthitic texture in alkali feldspar. (c) Zoned texture in plagioclase. Width of field of view is 3.2 mm (0.125 in.).

 

 

 In granites, alkali feldspar grains also commonly have a mineral texture called perthitic. Perthitic texture results when small, tabular, irregular, or worm-shaped grains of plagioclase feldspar separate from, but typically occur as subparallel blobs within, larger alkali-feldspar grains (Fig. 4b). Myrmekitic and granophyric textures involve wormlike quartz intergrowths within plagioclase and alkali feldspar, respectively.

Another important mineral texture is zoning (Fig. 4 c). In this texture, common in plagioclase feldspar, each successive layer of the growing crystal has a different composition, so that the mineral appears to have rings of different colors or brightness under polarized light and magnification.

 

Texture formation

Various factors control the formation of crystals (mineral grains) in a magma. The temperature at which the crystal nucleates (begins to form), how fast it grows, the nature of the “neighborhood” of growth (that is, whether the crystal is surrounded entirely by melt or mostly by crystals, and whether the crystal is able to complete or nearly complete its growth) influence the shapes of crystals.

Crystal formation in a rock occurs in two stages: nucleation and growth. Some magmas retain unmelted crystal fragments of the rock from which the magma formed, and these form nuclei (clusters of the particular atoms needed to form specific minerals). When atoms simply attach themselves to preexisting surfaces or nuclei to form a growing nucleus for a mineral grain, this is called heterogeneous nucleation. If no nuclei exist in a magma, nuclei must form by establishing bonds between atoms. Nucleation from a magma containing no remnants of former crystals is called homogeneous. Heterogeneous nucleation is more common than homogeneous nucleation.

Once nuclei form, mineral grains grow by adding atoms to the nucleus. Growth is influenced by factors such as the composition of the melt, the temperature of the melt, the cooling rate during growth, and the rate of migration of atoms through the melt to the growing crystal. In high-silica magmas, crystals grow more slowly than in low-silica magmas, if all other factors are equal.

Some minerals begin to form earlier than others as a magma cools. For example, iron-and magnesium-rich minerals tend to nucleate at higher temperatures than potassium- and sodium-rich minerals. Once adequate cooling occurs, different minerals may grow simultaneously but at different rates. The final texture of a rock results from the growth of different crystals under the influence of the various controlling influences.

 Volcanic structures

 Volcanic and plutonic structures vary widely in size. Some are less than 1 cm in diameter, and others are thousands of meters across.

Major volcanic structures include lava plateaus and plains, pyroclastic sheets, shield volcanoes, composite volcanoes, and calderas (Fig. 5). Lava plains and plateaus are large sheets of lava that cover up to a million or more square kilometers. Pyroclastic sheets are similarly shaped masses of ash and cinder that have fallen from the sky as a result of explosive eruptions. Shield and composite volcanoes are cone-shaped piles of lava. Shield volcanoes are flat cones dominated by low-silica flows of basalt, whereas composite volcanoes are steep cones dominated by alternating lava flows and layers of pyroclastic material (Fig. 5b). Massive explosive eruptions sometimes produce large, crudely circular holes; these calderas may be tens to thousands of square kilometers in area.  See also: Caldera; Volcano; Volcanology

 

Fig. 5  Some volcanic structures. (a) Lava plateau with flat-lying basalt lava flows, near Sprague, Washington. (b) Composite volcanoes, Atitlan (left) and Toliman in Guatemala. (c) Columnar jointing in andesite near Lost Creek Dam, Oregon. (d) Pillow basalts, Golden Gate National Recreation Area, California.

 

 

 

 

Intermediate-sized structures include small pyroclastic sheets, lava flows, pyroclastic (cinder) cones, and domes. Lava flows are sheets of lava that have flowed across the Earth's surface after erupting from a vent. Domes are piles of lava in the shape of an inverted cup that have not flowed far from the site of eruption because of their sticky character (high viscosity).

Small-sized structures range from tiny piles of lava (spatter cones) or cones of pyroclasts (small cinder cones) a few meters high to features less than a centimeter across that occur in hand specimens. Many of these structures are associated with lava flows. Small blobs of lava that squeeze out through cracks in a lava flow and solidify are called squeeze-ups. Larger bumps on lava flows are called pressure ridges. Where lavas cool in tabular sheets, they commonly form polygonal, columnar joints (Fig. 5c); but if they are extruded or flow into water, they may form pillow structures—tubular masses that are elliptical in cross section (Fig. 5d).

Other relatively small volcanic structures include craters, small lava flows, bombs, xenoliths, autoliths, and flow bands. Craters are the circular holes formed by eruption of magma. They range from a few meters to about 1 km (0.6 mi) across. Bombs are masses of lava of various shapes that have been blown into the air and have fallen to the ground as volcanic rock. Xenoliths (foreign stones) and autoliths (earlier crystallized parts of a magma) occur as inclusions in volcanic rocks. Flow bands are millimeter to centimeter thick layers that form during flow from concentrations of bubbles, crystals, or magma of a different composition. Vesicles (holes) in lava flows and amygdules (holes filled with minerals) are considered structures by some geologists.  See also: Xenolith

 

Plutonic structures 

Plutonic structures occur as both large- and small-sized features in magmas that intrude into rocks of the crust. The large features include batholiths, stocks, lopoliths, roof pendants, cupolas, and various types of dikes and sills. Batholiths are bodies of plutonic rock that are exposed at the surface over an area of 100 km2 (40 mi2) or more. Stocks are plutonic bodies of smaller dimension (Fig. 6). Batholiths are commonly lens-shaped. Lopoliths are special types of batholiths that are dish-shaped and typically consist of layers of silica-poor rock. Roof pendants are masses of country rock (the rock surrounding a batholith or stock) that hang down into the top of the plutonic rock body (the pluton). Cupolas are dome-shaped masses of the pluton that stick up into the country rock (Fig. 6).  See also: Pluton

 

 

Fig. 6  Some plutonic structures.

 

 

 

 

Various types of dikes and sills represent masses of magma that intruded and solidified within country rocks. Dikes are crosscutting, typically sheetlike structures that range from centimeters to kilometers of scale (Fig. 6). Some dikes are cone-shaped or cylindrical and are called cone sheets and ring dikes. Short fat dikes are called apophyses. Sills are intruded sheets of rock that parallel the layers in the country rocks (Fig. 6). When sills are relatively short, thick, and convex on their tops, they are called laccoliths.

Small-scale plutonic structures include dikes, xenoliths, and autoliths, as well as layers and foliation. Gabbroic magmas may undergo a process of fractional crystallization in which crystals form and settle to the bottom of the magma body to form layers of alternating composition. Where early formed crystals are dragged along and aligned by flow of the magma, the aligned crystals define foliation, a structure that also occurs in metamorphic rocks.  See also: Metamorphic rocks

 Magmas

In order to form igneous rocks, older rock must melt to form a magma. Usually only part of a rock melts, a process called anatexis. The magma never has the same chemical composition as the original rock, because some minerals melt before others. Generally, the newly formed magma is richer than the rock from which it came in elements such as silicon, aluminum, sodium, and potassium.

Origins

 Magmas form in two general regions: at the base of the crust and in the mantle below the crust. Magmas form when rocks melt as temperature rises (thermal melting), as pressure falls (decompression melting), and as water is added to hot rock (flux melting). Flux melting typically occurs where wet ocean crust is carried down into the mantle (subducted) as a result of plate movements, and water from suducting rocks migrates into hot mantle rocks above the subducted crust. The water causes flux melting that creates basaltic magma.  See also: Earth interior

Mantle rocks are hot and plastic. In some places they are cooler than in others, and the hotter rocks tend to rise or flow plastically upward. As they rise, the pressure from the weight of the overlying rock becomes lower. The drop in pressure causes decompression melting of mantle rocks, forming basaltic magmas. Decompression melting is most common in the mantle beneath mid-ocean ridges (spreading centers).  See also: Mid-Oceanic Ridge

Thermal melting occurs primarily at the bottom of the Earth's crust. Hot basaltic magmas rising from the mantle have extra heat. If they form a pool or pond at the base of the crust, they may have enough heat to melt the crust to form a rhyolitic or granitic magma.

 

Movement and modification

Once magmas form, they tend to rise because they are hotter and lighter than the rocks around them. Basaltic magmas are more fluid than rhyolitic magmas and can move more rapidly. Through fractures, they can move up 100 km (62 mi) from the mantle in one to a few days. Sometimes they move as diapirs—blobs shaped like an upside-down teardrop. Diapirs move more slowly than magmas in fractures because they must force their way through the overlying rocks.  See also: Diapir

Rhyolitic/granitic magmas also move through cracks and as diapirs, but because they are thicker (stickier) they move more slowly than basaltic magmas. In many cases, they do not reach the surface. Although they solidify if they move too slowly, calculations show that if rhyolitic/granitic magmas did not solidify, it might take them more than a million years to move 100 km.

As magmas move upward, they are commonly modified in composition. The three most common processes of modification are fractional crystallization, assimilation, and mixing. Mixing occurs where two magmas of different compositions come together at the same time and place. For example, where basaltic magma melts the bottom of the crust to form a rhyolitic magma, these two magma types may mix to form an andesitic magma.

Assimilation occurs if a magma melts or dissolves the rocks on the sides or walls of a diapir, fracture, or magma chamber, or if pieces of the walls fall into the magma and are absorbed. Generally, the chemistry of wall rocks is different from that of the magma, so assimilated material changes the chemistry of the magma. The chemistry of a magma can also change through fractional crystallization, one of several processes of differentiation. In this process, part (a fraction) of the magma crystallizes to form certain minerals. The minerals have a different chemistry than the whole magma, so when they form they are removing particular chemicals from the magma, changing its composition.

When modified magmas crystallize into rocks, they form rocks of compositions different from those that would have formed from the original magmas. Thus, from the two basic magma types—rhyolitic and basaltic—a whole range of rock types may form.

 

 

Bibliography

  • M. G. Best and E. H. Christiansen, Igneous Petrology, 2001
  • Ali Fazeli = egeology.blogfa.com
  • L. A. Raymond, Petrology: The Study of Igneous, Sedimentary, and Metamorphic Rocks, 2d ed., 2002
  • Ali Fazeli = egeology.blogfa.com
  • H. Sigurdsson (ed.), Encyclopedia of Volcanoes, 2000
  • Ali Fazeli = egeology.blogfa.com
  • J. D. Winter, An Introduction to Igneous and Metamorphic Petrology, 2001
  •  Ali Fazeli = egeology.blogfa.com

Additional Readings

  •  R. G. Cawthorn (ed.), Layered Intrusions, Elsevier, Amsterdam, 1996
  • Ali Fazeli = egeology.blogfa.com
  • R. J. Kirkpatrick, Crystal growth from the melt: A review: Amer. Mineralogist, 60:798–814, 1975
  • Ali Fazeli = egeology.blogfa.com
  • J. A. Pearce and D. W. Peate, Tectonic implications of the composition of volcanic arc magmas, Annu. Rev. Earth Planet. Sci., 23:251–285, 1995
  • Ali Fazeli = egeology.blogfa.com
  • W. S. Pitcher, The Nature and Origin of Granite, 2d ed., Chapman and Hall, London, 1997
  • Ali Fazeli = egeology.blogfa.com
  • H. M. Prichard et al. (eds.), Magmatic Processes and Plate Tectonics, Geol. Soc. London Spec. Publ., no. 76, 1993
  • Ali Fazeli = egeology.blogfa.com
  • A. Streckeisen, To each plutonic rock its proper name, Earth Sci. Rev., 12:1–33, 1976
  • Ali Fazeli = egeology.blogfa.com
  • Igneous Petrology

 
Chemical compositions of some igneous rocks in wt %
Compound
Dunitea
Basaltb
Andesitea
Foid-syenitec
Granitea
Rhyolited
SiO2
40.08
49.8
58.97
59.54
69.22
77.24
TiO2
0.01
2.6
1.04
0.14
0.48
0.20
Al2O3
0.29
14.0
17.17
18.60
15.50
10.81
Fe2O3
0.31
2.5
4.36
2.86
1.03
1.66
FeO
7.62
8.5
2.02
2.09
1.42
0.27
MgO
49.69
7.2
1.51
0.10
0.73
0.33
CaO
0.11
11.3
4.90
1.16
1.93
1.48
Na2O
0.05
2.2
4.23
8.96
4.15
2.59
K2O
0.01
0.62
2.90
4.24
4.42
4.12
P2O5
0.00
0.32
0.51
0.16
0.15
0.06
Other
0.58
0.35
1.55
1.80
0.30
0.65
TOTAL
98.86
99.6
99.26
99.87
99.37
99.80
aF. J. Flanagan (ed.), Descriptions and Analyses of Eight New USGS Rock Standards, U.S. Geol. Surv. Prof. Pap., no. 840, 1976.
bF. J. Flanagan et al., Basalt, BHVO-l, from Kilauea Crater Hawaii, in F. J. Flanagan (ed.), U.S. Geol. Surv. Prof. Pap., no. 840, 1976.
cP. D. Snavely, Jr., et al., Nepheline syenite, STM-1, from Table Mountain, Oregon, in F. J. Flanagan (ed.), U.S. Geol. Surv. Prof. Pap., no. 840, 1976.
dM. H. Staatz and W. J. Carr, Geology and Mineral Deposits of the Thomas and Dongway Ranges, Juab and Tooele Counties, Utah, U.S. Geol. Surv. Prof. Pap., no. 415, 1964.
SOURCE: From L. A. Raymond, Petrology: The Study of Igneous, Sedimentary, and Metamorphic Rocks, 2d ed., 2002.

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