آندزیت و نحوه تشکیل آن - Andesite

 

Andesite             

A typical volcanic rock erupted from a volcano associated with convergent plate boundaries. The process of subduction, which defines convergent plate boundaries, results in the sinking of oceanic lithosphere beneath either oceanic lithosphere or continental lithosphere. In the oceanic basins, arcuate chains of oceanic islands, such as the Aleutian Islands, the Marianas Islands, and the Izu-Bonin Islands, form oceanic island arcs. The islands of Japan, much of western North and South America, and the Taupo Volcanic Zone of Northern New Zealand (Fig. 1) form continental margin arcs. Andesites are common to all of these settings and are the archetypal volcano of the Pacific “Ring of Fire.”  See also: Lithosphere; Plate tectonics; Volcano

Mount Ruapehu (2797 m; 9140 ft) in the North Island, New Zealand, erupting July 1996. This active andesite...

 

Fig. 1  Mount Ruapehu (2797 m; 9140 ft) in the North Island, New Zealand, erupting July 1996. This active andesite volcano is at the southern end of the Taupo Volcanic zone continental margin arc system. Tephra (volcanic ash) is shown falling from the eruptive column to blanket the landscape to the north (left) of the volcano.

 

Andesite volcanoes have been responsible for some of the most destructive and globally significant eruptions of historic and modern times. Examples include the 1815 eruption of Tambora and the 1883 eruption of Krakatau in Indonesia, the 1902 eruption of Mount Pelée in the West Indies, the 1980 eruption of Mount St. Helens in the United States, the 1991 eruption of Pinatubo in the Philippines, and the 1995–1998 eruption of the Soufriere Hills in the West Indies. The activity of most active andesite volcanoes is monitored continuously, but loss of life can still occur, because eruptions may occur with very little warning or precursor activity. For example, mud flows (lahars) from Nevado del Ruiz volcano in Colombia caused approximately 25,000 deaths in November 1985.

Andesites are mostly dark-colored vesicular volcanic rocks that are typically porphyritic (containing larger crystals set in a fine groundmass). Phenocrysts (the larger crystals) are composed of plagioclase; calcium-rich, calcium-poor pyroxene; and iron-titanium oxides set in a fine-grained, frequently glassy, groundmass (Fig. 2). Some andesites contain phenocrysts of olivine, and some contain amphibole and biotite; these latter rocks generally contain more potassium. The porphyritic nature of andesites is derived from a complicated history of magmatic crystallization and evolution as the melts rise toward the surface from deep in the Earth. Phenocryst minerals commonly are strongly zoned and show evidence for disequilibrium during growth, consistent with an origin involving crystal fractionation and mixing processes. Andesites are readily classified in terms of their silicon dioxide (SiO2) content, between 52 and 63 wt %, and potassium oxide (K2O) content at a given SiO2 content (Fig. 3a; table). They can also be readily discriminated on a total alkali versus SiO2 diagram (the TAS diagram; Fig. 3b). Most andesite volcanoes erupt lavas and tephras (volcanic ash), which range in composition from basaltic andesite to dacite. Eruptions are often explosive, reflecting the relatively high water and gas content of the magmas. During an eruption, a column may rise tens of kilometers above the volcano, leading to stratospheric dispersion of tephra and volcanic gases and generation of pyroclastic flows (for example, Pinatubo). Pyroclastic flows are a particular feature of andesite-type volcanism and are among the most dangerous of volcanic hazards. Indeed, it was a pyroclastic flow from Mount Pelée on the island of Martinique in 1902 the pyroclastic flow engulfed the town of St. Pierre, killing approximately 28,000 people. More recently, in 1997, pyroclastic flows from the still-active Soufriere Hills volcano on the island of Montserrat in the Antilles killed 19 people and similar eruptions on Mount Merapi volcano on Java, Indonesia, killed approximately 150 people in November 2010.  See also: Basalt; Lava; Pyroclastic rocks. Thin section of an andesite lava from Mount Ruapehu, showing phenocrysts of plagioclase (elongate gray...

 

Fig. 2  Thin section of an andesite lava from Mount Ruapehu, showing phenocrysts of plagioclase (elongate gray mineral, right of center, with multiple twinning) and pyroxene (blue, orange, and red crystals, left of center) in a glassy groundmass. The phenocryst minerals crystallized from the cooling magma during ascent to the surface, and the glassy groundmass quenched on eruption. Field of view is 7 mm across crossed polars.

 

Igneous rock and andesite classification. (a) Igneous rocks, in terms of SiO2 versus K2O. Note that...

 

Fig. 3  Igneous rock and andesite classification. (a) Igneous rocks, in terms of SiO2 versus K2O. Note that a continuum exists between basalt, basaltic andesite, andesite, dacite, and rhyolite, and that it is possible to subdivide igneous rocks into low-, medium-, and high-K varieties. The data shown are andesites from Mount Ruapehu, which classify in this diagram as medium-K basaltic andesites and andesites. (b) Andesitic rocks, in terms of SiO2 versus (Na2O + K2O), showing a part of the total alkali versus silica (TAS) diagram. (After M. J. Le Bas et al., A chemical classification of volcanic rocks based on the total alkali-silica diagram, J. Petrol., 27:745–750, 1986)

 

Less commonly, rocks of andesite and basaltic andesite composition are associated with sites of intraplate volcanism, such as Iceland, Galápagos, and Hawaii, and are unrelated to subduction or orogenic processes. These rocks have basaltic-andesite to andesite composition (that is, in the composition range of 52–63% SiO2) and have been called icelandite and hawaiite. They have formed in response to fractional crystallization of basaltic magma.

It is generally recognized that most andesites cannot be direct partial melts of peridotitic mantle. As such, andesites are evolved magmas and the end product of a plethora of processes, including crystal fractionation, magma mixing, mingling, assimilation, and storage, which have acted on primary basaltic magmas produced by partial melting in the mantle wedge (Fig. 4).  See also: Magma

Schematic cross section through a typical subduction system. The major components of a convergent plate...

 

Fig. 4  Schematic cross section through a typical subduction system. The major components of a convergent plate boundary are the arc lithosphere, the subducting lithospheric slab, and the mantle wedge. Note that arc volcanoes overlie the subducting slab by around 110 km (68 mi); the volume of the mantle wedge is determined by arc lithosphere thickness and slab dip; and the mantle wedge is the principal site of magma production. After magmas form by partial melting, they can evolve further by fractional crystallization, mixing, and storage in reservoirs over a range of depths.

 

In more detail, the process of subduction recycles oceanic lithosphere back into the deep Earth; and as such, it complements the process of sea-floor spreading, which generates new basalt crust and oceanic lithosphere at the mid-ocean ridges. The zone, along which subduction takes place, is demarcated at the Earth's surface by a deep oceanic trench and within the Earth by an increase of earthquake intensity along an inclined plane, the Wadati-Benioff zone. This zone derives from the fact that cold, brittle material (the subducting oceanic lithosphere) is pushing into warmer upper mantle, and the earthquakes reflect the contrasting physical properties between the two mediums. The subduction process takes place at rates varying from a few tens to a few hundreds of millimeters per year and at angles of around 20° (shallow subduction) to nearly 90° (steep subduction). Typically, andesite volcanoes are located about 110 km (68 mi) above the surface of the subducting plate (Fig. 4). Between the surface of the subducting plate, which geologists refer to as the subducting slab, and the overlying plate is the mantle wedge, where basaltic melts form by the process of partial melting. Partial melting involves the generation of melt by dissolution of minerals, leaving a residual, more refractory, mineral assemblage. The melts aggregate and then move toward the surface by virtue of their lower density and viscosity. The melting occurs because the subducting slab, which may have spent many millions of years on the ocean floor as oceanic crust, contains minerals with seawater locked into their structure. When the minerals are subjected to higher pressure and temperature as subduction proceeds, chemical reactions in the minerals in the slab lead to loss of water and other volatile constituents, which diffuse upward into the mantle wedge, together with an array of readily soluble elements such as potassium, rubidium, cesium, barium, and uranium. The mantle wedge becomes relatively enriched in these elements. The introduction of water is especially significant as it depresses the melting temperature and facilitates partial melting among constituent mineral grains. Importantly, the mantle wedge is recognized as the source of most arc-related magmas. On rare occasions, where subducting oceanic lithosphere is young, is still hot, and is subducting rapidly, the slab itself may experience partial melting, generating a unique suite of high-silica, low-potassium rocks of andesite composition, called adakites (after Adak Island in the Aleutian island arc chain). Still another unique suite of andesitic rocks, called boninites, which have high-magnesium and typically low-titanium contents, lacking plagioclase phenocrysts but with magnesium-rich, calcium-poor pyroxene phenocrysts, are associated with some suites of island arc rocks (for example, the Izu-Bonin Island arc, which is the type area), where water-saturated melting of depleted mantle peridotite produced partial melts of high-silica and high-magnesium content.  See also: Subduction zones

The bulk composition of typical andesites is similar to that of continental crust (see table), contrasting with oceanic crust, which is basaltic. Over geological time, subduction and the resulting andesite volcanism has proved a means of generating new continental crust, possibly at continental margin arc systems.

 

Bibliography

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    * P. W. Francis, Volcanoes: A Planetary Perspective, Oxford University Press, New York, 1993

    * J. A. Gamble et al., A fifty year perspective of magmatic evolution on Ruapehu Volcano, New Zealand: Verification of open system behaviour in an arc volcano, Earth Planet. Sci. Lett., 170:301–314, 1999

    * J. B. Gill, Orogenic Andesites and Plate Tectonics, Springer-Verlag, New York, 1981

    * A. J. R. Kent et al., Preferential eruption of andesitic magmas through recharge filtering, Nat. Geosci., 3: 631–636, 2010 DOI:10.1038/ngeo924

    * M. J. Le Bas et al., A chemical classification of volcanic rocks based on the total alkali-silica diagram, J. Petrol., 27:745–750, 1986

    * A. R. McBirney, Igneous Petrology, Jones and Bartlett, Sudbury, MA, 1993

    * O. Reubi and J. Blundy, A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites, Nature, 416: 1269–1274, 2009 DOI:10.1038/nature08510

    * R. L. Rudnick, Making continental crust, Nature, 378:571–578, 1995 DOI:10.1038/378571a0

    * B. M. Wilson, Igneous Petrogenesis: A Global Tectonic Approach, Springer, New York, 1989

    * [e-mail] = egeology.blogfa.com