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  • Print publication year: 2012
  • Online publication date: November 2012

Chapter 8 - How do igneous rocks form?

Summary

Igneous rocks are those formed by the solidification of molten rock. This molten material, which we call magma, is formed at depth in the Earth and rises toward the surface, where it cools and solidifies, either beneath the surface, where it usually has time to crystallize, or on the surface as volcanic rocks, where cooling may be rapid enough to form glass. We classify igneous rocks on the basis of the minerals they contain, which are determined by the composition of the magma. Magma compositions are determined in the source region by the nature of the rock that undergoes partial melting, but they can be modified during ascent and solidification, especially in large magma chambers, where solidification can take thousands of years. Throughout geologic time, the rise of magmas from the mantle has slowly generated the Earth’s crust, whose composition is, therefore, determined by the composition of magmas.

We can think of an igneous rock as the end product of a lengthy series of processes, all of which play important roles in determining the rock’s composition, its appearance and mineral makeup, and the shape and position of igneous bodies it forms. It is these processes that we discuss in this chapter, leaving the mode of occurrence and classification of igneous rocks to Chapter 9. Many things can happen to magma between its formation as a partial melt in the source region and its eventual solidification to form a rock. In Box 8.1, we trace the three main types of magma from their sources to their final resting places. These magmas solidify to form the important rock types, basalt, andesite, and granite, which were introduced in Chapter 2. The first two of these are commonly associated with divergent and convergent plate boundaries, respectively.

ONLINE RESOURCES
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FURTHER READING
Bowen, N. L. 1928 The Evolution of the Igneous RocksPrinceton University PressPrinceton, NJ
Philpotts, A. R.Ague, J. J. 2009 Principles of Igneous and Metamorphic PetrologyCambridge University PressCambridge
Bowen, N. L. 1913 The melting phenomena of the plagioclase feldsparsAmerican Journal of Science 34 577
Bowen, N. L.Anderson, O. 1914 The binary system MgO-SiO2American Journal of Science 37 487
Bowen, N. L.Schairer, J. F. 1935 The system MgO-FeO-SiO2American Journal of Science 29 151
Bowen, N. L.Tuttle, O. F. 1950 The system NaAlSi3O8-KAlSi3O8-H20Journal of Geology 58 489
Burnham, C. W. 1979 The importance of volatile constituentsThe Evolution of the Igneous Rocks: Fiftieth Anniversary PerspectivesYoder, H. S.Princeton University PressPrinceton, NJ439
Dixon, J. E.Stolper, E. M.Holloway, J. R. 1995 An experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids. Part I: Calibration and solubility modelsJournal of Petrology 36 1607
Greig, J. W.Barth, T. F. W. 1938 The system Na2O·Al2O3·5SiO2 (nephelite, carnegieite)– Na2O·Al2O3·6SiO2 (albite)American Journal of Science 35A 93
Holtz, F.Behrens, H.Dingwell, D. B.Johannes, W. 1995 H2O solubility in haplogranitic melts: Compositional, pressure, and temperature dependenceAmerican Mineralogist 80 94
King, S. D. 1995 227
Kushiro, I.Syong, Y.Akimoto, S. 1968 Melting of a peridotite nodule at high pressures and high water pressuresJournal of Geophysical Research 73 6023
Moore, G.Vennemann, T.Carmichael, I. S. E. 1998 An empirical model for the solubility of water in magmas to 3 kilobarsAmerican Mineralogist 83 36
Peck, D. L.Moore, J. G.Kojima, G. 1964 Temperatures in the crust and melt of Alae lava lake, Hawaii, after the August 1963 eruption of Kilauea volcano–a preliminary reportU.S. Geological Survey Professional Paper 501D 1
Petford, N. 2003 Rheology of granitic magmas during ascent and emplacementAnnual Review of Earth and Planetary Sciences 31 399
Sclater, J. G.Parsons, B.Jaupart, C. 1981 Oceans and continents: similarities and differences in the mechanisms of heat lossJournal of Geophysical Research 86 11535
Silver, L.Stolper, E. M. 1989 Water in albitic glassJournal of Petrology 30 667
Takahashi, E. 1986 Melting of a dry peridotite KLB-1 up to 14 GPa: implications on the origin of peridotitic upper mantleJournal of Geophysical Research 91 9367
Tuttle, O. F.Bowen, N. L. 1958 Origin of granite in the light of experimental studies in the system NaAlSi3O8– KalSi3O8–SiO2–H2OGeological Society of America Memoir,
Weill, D. F.Hon, R.Navrotsky, A. 1980 The igneous system CaMgSi2O6-CaAl2Si2O8-NaAlSi3O8: variations on a classic theme by BowenPhysics of Magmatic ProcessesHargraves, R. B.Princeton University PressPrinceton, NJ49
Wright, T. J.Ebinger, C.Biggs, J. 2006 Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episodeNature 442 291
Wyllie, P. J. 1977 Crustal anatexis: an experimental reviewTectonophysics 13 41