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Igneous Processes I: Igneous Rock Formation, Compositions, and Textures Crustal Abundances of Rock Types Igneous Rocks • Form by the cooling and hardening (crystallization/glassification) of magma. • Most magma crystallizes before it can reach the surface, producing bodies called plutons made of intrusive (plutonic) igneous rock. • Some magma (known as lava) reaches the surface while still at least partially molten, producing volcanic eruptions and extrusive (volcanic) igneous rocks. Classifying Igneous Rocks A magma is a multi-component material with a bulk composition which almost always changes as it moves and cools. • Composition: types and abundances of different minerals and non-minerals • Texture: sizes, shapes, and boundary relationships of the mineral grains and other components (i.e. flow patterns • Method of Cooling: Temperature at eruption and/or rate of cooling in a magma chamber • Magmatic Sources and Pathways: determines final product that appears on Earth’s surface Igneous Composition Various igneous environments will produce magmas which differ in silica content and the abundances of metals such as Fe, Mg, Ca, Na, and K. • Mafic: poor in silica (~50%), rich in Fe, Mg, Ca, poor in Na and K • Felsic: rich in silica (~70%), poor in Fe, Mg, Ca, rich in Na and K • Intermediate: between mafic and felsic (50-70% silica) • Ultramafic: “beyond mafic,” even more mafic than mafic (<50% silica). Composition Pahoehoe flow, Hawaii Magma (or lava if erupted to the surface) is composed of liquid, solid (mineral crystals) and gas. Its composition is largely controlled by its source. Glassy Scoria Obsidian flow, Oregon • Magmas are subdivided largely by silica (SiO2)content. As silica content increases, iron (Fe), magnesium (Mg), and calcium (Ca) content decreases. • Lighter elements, such as sodium (Na) and potassium (K) content follow the silica trends. Chemical compositions are often described in terms of oxides. Recognizing Igneous Composition • Need to be able to identify the common minerals in igneous rocks: olivine, pyroxene, amphibole, micas, feldspars, and quartz. • If grains are not apparent, can fall back on the observation that mafic minerals tend to be dark or green, whereas felsic minerals tend to be light grey or pink. • Note that the above point applies to minerals, not glasses, which can be strongly colored by submicroscopic inclusions. Obsidian is felsic, but is usually black in color. Silicate Behavior Bowen (1925) recognized that mafic minerals tend to have higher melting points and less polymerization (chain-forming) between silicate tetrahedra. Bowen’s Reaction Series summarizes these trends, along with the effects of dissolution (dissolving), precipitation (forming crystals), and solid-state diffusion (of elements between or within crystals) in determining which minerals will be produced for a magma of a given bulk composition. As magma cools, minerals form at different temperatures. Along the discontinuous series, there are distinct “steps” at which minerals will begin crystallizing (and perhaps later dissolving). Along the continuous series, the composition of the plagioclase shifts from Ca-rich to Na-rich. The steps described by Bowen’s Reaction Series may end up interrupted if temperatures fall too quickly. Olivine, for example, may only be partially dissolved before the texture and composition becomes “frozen” when the reaction rates are too slow. Such features are themselves useful in determining the conditions under which the rock formed. The “continuous” replacement of high-temperature Ca-spar by low- temperature Na-spar often is incomplete, since it relies upon very slow diffusion of atoms through already-solid crystals. The result is “zoned” plagioclase feldspar, with Ca-rich centers and Na-rich rims. Changes in Bulk Chemistry • Further complications arise if materials are removed during solidification. • Several fractionation processes: 1) Gravitational settling of initial solids 2) Flow segregation as the magma moves 3) Filter pressing of residual fluid 4) Loss of volatiles (water, gases) along with readily-dissolved elements which don’t fit well in the crystallizing silicate minerals Differentiation of magma can occur from fractional crystallization involving the removal of crystals as they accumulate. The solid phase will have a composition that is relatively more mafic than the remaining melt phase. Animation From Pearson ebook • file:///C:/Users/Patty%20weston/Desktop/C lass%20Docs%202013- 2014/ESS%20101/Pearson%20Animation s/resources/anim/FractionalCrystallization _GL.html • Fractional crystallization Magmatic differentiation of magma by fractional crystallization. Note how the composition of the magma changes as more mineral crystals form. Think of the yellow atoms forming to Fe-Mg silicate minerals that crystallize first during the differentiation process. Think of the red atoms comprising the silica-rich melt. As earlier formed minerals are removed from the magma by fractional crystallization, a greater proportion of the denser elements (Fe and Mg) are removed leaving a residual melt that is more enriched in silica and lighter elements. Minerals and rocks that form later will have a greater proportion of the lighter elements (SiO, Al, Na and K). Gold ore in a quartz vein Several metals of economic interest, such as gold, silver, and copper, do not “fit” well in the growing silicate minerals. Instead, they often are carried away from the magma in aqueous fluids and become deposited in cracks (veins) as pressures and temperatures decrease towards the surface. Silica also is carried this way, precipitating as quartz. Igneous Rock Classification Silica Content and Color • High silica rocks are light in color (pale grey to pink) • Low silica rocks are dark (due to more dark minerals containing Mg and Fe) Low Silica Medium Silica High Silica Basalt Andesite Rhyolite Extrusive Granite Gabbro Diorite Intrusive Silica Content and Viscosity • Even when molten, the silica tetrahedra will polymerize into chains. These will become entangled and thereby inhibit flow. • Over the range of 50-70% silica content, this extent of tangling results in a change of about 7 orders of magnitude in viscosity:10,000,000 times! • Mafic (basaltic) magmas can flow almost like water. Felsic (rhyolitic) magmas are far more sluggish than toothpaste! Mafic lavas often erupt in a gentle fashion. Their low viscosities make it less likely that gas pressure will build to the point of explosiveness. Due to their low viscosities, basaltic composition magma (lava) will flow great distances from its vent. Intermediate (andesitic) and felsic (rhyolitic) lavas often erupt with great violence (as at Pinatubo above) in large part because gases cannot easily escape them. When they do not explode, they instead ooze slowly and do not travel far. Rhyolite/dacite flows will retain steep slope fronts because of their high viscosity. Silica content and Volcano Type • High silica volcanoes are explosive, due to build-up of pressure within volcano. Viscous lava won’t flow far, so volcanoes are tall and pointy (stratovolcanoes). • Low silica volcanoes are non-explosive. Lava is runny, so volcanoes are broad and non-pointy (shield shape) Summary of Trends with Composition Mafic (Basalt/Gabbro) Felsic (Rhyolite/Granite) • Density about 3.3 g/cm3 • Density about 2.7 g/cm3 • Crystallization ~1200°C • Crystallization ~700°C • Low Silica • High Silica • Rock color = dark grey to • Rock color = pale black grey/pink • Low viscosity • High viscosity • Typically mild eruptions • Typically violent eruptions • Shield Volcanoes (low, • Stratovolcanoes (tall, wide) pointy) Igneous Textures • Slow cooling produces large grains, rapid cooling produces small (or no) grains. • Terms for Crystal Size: • Phaneritic: visible to unaided eye, also called coarse-grained. Usually intrusive. • Aphanitic: crystalline, but not visible, also called fine-grained. Usually extrusive. • Glassy: not crystalline. Extrusive. • Porphyritic: coarse grains (phenocrysts) surrounded by fine grains (groundmass). Began crystallizing underground, then erupted and finished solidifying on surface. Extrusive. Gabbro Diorite Granite Phaneritic igneous rocks crystallize slowly (usually underground). Chemical composition also plays a role in determining the specific rock type. Phaneritic grains are distinguishable to the unaided eye. This rock contains quartz (light gray), plagioclase feldspar (white) and biotite (black) crystals. A pink granite is dominated by potassium feldspar (pink crystals), quartz (gray glassy appearance), plagioclase (porcelain white mineral) and biotite (black sheets). Aphanitic rocks contain mineral grains which are too small to distinguish clearly with the unaided eye. Same magnification as the previous image. Obsidian has a glassy texture. It may contain a few isolated mineral grains or even an abundance of submicroscopic crystal “seeds” (crystallites), but it is mostly amorphous, lacking the long-range order of crystal structure. Note the characteristic concoidal fracture diagnostic of obsidian. Porphyritic rock is partially coarse and partially fine. The large phenocrysts formed first, slowly, in the subsurface, whereas the groundmass crystallized quickly after eruption onto the surface. This is often referred to a two-stage cooling process Other Igneous Textures Pyroclastic “Broken by Fire”: • Violent volcanic eruptions produce an explosive spray of lava which hardens (at least partially) while in flight. • The resulting fragments may or may not weld
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