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Home , Felsic, Igneous rock, Igneous textures, Intermediate composition, Magma

Igneous Processes I: Igneous Formation, Compositions, and Textures Crustal Abundances of Rock Types Igneous Rocks • Form by the cooling and hardening (/glassification) of .

• Most magma crystallizes before it can reach the surface, producing bodies called plutons made of intrusive (plutonic) .

• Some magma (known as ) 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 and non-minerals • Texture: sizes, shapes, and boundary relationships of the grains and other components (i.e. flow patterns • Method of Cooling: Temperature at eruption and/or rate of cooling in a • Magmatic Sources and Pathways: determines final product that appears on ’s surface Igneous Composition Various igneous environments will produce which differ in silica content and the abundances of metals such as Fe, Mg, Ca, Na, and K. • : poor in silica (~50%), rich in Fe, Mg, Ca, poor in Na and K • : 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,

Magma (or lava if erupted to the surface) is composed of , solid (mineral ) and . Its composition is largely controlled by its source.

Glassy flow, Oregon • Magmas are subdivided largely by silica (SiO2)content. As silica content increases, (Fe), (Mg), and (Ca) content decreases. • Lighter elements, such as (Na) and (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: , , , , , and . • 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. 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 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 , 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 (water, ) 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 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 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 Extrusive

Granite

Diorite Intrusive Silica Content and • 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 often erupt in a gentle fashion. Their low make it less likely that gas 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/ flows will retain steep slope fronts because of their high viscosity. Silica content and 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 ( shape) Summary of Trends with Composition

Mafic (/Gabbro) Felsic (Rhyolite/) • 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) • Slow cooling produces large grains, rapid cooling produces small (or no) grains. • Terms for 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. • : coarse grains () surrounded by fine grains (groundmass). Began crystallizing underground, then erupted and finished solidifying on surface. Extrusive. Gabbro 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 (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 to one another upon landing, but usually retain the outlines of their initial crusts. • Individual particles range from dust-sized, called ash, to building-sized, called bombs, and are typically a mixture of minerals and glass. A large pyroclastic eruption of in the Philippines (1992). The ash and other volcanic derived clasts can become welded together to form fine-grained or coarse-grained volcanic . () derived from the Mount Mazama (Crater Lake, Oregon) eruption 6800 ago. Welded tuffs in : The triangular fragments are created when the magma between gas bubbles is blown apart. The fragments then get flattened and welded together from the heat and weight of the flow. Hand Sample

Volcanic breccia forms from a welded, mixture of large, angular volcanic clasts within a of fine ash. This photo was taken on Lipari Island, Italy by Raymond Coveney. Volcanic Bombs: molten rock aerodynamically shaped due lava freezing while in flight. Other Igneous Textures

Vesicular: As a magma approaches the surface, it undergoes decompression and cooling. This decreases its ability to hold various gases

(H2O, CO, CO2, etc.) in solution.

These gases will separate as bubbles which will either escape or remain trapped as the magma hardens around them. Trapped bubbles are called vesicles. (shown) or scoria (darker) form when gas bubbles are trapped in rapidly cooling pyroclastic materials. The rocks are glassy and frothy. Scoria often forms in basaltic magmas where gases are escaping— often near the tops of flows. Bubble size can get quite large, since the lower viscosity lavas allow gases to coalesce into larger bubbles compared to a felsic lava (which will form pumice) can be a deep red when the iron in the mafic lava is oxidized by the escaping gases. Other Igneous Textures

Aa Flow (Think about what you would say if you had to walk on this aa flow (ah, ah).

Pahoehoe Flow (Smooth word, smooth flow).

Pahoehoe (ropey textured) basalt flows have a lower viscosity than aa (blocky textured) flows, which have degassed and cooled. Other Igneous Textures

Pillow : when basaltic lava erupts underwater or flows into water, it will form into pillow-like shapes, often with a glassy rind, since the exterior of the pillow is in with cold water and freezes rapidly. Other Igneous Textures : fracture pattern into the shape of hexagonal columns that happens when lava (usually basaltic) cools and contracts. The columns will be perpendicular to the cooling surfaces, such as the air and ground. Columnar Jointing at Devil’s Postpile, near Mammoth Lakes, CA. The direction of the columns changes near the front of the flow Typical Magmatic Sources • The is ultramafic. Unusually extensive melting will produce ultramafic magmas, but “routine” produces mafic magmas. • Partial melting of subducting oceanic (mafic) and its associated sediments produces mafic and intermediate magmas. • Interaction with continental material is required for the production of felsic magmas. Sources of Magma • In nearly all of the crust and mantle, temperatures are too low for melting to occur at the surrounding pressures.

• Magma production occurs when:

– warm rock travels upwards (decompression melting), as at divergent zones and hotspots, or

– cold rock is forced downwards and absorbs heat from its new surroundings, as at zones Mafic Magma Formation Mafic magma forms from a partial melt of the asthenosphere, which occurs at a depth (100-350 km) where the intersects the melting temperature curve for upper mantle rock ( ). •Note that the geothermal gradient is dependent upon pressure (depth), while the melting temperature curve is dependent upon pressure (depth) and composition of the rock involved. The curve is for a “dry” melt, with no water involved. •Even in the region of melting, only a small fraction (1-5%) of the rock actually melts– this is the portion with the lowest melting point. The product is a relatively low-density mafic magma from an ultramafic starting material. This magma will tend to be displaced upwards by buoyancy. Mafic magma forms at four different tectonic settings. Mafic (basaltic) magma is always derived from a partial melt of the ultramafic asthenosphere. Felsic Magma Formation Felsic (granitic) magma forms from a partial melt of , which contains dissolved water. Dissolved water content in a magma reduces its melting temperature with increasing pressure (water molecules will inhibit the silicate tetrahedra from forming bonds). Note that the melting temperature curve for a wet granitic melt increases with decreasing pressure (opposite of basaltic dry melt). Melting occurs at a depth of 35- 45 km within continental crust. As granitic magma rises it solidifies (point X) as its melting temperature increases while the geothermal gradient (actual temperature) decreases. Granitic composition magmas rarely reach the surface as volcanic rhyolite flows because of the high water content and corresponding increase in melting temperature as it rises towards the surface. Felsic Magma Formation Granitic composition magma is produced at margins. As the continental crust thickens it begins to partially melt at depth. Igneous intrusions (plutons) form below the belts. is rare in continental collision boundaries. As collisional tectonic mountain ranges are uplifted the overlying marine sedimentary and metamorphic rocks are eroded exposing the underlying granitic plutons. The granitic rocks of New Hampshire and Vermont represent old granitic plutons that were intruded when the Appalachian formed 300 million years ago as North American collided with proto-European continent. Granitic rock excavated from a quarry in Barre, Vermont formed as plutons beneath the Appalachian Mountains when North Africa collided with eastern North America 300 million years ago. Roof pendant of remnant “” (dark ) lies above the intruded (light colored ). Granitic composition magma reaches to the surface in Yellowstone Park because the continental crust is being heated closer to the surface by upwelling magma generated from a in the asthenosphere. The Yellowstone (Wyoming) formed following a very large eruption ~600,000 years ago. The rhyolite flows are very viscous and internal gas pressures can be very high Intermediate Magma Formation Intermediate (andesitic) composition magma can crystallize below the surface beneath subduction zones and create large coarse-grained plutonic bodies. Compositions can range from granite to diorite. shown on the left is part of the Sierra Nevada intrusive complex that formed over 90 million years ago when a subduction zone existed along the margin of California. The plutonic bodies comprising the Sierra Nevada are similar in origin to the plutonic bodies forming under the modern Cascades. Grano-diorite rock from the Sierra Nevada Andesitic magma is produced from a partial melt of along subduction zones. Introduction of water forced out of the subducting plate lowers the melting temperature of the upper mantle, which rises and partially melts the overlying asthenosphere. In an ocean-continental convergent margin it may mix with partially melted continental crust, increasing the magma’s silica content (becomes more felsic). Mount St. Helens are more silica rich than Mt. Rainier andesite, likely due to continental source. Mt. St. Helens is composed of dacitic flows. Dacite is slightly more felsic (has greater silica content) than andesite, but more mafic (higher Fe and Mg content) than rhyolite.