Igneous Processes I: Igneous Rock Formation, Compositions, and Textures

Transcription

1 Igneous Processes I: Igneous Rock Formation, Compositions, and Textures

2 Crustal Abundances of Rock Types

3 Igneous Rocks Form by the cooling and hardening (crystallization/glassification) of magma. There are 2 categories: ( ): Most magma crystallizes before it can reach the surface, producing large rock deposits called. ( ): Some magma (aka lava) reaches the surface at least partially molten, producing eruptions and volcanoes.

4 Classifying Igneous Rocks Classification of igneous rocks is based on the following four factors: 1. : types and abundances of different minerals and non-minerals. A magma evolves and changes composition as it moves and cools. 2. : sizes, shapes, and boundary relationships of the mineral grains and other components (i.e. flow patterns) 3. Method of : Temperature at eruption and/or rate of cooling in a magma chamber. This will mostly affect the texture and types of minerals present. 4. Magmatic : determines final product that on Earth s surface appears

5 1. 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. Glassy Scoria Obsidian flow, Oregon

6 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. : poor in silica (SiO 2 ) (~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 : between mafic and felsic : beyond mafic, even more mafic than mafic (rare at Earth s surface) Mafic Felsic

7 Igneous Composition

8 (Mafic) (Intermediate) (Felsic) Magmas are subdivided largely by 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.

9 Recognizing Igneous Composition Common minerals in igneous rocks: olivine, pyroxene, amphibole, micas, feldspars, and quartz. Mafic minerals tend to be, whereas felsic minerals tend to be. Exception: volcanic, such as obsidian. The color of the glass is misleading. Obsidian is felsic, but is usually black in color.

10 Silicate Behavior Bowen (1925) recognized that mafic minerals tend to have higher points and less polymerization ( ) between silicate tetrahedra. Bowen s Reaction Series summarizes these trends, along with the effects of dissolution, precipitation, and solid-state diffusion in determining which minerals will be produced for a magma of a given bulk composition.

11 As magma cools, minerals form at different. 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.

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13 The steps described by Bowen s Reaction Series may be 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. Microscopic vies of an olivine crystal with partially dissolved edges. This indicates that the temperature of the magma at the time this lava was erupted was too low for olivine to be stable. Such features are themselves useful in determining the conditions under which the rock formed.

14 The continuous replacement of high-temperature Ca-spar by lowtemperature 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.

15 Changes in Bulk Chemistry Fractionation: materials are during solidification. This can change the composition of the melts in addition to simple cooling. Several fractionation processes: 1) Gravitational of initial solids 2) Flow as the magma moves 3) Filter pressing of residual fluid 4) Loss of (water, gases) along with readily-dissolved elements which don t fit well in the crystallizing silicate minerals 5) that partially empty the magma chamber

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17 This makes the remaining melt more, while the settled crystals at the bottom of the chamber are More Felsic More Mafic Differentiation of magma can occur from fractional crystallization involving the more. removal of crystals as they accumulate. The solid phase will have a composition that is relatively more mafic than the remaining melt phase.

18 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.

19 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 and become deposited in cracks ( ) as pressures and temperatures decrease towards the surface. Silica also is carried this way, precipitating as.

20 Igneous Rock Classification Ultra- Mafic Mafic Felsic

21 Silica Content and Color silica rocks (felsic) are light in color (pale grey to pink). silica rocks (mafic) are dark (due to more dark minerals containing Mg and Fe) Low Silica Medium Silica High Silica Extrusive Basalt Andesite Rhyolite Intrusive Gabbro Granite Diorite

22 Silica Content and Viscosity Even when molten, the silicate tetrahedra will form. These will become entangled and inhibit. Over the range of silica content, this extent of tangling results in a change of about seven orders of magnitude in viscosity (10,000,000x). Mafic (basaltic) magmas can flow almost like water. Felsic (rhyolitic) magmas are far more sluggish than toothpaste.

23 Silica Content and Viscosity

24 Silica content and Volcano Type High silica volcanoes are, due to build-up of pressure within volcano. Viscous lava won t flow far, so volcanoes are tall and pointy ( ). Low silica volcanoes are non-explosive. Lava is runny, so volcanoes are broad and non-pointy ( shape)

25 Mafic lavas often erupt in a gentle fashion. Their low viscosities make it less likely that will build to the point of explosiveness. Due to their low viscosities, basaltic composition magma (lava) can flow great distances from its vent.

26 Intermediate and felsic lavas often erupt with great violence in large part because gases cannot easily escape them. When they do not explode, they instead ooze slowly and do not travel far.

27 Rhyolite/dacite flows will retain steep slope fronts because of their high viscosity.

28 Summary of Trends with Composition Mafic (Basalt/Gabbro) Density about 3.3 g/cm 3 Felsic (Rhyolite/Granite) Density about 2.7 g/cm 3 Crystallization ~ o C Silica (SiO 2 ) Rock color = dark grey to black viscosity Typically eruptions Volcanoes (low, wide) Crystallization ~ o C Silica Rock color = pale grey/pink viscosity Typically eruptions (tall, pointy)

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30 2. Igneous Textures Slow cooling produces large grains. Rapid cooling produces small (or no) grains. Terms for Crystal Size: Phaneritic: visible to unaided eye, coarse-grained. Usually. Aphanitic: crystalline, but crystals not visible, finegrained. Usually. Glassy: not crystalline.. Porphyritic: coarse grains (phenocrysts) surrounded by fine grains (groundmass). Began crystallizing underground, then erupted and finished solidifying on surface..

31 Composition: Mafic Intermediate Felsic Rock Name: Gabbro Diorite Granite Phaneritic igneous rocks crystallize slowly (usually underground). The rock name depends on the chemical composition and whether it is intrusive or extrusive.

32 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.

33 Snowflake obsidian Obsidian with conchoidal fracture Obsidian has a glassy texture because it cools to form crystals. It may contain isolated mineral grains (i.e. snowflake obsidian) or even an abundance of submicroscopic crystal seeds (crystallites), but it is mostly amorphous, lacking the long-range of crystal structure. Obsidian usually exhibits conchoidal fracture (Obsidian has often been used to make arrowheads).

34 Porphyritic rock is partially coarse and partially fine. The large (crystals within the fine-grained matrix) formed first, slowly, in the magma chamber. Then the rock was erupted before it fully crystallized, so the matrix (groundmass) is very fine-grained. This is often referred to a cooling. process

35 Sample Igneous Textures and Where they Form.

36 Other Igneous Textures Pyroclastic Broken by Fire : Violent volcanic eruptions produce an explosive spray of lava which hardens (at least partially) while in. The resulting fragments may or may not to one another upon landing, but usually retain the outlines of their initial crusts. Individual particles range from dust-sized, called, to building-sized, called, and are typically a mixture of minerals and glass.

37 The ash and other volcanic derived clasts can become welded together by heat to form fine-grained tuff or coarse-grained volcanic breccia. Volcanic ash (tephra) derived from the Mount Mazama (Crater Lake, Oregon) eruption 6800 years ago. thin section of a welded tuff (ash)

38 Volcanic Bombs: molten rock aerodynamically shaped due lava while in flight. Can be any size Volcanic Breccia Hand Sample of a breccia

39 Other Igneous Textures Vesicular: As a magma approaches the surface, it undergoes decompression and cooling. This its ability to hold various gases (H 2 O, CO, CO 2, etc.) in solution. These gases will separate as which will either escape or remain trapped as the magma hardens around them. Trapped bubbles are called vesicles. If the frozen bubbles are large enough, they can later contain, which can crystallize and form geodes.

40 Pumice or scoria (darker) form when gas bubbles are in rapidly cooling pyroclastic materials. The rocks are glassy and frothy. 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). Scorias can be a deep red when the iron in the mafic lava is oxidized by the escaping volatiles.

41 Other Igneous Textures Aa Flow (Think about what you would say if you had to walk on this aa flow (ah, ah). Pahoehoe (ropey textured) basalt flows have a viscosity than aa (blocky textured) flows, which have degassed and. Pahoehoe Flow (Smooth word, smooth flow). Pele's is abundant around Halema'uma'u Crater, and originates from the active, spattering lava lake. When the lava spatters into the air, some of it stretches into thin fibers, which quickly cool into amber strands of glass, and can be carried off by the wind. (USGS/Hawaiian Volcano Observatory)

42 Other Igneous Textures Pillow Basalts: when basaltic lava erupts underwater or flows into water, it will form into pillow-like shapes, often with a rind, since the exterior of the pillow is in contact with cold water and rapidly.

43 Other Igneous Textures Columnar Jointing: fracture pattern into the shape of hexagonal columns that happens when lava (usually basaltic) cools and. The columns will be to the cooling surfaces, such as the air and ground.

44 Interaction with continental material is required for the production of magmas. 4. Typical Magmatic Sources The mantle is ultramafic. Extensive melting can produce ultramafic magmas, but this is fairly rare. Routine melting produces magmas. Partial melting of subducting oceanic crust (mafic) and its associated sediments produces mafic and magmas.

45 Typical Magmatic Sources

46 Sources of Magma In nearly all of the crust and mantle, temperatures are too for melting to occur at the surrounding. Magma production occurs when: Warm rock travels (decompression melting), as at divergent zones and hotspots, or Cold rock is forced downwards and absorbs from its new surroundings, as at subduction zones

47 in the melt will lower the melting temperature of the rock. Mafic Magma Formation Mafic magma forms from a partial melt of the ultra-mafic asthenosphere, which occurs at a depth ( km). Melting temperature is dependent upon pressure ( ) and of the rock involved. Only a small fraction ( ) of the rock actually melts the portion with the lowest melting point. The melt is a -density mafic magma from an ultramafic starting material. This magma will tend to rise due to buoyancy.

48 Mafic magma forms at four different tectonic settings. Mafic (basaltic) magma is always derived from a partial melt of the ultramafic asthenosphere.

49 Felsic Magma Formation Felsic (granitic) magma forms from a partial melt of crust, 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 ). Melting occurs at a depth of km within continental crust.

50 Felsic Magma Formation at Continent- Continent Boundaries Granitic composition magma is produced at continental collision margins. As the continental crust thickens it begins to partially melt at depth. Granite intrusions (felsic ) form below the mountain belts. As collisional tectonic mountain ranges are uplifted, the overlying marine sedimentary and metamorphic rocks are eroded exposing the underlying granitic plutons.

51 Granitic composition magma reaches the surface in Yellowstone Park because the continental crust is being melted closer to the surface by upwelling magma generated from a in the asthenosphere.

52 Intermediate Magma Formation Intermediate ( ) composition magma can crystallize below the surface beneath subduction zones and create large coarse-grained plutonic bodies. Compositions can range from granite to diorite. El Capitan 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

53 Andesitic magma is produced from a partial melt of crust along zones. Introduction of forced out of the subducting plate lowers the melting temperature of the upper mantle, which rises and partially melts the overlying crust. 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 dacites are more silica rich than Mt. Rainier andesite, likely due to continental source.