Lab%203---Igneous%20Rocks%20AND%20Volcanic%20Hazards(2)

Lab%203---Igneous%20Rocks%20AND%20Volcanic%20Hazards(2) -...

Info iconThis preview shows pages 1–14. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 10
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 12
Background image of page 13

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 14
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: LABORATORY FIVE Igneous Rocks and Volcanic Hazards -CONTR|BUTING AUTHORS? Harold E. Andrews - Wellesley College James R. Besancon - Wellesley College Claude E. Bolze - Tulsa Junior College Margaret D. Thompson - Wellesley College OBJECTIVES A. Explore the geometry and origin of some intru— sive and extrusive bodies of igneous rock. B. Be able to describe and interpret textural fea- tures of igneous rocks. C. Be able to describe compositional features of ig- neous rocks (mineralogy, color index). D. Be able to classify common igneous rocks based on their texture, mineralogy, and color index. E. Infer the origin of common igneous rocks. F. Explore Internet resources to investigate hazards and human risks associated with active modern volcanoes. MATERIALS Pencil, eraser, laboratory notebook, hand magnifying lens (optional), mineral-identification tools of your choice, metric ruler and a chart for visual estimation of percent (from GeoTools Sheet 1 at back of manual), and samples of igneous rocks (obtain as directed by your instructor). INTRODUCTION Igneous rocks form when molten rock (rock liquefied by intense heat and pressure) cools to a solid state. When the molten rock cools, it always forms a mass of intergrown crystals and/ or glass. Therefore, all igneous rocks and fragments of igneous rocks have crystalline or glassy textures. Even volcanic ash is microscopic fragments of igneous rock (mostly volcanic glass pul- verized by an explosive volcanic eruption). PART 5A: IGNEOUS PROCESSES AND ROCKS Molten (heated until liquefied) rock exists in isolated bodies below Earth’s surface, where it is called magma. In addition to its liquid molten rock portion, or melt, magma also contains dissolved gases (e.g., water, carbon dioxide, sulfur dioxide) and tiny crys- tals that may grow in size or abundance as the magma cools. Magma is under great pressure (like a bottled soft drink that has been shaken) and is less dense than the rocks that confine it. Like the blobs of heated “lava” in a lava lamp, the magma tends to rise and squeeze into Earth’s cooler crust along any frac- tures or zones of weakness that it encounters. Abody of magma that pushes its way into Earth’s crust is called an intrusion, and it will eventually cool to form a coarse-grained intrusive igneous rock comprised of visible mineral crystals. If an intrusion of magma ap— proaches Earth’s surface, then the decrease in pres- sure allows its dissolved gases to separate from the magma as gas bubbles. This is like the bubbles of car- bon dioxide that form when you open (release the pressure from) a pressurized bottle of beer or soft drink (containing dissolved carbon dioxide). When this happens to magma, the bubbly magma is called 91 92 ‘ LaboratoryFive lava, which may erupt (extrude) onto Earth’s surface at volcanoes and cool to form a fine-grained extrusive igneous rock comprised of tiny crystals and / or glass. Intrusions have different sizes and shapes. Batholiths (Figure 5.1) are massive intrusions (often covering regions of 100 km2 or more in map View) that have no visible bottom. They form when small bodies of lava amalgamate (mix together) into one large body. To observe one model of this amalgama- tion process, watch the blobs of “lava” in a lighted lava lamp as they rise and merge into one large body (batholith) at the top of the lamp. Smaller intrusions (see Figures 5.1, 4.2) include sills (sheet—like intrusions that force their way between layers of bedrock), laccoliths (blister-like sills), pipes (vertical tubes or pipe-like intrusions that feed volca- noes), and dikes (sheet-like intrusions that cut across layers of bedrock). The dikes can occur as sheet dikes (nearly planar dikes that often occur in parallel pairs or groups), ring dikes (curved dikes that form circular patterns when viewed from above; they typically form under volcanoes), or radial dikes (dikes that de- velop from the pipe feeding a volcano; when viewed from above, they radiate away from the pipe). Study these three kinds of dikes in Figure 5.1. Extrusive igneous processes produce lava flows and pyroclastic deposits (accumulations of rocky mate— rials that have been fragmented and ejected by explo- sive volcanic eruptions). Extrusive (volcanic) igneous processes also present geologic hazards that place hu- mans at risk. Textures of Igneous Rocks Texture of an igneous rock is a description of its con- stituent parts and their sizes, shapes, and arrangement. You should know the common textures of igneous rocks (highlighted in bold text below) and understand how they form (Figures 5.2 and 5.3). This will help you to classify and infer the origin of igneous rocks. The size of mineral crystals in an igneous rock generally indicates the rate at which the lava or magma cooled to form a rock and the availability of the chemicals required to form the crystals. Large crystals require a long time to grow, so their presence generally means that a body of molten rock cooled slowly and contained ample atoms of the chemicals required to form the crystals. Tiny crystals generally indicate that the magma cooled more rapidly (there was not enough time for large crystals to form). Vol- canic glass (no crystals) can indicate that a magma was quenched (cooled immediately), but most vol- canic glass is the result of poor nucleation as de- scribed below. The crystallization process depends on the ability of atoms in lava or magma to nucleate. Nucleation is the initial formation of a microscopic crystal, to which other atoms progressively bond. This is how a crystal grows. Atoms are mobile in a fluid magma, so they are free to nucleate. If such a fluid magma cools slow- ly, then crystals have time to grow—sometimes to many centimeters in length. However, if a magma is very viscous (thick and resistant to flow), then atoms cannot easily move to nucleation sites. Crystals may not form even by slow cooling. Rapid cooling of very viscous magma (with poor nucleation) can produce igneous rocks comprising volcanic glass, which are said to have a glassy texture (see Figure 5.2). Several common terms are used to describe ig- neous rock texture on the basis of crystal size (Figure 5.2). Igneous rocks composed of crystals too small to see without a hand lens (generally <1 mm) have a fine-grained or aphanitic texture (from the Greek word for invisible). Those composed of visible crys- tals have a phaneritic texture (coarse-grained; crystals 1—10 mm) or pegmatitic texture (very coarse-grained; >1 cm). Some igneous rocks have two distinct sizes of crystals. This is called porphyritic texture (see Figure 5.2). The large crystals are called phenocrysts, and the smaller, more numerous crystals form the groandmass, or matrix. Porphyritic textures may generally indicate that a body of magma cooled slowly at first (to form the large crystals) and more rapidly later (to form the small crystals). However, recall from above that crys- tal size can also be influenced by changes in magma composition or Viscosity. Combinations of igneous-rock textures also occur. For example, a porphyritic-aplzanitic texture signifies that phenocrysts occur within an aphanitic matrix. A porphyritic-phaneritic texture signifies that phenocrysts occur within a phaneritic matrix. When you examine an unopened pressurized bot- tle of soft drink or beer, no bubbles are present. But when you open the bottle (and hear a “swish” sound), you are releasing the pressure on the drink and allow- ing bubbles of carbon dioxide gas to escape from the liquid. Recall that magma behaves similarly. When its pressure is released near Earth’s surface, it turns into bubbly lava that may erupt from a volcano. In fact, early stages of volcanic eruptions are eruptions of steam and other gases separated from magma. If the hot; bubbly lava cannot escape normally from the vol- cano, then the volcano may explode (like the top blowing off of a champagne bottle). When gas bubbles get trapped in cooling lava they are called vesicles, and the rock is said to have a vesicular texture. Scoria is a textural name for a rock Igneous Rocks and Volcanic Hazards it 93 _ 1:"? I; Volcanic cone - - q" of pyroclasts . ' FIGURE 5.1 Illustration of the main types of intrusive and extrusive bodies of igneous rock. having so many vesicles that it resembles a sponge. Pumice is glassy and has so many tiny vesicles that it resembles a frothy meringue and will float in water. Pyroclasts (from Greek meaning “fire broken”) are rocky materials that have been fragmented and / or ejected by explosive volcanic eruptions. They include volcanic ash fragments (pyroclasts <2 mm), lapilli or Cinders (pyroclasts 2—64 mm), and volcanic bombs or blocks (pyroclasts >64 mm). Igneous rocks composed of pyroclasts have a pyroclastic texture (see Figure 5.2). They include taff (made of volcanic ash) and volcanic breccia (made chiefly of Cinders and volcanic bombs). Mineral Composition of Igneous Rocks Mineral composition of an igneous rock is a descrip- tion of the kinds and abundance of the mineral crys- tals that comprise the rock. You can estimate the abundance of any mineral in the rock using the charts for visual estimation of percent, provided in GeoTools Sheet 1 at the back of this manual. Eight rock-forming minerals comprise most ig- neous rocks (Figures 5.2, 5.3): quartz, potassium feldspar (K-spar), plagioclase feldspar, muscovite mica, biotite mica, amphibole, pyroxene, and olivine. They are commonly divided into two groups on the basis of the darkness of their color (which is a func- tion of their chemical compositions). Quartz, plagioclase feldspar, potassium feldspar, and muscovite mica are generally light-colored and form a group of felsic minerals. The name felsic refers to feldspars (fel-) and other silica-rich (-sic) minerals. Biotite mica, amphibole, pyroxene, and olivine are generally dark- colored and form a group of mafic minerals. The name mafic refers to the magnesium (ma-) and iron (—fic) in their chemical formulas, so they are also called ferromagnesian minerals. Notice at the top of Figure 5.3 that the mineralogy of an igneous rock can be approximated based on a color index. Color index (CI) is the percentage (by IGNEOUS ROCK ANALYSIS AND CLASSIFICATION :l d m _ f’smn- STEP 3: Texture Pegmatitic (crystals >1 cm): very slow cooling, viscous magma, and/or good nucleation Quartz (gray) Phanefific (crystals 1—1 0 mm): slow cooling, viscous magma, and/or good nucleation Plagioclase Feldspar (White) a 9 I: O 5 U3 3 E a Porphyritic (large and small crystals): slow, then rapid cooling and/or change in magma viscosity or composition Potassium Feldspar (K-Spar) (pink) E i Aphanitic (crystals <1 mm): rapid cooling, fluid lava, and/or good nucleation Muscovite Mica (brown) Glassy rapid cooling and/or very poor nucleation Biotite Mica (black) Vesicular (like meringue): rapid cooling of gas-charged lava Amphibole (dark gray) Vesicular (some bubbles): roxene dark reen Py ( g ) gas bubbles in lava Color Index (CI): the percent, by volume. of mafic minerals in a rock. MAFIC MINERALS (dark-colored) Pyroclastic or _ . . Fragmental: OIIVIne (green) geseatgg agglggg particles emitted from I. ' ' .. sheet 1 volcanoes .5“ .'. _ _, a n‘ STEP 4: Igneous Rock Classification Flowchart _ . 1,2 Feldspar ) mafic minerals K spar > PlagIoclase <quartz present... GRANITE no quartz ......... .. SYENITE‘2 Texture is eggnetitic.I K-spar < Plagioclase .............................. .. DlOFtlTE1'2 or p aneflt c . Feldspar 4 meme minerals CI = 45—85 .................... ..... ............. .. GABBR01-2 CI = 85—100(< 15% feISIc mIneraIS).. .... .. PEFIIDOTITE felsic (CI = 0—15)and/or pink, white, or pale brown ..................................... .. RHYOLITEQ-a gggrgegegto Texture 53 intermediate (CI = 15—45) and/or green to gray. ANDESITE” ' aphanittc andfor - mafic (Cl 2 45) and/or dark gray to black ............... .. .. BASAL‘I'Q-a vesicular mafic with abundant vesicles (resembles a sponge .............................................. .. SCORIA intermediate or felsic with abundant tiny vesicles—like meringue, floats in water... PUMICE ............................................................................................................................................... ..OBSIDIAN Pyroclastic (fragmentau texture fragments 5 2mm ................................................................ .. VOLCANIC TUFF fragments 2» 2mm ................................................................ ..VOLCANIC BREOCIA ‘Add pegmatite to end of name if crystals are > 1 cm (e.g., granite-pegmatite). ZAdd porphyn'tl‘c to front of name when present (e.g., porphyritic granite, porphyritic myolite). 3Add vesicular to front of name when present (e.g., vesicular basalt). V FIGURE 5.2 Igneous rock analysis and classification. Step 1—Estimate the rock’s color index. Step 2—ldentify the main rock-forming minerals if the mineral crystals are large enough to do so, and estimate the relative abundance of each mineral (using a Visual Estimation of Percent chart from GeoTools Template 1). Step 3—ldentify the texture(s) of the rock. Step 4— Use the Igneous Rock Classification Flowchart to name the rock. Start on the left side of the flowchart, and work toward the right Side to the rock name. IGNEOUS ROCKS CLASSIFICATION 1. Color Index \ Estimate the rock’s _ _ - Ultramafic color index (Cl): I _ _ _ _ I _ _ _ v _ _ % of mafic mineral - ' ‘ ‘ ' ' ‘ ' - ' .- - - . crystals or darkness of the rock. _ .' . . . . ' - - - - 0 15 45 85 100% Felsic ‘ Intermediate ' Mafic ' Ultramafic (O to 15% mafic crystals) : (16 to 45% mafic crystals) ' (46 to 85% mafic crystals) ' (> 85% mafic crystals) 100% | | | | | | 80 - r I Plagioclase Felds ar 2. Minerals p identify minerals in the rock, if possible, and 60 their percent (by volume) of the whole rock. Skip this step if mineral crystals are not visible or 40 are too small to identify. Potassium Feldspar 3. Texture(s) Identify the rock’s texture(s). 4. Rock Name: Select name below, based on data from steps 1—3. P9952?“ PEGMATITIC PEGMATITIC PEGMATITIC PEGMATITIC 5 coarsegrained GRANITE DIORITE GABBRO PERIDOTITE E o Phaneritic: g coarsegrained GRANITE DIORITE GABBRO PERIDOTITE a (SYENITE, if no quartz) D 1 E C PORPHYRITIC PORPHYRITIC PORPHYFilTIC PORPHYRITIC E groufldmass GRANITE DIORITE GABBRO PERIDOTITE PORPHYRITIC PORPHYRITIC PORPHYRmc groundmass RHYOLITE ANDESITE BASALT Aphanitic: __ finegrained RHYOLITE ANDESITE BASALT z ease-t2; l uns 8 from basalt in > SCORIA hand samples :7: PUM'CE (resembles a sponge) (KOMATITE) :> Vesicular (abundant tiny vesicles-like meringue; '3 very lightweight; white or gray; floats in water) VESICULAR BASALT E (has few scattered veSIcles) VOLCANIC TUFF (fragments s 2 mm) VOLCANIC BRECCIA (fragments > 2 mm) Pyrocias‘tic or Fragmental 1Phenocrysts are crystals conspicuously larger than the finer grained groundmass (main mass, matrix) of the rock. FIGURE 5.3 Igneous rock classification chart. Obtain data about the rock in steps 1—3, then use that data to select the name of the rock (step 4). Also refer to Figure 5.2 and the examples of classified igneous rocks in Figures 56—518. 95 96 ' LaboratoryFive volume) of the mafic (ferromagnesian, dark-colored) mineral crystals in the rock. Classification of an ig- neous rock depends on its mineralogy as estimated by color index or determined by direct identification of crystals of the eight rock-forming minerals (Figure 5.3). The large (coarse-grained) mineral crystals in phaneritic igneous rocks are generally easy to identify with the naked eye or a hand lens. But the tiny (fine- grained) mineral crystals in aphanitic igneous rocks are usually too small to identify with the naked eye, and one must rely on color index to estimate mineral composition. Classifying Igneous Rocks The complete classification of any igneous rock re- quires knowledge of its texture, color index, and the identity and abundance of specific minerals that comprise it (Figures 5.2, 5.3). Some igneous rocks are named on the basis of their texture, but most are named on the basis of their texture and mineral composition. Color index (CI) is used to estimate the propor- tion of mafic and felsic mineral crystals in an igneous rock. If possible, the specific minerals and their abun- dance are then identified to complete the mineralogi- cal analysis of the rock. Both color index and specific mineral identities are required for complete miner- alogical analysis and classification of the rock. Felsic igneous rocks have only 0—15% mafic min- eral crystals (CI = 0—15), so they are generally very light-colored (Figure 5.3). Quartz and potassium feldspar are usually the most abundant mineral crys- tals in felsic rocks. Intermediate igneous rocks have 16—45% mafic mineral crystals (CI = 16—45), so they are more light-colored than dark (Figure 5.3). Plagio- clase or potassium feldspar are generally the most abundant mineral(s) in intermediate rocks. Mafic igneous rocks have 46—85% mafic mineral crystals (CI = 46—85), and they are dark-colored (Figure 5.3). Pyroxene and plagioclase are generally the most abundant mineral crystals in mafic igneous rocks. Ultramafic igneous rocks have 86—1000/0 mafic minerals (CI = 86—100), so they are usually very dark-colored (Figure 5.3). Olivine and pyroxene are generally the most abundant mineral crystals in ultra- mafic igneous rocks. The color index of an igneous rock is only an ap- proximation of the rock’s mineral composition. Whenever possible, the specific mafic or felsic min- erals should also be identified. This is particularly important, because some mafic minerals (olivine) can be light-colored and some felsic minerals (labradorite feldspar) are dark-colored. Olivine is sometimes a pale yellow-green color (instead of dark green), and it could be mistaken for a mineral of the felsic group. Labradorite is a dark gray or black vari- ety of plagioclase that could easily be mistaken for a mafic mineral. Obsidian (volcanic glass) is also an exception to the color index rules. Its black color suggests that it is ultramafic when, in fact, most obsidian has less than 15% ferromagnesian constituents. (Ferromagnesian— rich obsidian does occur, but only rarely.) The complete classification of any igneous rock requires classification charts like the ones in Figures 5.2 and 5.3, plus knowledge of the rock’s color index, the identity and abundance of its specific minerals, and its texture(s). Follow these steps to classify an igneous rock: Steps 1 and 2: Identify the rock’s color index (CI). Then, if possible, identify the minerals that make up the rock, and estimate the percentage of each. 0 If the rock is fine-grained (aphanitic or porphyrit- ic-aphanitic), or if you cannot identify its miner- als, then you must estimate mineralogy based on the rock’s color index. Felsic fine-grained rocks tend to be pink, white, or pale brown. Intermediate fine-grained rocks tend to be greenish gray. Mafic and ultramafic fine-grained rocks tend to be dark gray to black. 0 If the rock is coarse-grained (phaneritic or peg- matitic), then estimate the color index and per- centage abundance of quartz, feldspars, and mafic minerals. With this information, you can also characterize the rock as felsic, intermediate, mafic, or ultramafic. Step 3: Identify the rock’s texture(s) using Figure 5.2. Step 4: Classify the rock using the flowchart in Figure 5.2 or the expanded classification chart in Figure 5.3. - Use textural terms, such as porphyritic or vesicu- lar, as adjectives. For example, you might identify a pinkish, aphanitic (fine-grained), igneous rock as a rhyolite. If it contains scattered phenocrysts, then you would call it a porphyritic rhyolite. Simi- larly, you should call a basalt with vesicles a vesicular basalt. 0 The textural information can also be used to infer the origin of a volcanic rock. For example, vesicles (vesicular textures) imply that the rock formed by cooling of a gas-rich lava (vesicular and aphanitic). Pyroclastic texture implies Violent vol- canic eruption(s). Aphanitic texture implies more rapid cooling than phaneritic texture. Bowen’s Series of Mineral Crystallization and Reaction in Magma When magma intrudes Earth's crust, it cools into a mass of mineral crystals and/ or glass. Yet when geol— ogists observe and analyze the igneous rocks in a sin- gle dike, sill, or batholith, they usually find that it contains more than just one kind of igneous rock. Ap- parently, more than one kind of igneous rock can differentiate (separate) from a single homogenous body of magma as it cools. American geologist, Norman L. Bowen made such observations in the early 19003. He then devised and carried out laboratory experiments to study how magmas might evolve in ways that could explain the differentiation of multiple rock types from a single magma. Other geologic investigations had already sug- gested that peridotite may comprise the top of Earth’s mantle (Figure 1.18). So Bowen placed pieces of peri- dotite into bombs, strong pressurized ovens used to melt the rocks at high temperatures (1200—1400° C). Once melted, he would allow the molten rock (magma) to cool to a given temperature and remain at that temperature for a while in hopes of having it begin to crystallize. The rock was then quickly re— moved from the bomb and quenched (cooled by dunking it in water) to make any remaining molten rock (magma) form glass. Bowen then identified the mineral crystals that had formed at each temperature. His experiments showed that, as magmas cool, differ- ent silicate minerals crystallize in predictable series that are often summarized in a Bowen’s Reaction Se- ries diagram (Figure 5.4). One series is the continuous crystallization of plagioclase feldspar (on the right in the figure). Another series is the discontinuous crys- tallization of various mafic (ferromagnesian) silicate minerals (left). Notice in Figure 5.4 how plagioclase feldspar (Figure 3.19) crystallizes continuously from high to low temperatures, yet the high-temperature plagio- clase is calcium-rich (sodium-poor) and the low tem- perature plagioclase is sodium-rich (calcium-poor). As the magma begins to cool, calcium is depleted from it to form the calcium-rich plagioclase. But as the magma becomes calcium-poor, sodium takes its place in forming the plagioclase crystals. This is also observed in the field. Naturally formed plagioclase crystals are normally more calcium—rich (sodium- poor) at their centers and more sodium-rich (calci- um-poor) in their outer edges. Now notice the discontinuous series of crystalliza- tion in Figure 5.4. Bowen found that when one of these mafic minerals formed at high temperature it reacted Igneous Rocks and Volcanic Hazards ° 97 with the magma at lower temperature to produce a different mineral. Olivine forms at the highest temper- atures. If the olivine crystals remain in the magma as it cools (and do not settle out), then they react with the magma and are replaced by mineral crystals of pyrox- ene. If these pyroxene crystals remain in the magma as it cools, then they react with the magma and are re- placed by mineral crystals of amphibole and biotite. Finally, notice what happens at the bottom of Bowen’s Reaction Series (Figure 5.4). Bowen found that at the lowest temperatures, where the last crystal- lization occurs, the remaining elements commonly form abundant potassium feldspar, muscovite, quartz, and rare gems such as emerald. Bowen’s laboratory investigations revealed one way that different kinds of igneous rocks can differen- tiate from a single, homogenous body of magma as it cools. If a mineral of the discontinuous series remains in the magma as it continues to cool, then it will react with the magma at a lower temperature and a differ- ent mineral ‘will form. However, crystals that form continuously, and / or those that settle out of the magma as it cools, no longer react with the remaining magma. These crystals also take with them some of the chemicals that originally existed in the magma. In this way, crystallization and crystal settling remove chemical elements from the magma. These processes change the magma’s composition and leave the body of cooling magma with a different combination of ele— ments to form the next crystals. This is one way that intermediate and felsic magmas / rocks can differenti- ate from what started out as a mafic magma. Bowen’s Reaction Series clearly suggests a rela- tionship between temperature, the composition of magmas, and the mineralogy and names of igneous rocks. It is laboratory-based evidence that ultramafic igneous rocks (peridotite, komatite) form at the high- est temperatures, followed at lower temperatures by mafic rocks (gabbro, basalt), intermediate rocks (dio- rite, andesite), and felsic rocks (granite, rhyolite) (Figure 5.4). This makes Bowen’s Reaction Series a useful conceptual model (like the rock cycle) for inter- preting the origin of igneous rocks. For example, geol- ogists have identified many natural examples where mafic magmas have differentiated into mafic and an- desitic rocks and where andesitic magmas have differ- entiated into andesitic and felsic rocks. However, there is no known natural example of where a single ultramafic magma cooled and differentiated into all four main groups of igneous rocks (ultramafic, mafic, intermediate, felsic). This suggests that other factors are significant in changing the composition and tem- perature of a magma. For example, crystal settling may remove chemicals from a magma, while new Temperature' . , . . ' I Bowen 5 Reaction Series 1200°C Ultramafic magma 900°C Intermediate Diorite Andesite 800°C . l - - . - - Granite Low temperature J' _ "f _ " ', _ l Eels? mgr-1.1a ' Rhyolite (last to _ ' fl crystallize) ' ' . o No magma 500 C remaining FIGURE 5.4 Bowen’s Reaction Series—a laboratory-based conceptual model of one way that different kinds of igneous rocks can differentiate from a single, homogenous body of magma as it cools. See text for discussion. DIVERGENT PLATE BOUNDARY Andesitic Andesitic Linear eruption of Basaltic Basaltic volcanoes - lava flow basaltic magma submarine volcanic at mid-ocean volcano island CONVERGENT PLATE BOUNDARY FIGURE 5.5 Formation of igneous rocks at a hot spot (such as the Hawaiian Islands), divergent plate boundary (mid-ocean ridge), and convergent plate boundary (subduction zone). See text for discussion. 98 chemicals are added to the magma as it melts and in- corporates host bedrock—a process called assimilation. Assimilation may explain how mafic magmas at con- vergent plate boundaries evolve into larger bodies of intermediate and felsic magma by melting and incor- porating crustal rocks that are rich in quartz and feldspar (Figure 5.5). Bowen’s Reaction Series is very generally re- versed when rocks are heated. Earth materials react with their surroundings and melt at different temper- atures as they are heated. An analogy is a plastic tray of ice cubes, heated in an oven. The ice cubes would melt long before the plastic tray would melt (i.e., the ice cubes melt at a much lower temperature). As rocks are heated, their different mineral crystals melt at dif- ferent temperatures. Therefore, at a given tempera- ture, it is possible to have rocks that are partly molten and partly solid. This phenomenon is known as partial melting and Bowen’s Reaction Series can be used to predict the sequence of melting for mineral crystals in a rock that is undergoing heating. Mineral crystals formed at low temperatures will melt at low tempera- tures, and mineral crystals formed at high tempera- tures will melt at high temperatures. However, the minerals in a particular group, say felsic or intermedi- ate, do not all melt at once. Each mineral in the group FIGURE 5.6 Granite—an intrusive, phaneritic igneous rock that has a low color index (light color) and is com- posed chiefly of quartz and feldspar mineral crystals (see also Figures 1.18A and 3.10). Ferromagnesian mineral crystals in granites generally include biotite and amphibole (hornblende). This sample contains pink potassium feldspar (K—spar), white plagioclase feldspar, gray quartz, and black biotite mica. Granites rich in pink potassium feldspar appear pink like this one, whereas those with white K-spar appear gray or white. Felsic rocks that resemble granite, but contain no quartz, are called syenites. Quartz crystals “El-7.1"- . .. ."ii Mica crystals ~--—.___ " Feldspar crystals Photomicrograph (X 26.6) Original sample width is 1.23 mm Igneous Rocks and Volcanic Hazards ° 99 has its own unique melting point at specific pressures. Thus, partial melting of mantle peridotite beneath hot spots and mid-ocean ridges produces mafic magma rather than ultramafic magma. When the mafic magma erupts as mafic lava along the mid-ocean ridges and hot spots (e.g., Hawaiian Islands), it cools to form basalt (Figure 5.5). A contributing factor in melting is water, which can lower the melting point of rocks. This may be how peridotite and/ or basalt are partially melted in subduction zones at convergent plate boundaries (Figure 5.5). Questions 1. Review the textures of rocks in Figures 5.6—5.18. Refer to Figure 5.19 and Step 3 of Figure 5.2. For each rock in Figure 5.19, identify the texture(s) present and infer the origin of the texture(s). 2. Review the textures, mineralogy, color indices, and classification of rocks in Figures 5.6—5.18. Then refer to the four rock samples in Figure 5.20. For each sample, identify the texture(s), color index, and mineralogical composition as requested. Use this data, Figures 5.2 and 5.3, and the steps listed on page 96 to classify and determine an igneous rock name for each sample. FIGURE 5.7 Rhyolite—a felsic, aphanitic igneous rock that is the extrusive equivalent of a granite. It is usually light gray or pink. Some rhyolites resemble andesite (see Figure 5.9), so their exact identification must be finalized where possible by microscopic examination to verify the abundance of quartz and feldspar mineral crystals. Feldspar crystals Photomicrograph (x 26.6) Original sample width is 1.23 mm FIGURE 5.8 Diorite—an intrusive, phaneritic igneous rock that has an intermediate color index and is composed chiefly of plagioclase feldspar and ferromagnesian mineral crystals. The ferromagnesian mineral crystals are chiefly amphibole (hornblende). Quartz is only rarely present and only in small amounts (<5%). Feldspar crystals Photomicrograph (X 26.6) Original sample width is 1.23 mm 100 FIGURE 5.9 Andesite—an intermediate, aphanitic igneous rock that is the extrusive equivalent of diorite. It is usually medium—to-dark gray. Some andesites resemble rhyolite (Figure 5.7), so their identification must be finalized by microscopic examination to verify the abundance of plagioclase feldspar and ferromagnesian mineral crystals. This sample has a porphyritic-aphanitic texture, because it contains phenocrysts of black amphibole (hornblende) set in the aphanitic groundmass. Amphiboie phenocryst Groundmass of feldspar and ferromagnesian mineral crystals Feldspar 1 I h t " " p enocrys s V _ Photomicrograph (x 26.6) Original sample width is 1.23 mm FIGURE 5.10 Gabbro—a mafic, phaneritic igneous rock composed chiefly of ferromagnesian and plagioclase min- eral crystals. The ferromagnesian mineral crystals usually are pyroxene (augite). Quartz is absent. Plagioclase feldspar crystals Pyroxene crystals Photomicrograph (x 26.6) Original sample width is 1.23 mm 101 FIGURE 5.11 Basalt—a mafic, aphanitic igneous rock that is the extrusive equivalent of gabbro, so it is dark gray. This sample has a vesicular (bubbly) texture. Microscopic examination of basalts reveals that they are composed chiefly of plagioclase and ferromagnesian mineral crystals. The ferromagnesian mineral crystals generally are pyrox- ene, but they also may include olivine or magnetite. Glass also may be visible between mineral crystals. Basalt forms the floors of all modern oceans (beneath the mud and sand) and is the most abundant aphanitic igneous rock on Earth. Ferromagnesian mineral crystals Plagioclase feldspar crystals Glass Photomicrograph (x 26.6) Original sample width is 1.23 mm 102 FIGURE 5.12 Peridotite—an intrusive, phaneritic igneous rock having a very high color index (>95%) and com- prised essentially of ferromagnesian mineral crystals. This sample is a peridotite composed of olivine mineral crystals; such a peridotite also is called dunite. Similarly, a peri- dotite composed of pyroxene mineral crystals is called pyroxenite. Also refer to Figure 1.18B (peridotite xenoliths). FIGURE 5.13 Obsidian—an extrusive igneous rock com— posed of dark glass (volcanic glass). It forms when very viscous lava is cooled very suddenly, or quenched. Such a glassy texture is also called hyaline (the Greek word for glass) texture. Some obsidian contains phenocrysts of feldspar that are visible in hand samples; such texture is called porphyritic-g/ass or porphyritic-hyaline. Some obsid- ian also contains microscopic plagioclase feldspar crystals, which impart a glittery reflectiveness; gemstone manufac- turers call this “golden-sheen” obsidian. Glass Feldspar and ferromagnesian mineral crystals Photomicrograph (x 26.6) Original sample width is 1.23 mm FIGURE 5.14 Scoria—an extrusive igneous rock with a mafic color index and such abundant adjacent vesicles that it resembles the texture of a sponge. Scoria can form from the cooling of lava flows that are dense and frothy (bubbly, like whipped egg whites). Scoria also can develop from the cooling of gas-charged lava that is explosively ejected from volcanoes, forming scoria cinders. FIGURE 5.15 Pumice—a glassy extrusive igneous rock, generally white to dark gray, having very abundant adja- cent vesicles. In these properties, pumice is similar to sco- ria (Figure 5.14). However, pumice is less dense than sco- ria. Its density is so low that it floats on water. In 1992 a submarine volcanic eruption near Fiji produced a floating boulder field of pumice in the Pacific Ocean. 103 FIGURE 5.16 Volcanic breccia—an extrusive igneous rock composed chiefly of pyroclasts more than 2 mm in diameter. Recall that pyroclasts (from the Greek, “fire bro- ken”) are rocky materials that have been fragmented and ejected by explosive volcanic eruptions. They include volcanic ash fragments (pyroclasts <2 mm), lap/Ill and Cinders (pyroclasts 2—64 mm), and volcanic bombs or blocks (pyroclasts >64 mm, Figure 5.17). The pyroclasts in this sample are angular pieces of obsidian (volcanic glass, Figure 5.13). FIGURE 5.18 Tuff—an extrusive, pyroclastic igneous rock composed chiefly of volcanic ash (pyroclasts <2 mm). Tul'f has a dull, earthy appearance, as in this example. This sample also includes tiny shards of volcanic glass and brown lapi/li' (pyroclasts 2—64 mm) in the tuff. Coarse ash Feldspar crystals Fine ash Glass shard Photomicrograph (x 27.8) Original sample width is 1.17 mm 104 J‘— Warn—I- FIGURE 5.17 A volcanic bomb. Volcanic bombs are aphanitic masses of cooled lava that were violently ejected from volcanoes and then solidified while in the air. As such, many volcanic bombs have the shapes of teardrops, rib- bons, and raindrops (this example). Actually, they are cooled “lava drops”! ...
View Full Document

Page1 / 14

Lab%203---Igneous%20Rocks%20AND%20Volcanic%20Hazards(2) -...

This preview shows document pages 1 - 14. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online