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Textures of Igneous Rocks: Composition of Igneous Rocks

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Textures of Igneous Rocks: Composition of Igneous Rocks LAST NAME (ALL IN CAPS): _________________________________ FIRST NAME: _____________________ 7. ROCKS AND THE ROCK CYCLE Instructions: Refer to Exercise 2 in your Lab Manual on pages 25-46 to answer the questions in this work sheet. Your work will be graded on the basis of its accuracy, completion, clarity, neatness, legibility, and correct spelling of scientific terms. If you have to test a mineral for effervescence and want to apply HCl, you must first ask permission from your instructor (remember HCl is hazardous to health; it may also discolor & decompose your clothes & books). Some rocks that you will be working with have sharp edges and corners, therefore, be careful when working with them! When you are done with your lab work, if smaller fragments and dust of minerals had fallen-off while working with them, clean the desk and leave all materials you worked with in the same way you found them! INTRODUCTION IGNEOUS ROCKS: Formed when molten rock (magma/lava) cools down and solidifies; minerals are arranged in a dense interlocking mass with no alignment; made of intergrown/interlocking crystals. {Born of fire} Examples of Igneous rocks: Granite, Diorite, Gabbro, Rhyolite, Andesite, Basalt, Pumice, Obsidian Textures of igneous rocks: Aphanitic texture: produced when lava cooled quickly on Earth’s surface; rock made up of microscopic minerals. Vesicular texture: produced on fine grained rocks when gas escapes from a cooling lava. Phaneritic texture: produced when magma cooled slowly at depth; rock made up of large (visible) minerals. Porphyritic texture: produced by slow then rapid cooling; rock made up of fine and coarse minerals. Glassy texture: produced by very rapid cooling; rock made up of natural glass – doesn’t contain any crystals at all! Fragmental texture: Produced during explosive eruption of a volcano Composition of igneous rocks: Igneous rocks (and the magma from which they come) can be divided into four groups, based on the proportion of light and dark minerals. Felsic (or granitic): composed mainly of light-colored minerals such as quartz, K-feldspar, with lesser amounts of plagioclase feldspar. Dark-colored minerals account for no more than 15% of the minerals in this groups of rocks. Intermediate (or andesitic): are rock mixtures of both light-colored minerals (mainly plagioclase feldspar) and dark colored minerals (mainly amphibole). Dark-colored minerals comprise between 15% and 45% of these rocks. Mafic (or basaltic): contain abundant dark-colored minerals such as pyroxene and olivine that account for between 45% and 85% of their composition. Plagioclase feldspar makes up the bulk of the remainder. Ultramafic: composed almost entirely of the dark minerals as pyroxene and olivine and are seldom observed at Earth’s surface. Color Index: A value based on the % of dark silicate minerals in a rock (shown at bottom of Figure 2.10, Lab Manual). Rocks with a low color index (light in color) are felsic in composition, where as those with high color index (dark in color) are mafic. Those in between have intermediate composition. Ultramafic compositions are rare on Earth, thus we will not consider them any further in this lab. The following features, if they occur, are distinctive of igneous rocks: Vesicles, glassy texture, and very light-weight (pumice floats in water). Page 1 of 7 SEDIMENTARY ROCKS: Formed when sediments are accumulated (deposited) in an ocean/sea & get cemented together; made up of randomly arranged sediments or rock fragments. {Born of Water} Examples of sedimentary rocks: Conglomerate, breccia, sandstone, shale, limestone, rock salt, coquina Classification of sedimentary rocks Detrital: made up of fragments of rocks and minerals Chemical: made of minerals produced by precipitation or evaporation Biochemical: Contain fossils Composition of sedimentary rocks Clay Minerals: rocks composed of clay minerals are very fine grained, soft, and can be scratched with an iron nail Quartz: Sedimentary rocks composed mainly of quartz usually consist of sand-sized particles that are hard and can scratch glass Calcite: Rocks composed of calcite effervesce when a drop of dilute HCl is applied. They can also be easily scratched with an iron nail Evaporite Minerals: These sedimentary rocks contain salts, usually halite or gypsum that are deposited when saltwater evaporates. They are crystalline and can be scratched by an iron nail. Gypsum is soft enough to be scratched by a fingernail Altered Plat Fragments: Rocks composed of organic material are usually black in color, have a low density, and are easily broken The following features, if they occur in rocks are distinctive of sedimentary rocks: layering, fossils, and rock entirely made up of salt. METAMORPHIC ROCKS: Formed when any kind of rock is subjected to high temperature and pressure (usually at greater depths within the Earth). Minerals aligned in the same direction; made of intergrown crystals. [One rock is the raw material for another!] Textures of metamorphic rocks: Foliated: rock shows layering. Examples include slate, phyllite, schist, gneiss Nonfoliated: rock is massive (lacks layering). Examples include Marble and Quartzite The following features, if they occur in rocks are distinctive of metamorphic rocks: stretched pebbles, folds, foliation, porphyroblasts, and metamorphic minerals (talc, chlorite, serpentine, etc.). QUESTIONS Q1. (I) What is magma? How does magma differ from lava? {Hint: Read page 26} (II) The terms intrusive and extrusive are used to describe which one of the three rock groups? Choose one and circle your answer. A) Igneous B) Sedimentary C) Metamorphic (III) In your own words, write the definition of rock texture. {Hint: Read page 28} Page 2 of 7 (IV) Name the process by which sediment is transformed into sedimentary rocks. {Hint: purple arrows in figure 2.2, page 27 are “processes” that lead to the formation of different rock groups} (V) What are some of the most recognizable (or distinctive) feature of sedimentary rocks? {Hint: Read notes in this worksheet} (VI) When rocks that formed near the surface become deeply buried and subjected to intense pressure and/or heat new group (class) of rocks are formed. Classify and describe this newly formed group of rocks. {Hint: Do they belong to Igneous, Sedimentary or Metamorphic rocks? Give reason/s to your answer.} (VII) When determining rock composition, what test can be used to find out if it contains the mineral calcite? Describe the process. {Hint: Recall special properties of minerals from Lab # 6} Q2. Use the close-up photos of igneous rocks in Figure 2.9 (Lab Manual) to answer the following questions. (I) Which sample(s) (A-H) exhibit porphyritic texture? (II) Of the samples you listed in Q2(i) above, what terms are used to describe the larger crystals and surrounding smaller crystals? {Hint: Read page 29} Larger crystals: _________________ Smaller crystals: _________________ (III) Which sample(s) exhibit(s) coarse-grained texture? (IV) Complete the description of the environment in which a coarse-grained igneous rock forms by choosing the appropriate terms (1. at great depth or on the surface, 2. slowly or rapidly, 3. extrusive or intrusive) When molten rock solidifies 1. __________________________, it cools 2. __________________, and produces 3. ___________________________igneous rock. (V) Which sample(s) exhibit(s) fine-grained texture? (VI) Complete the description of the environment in which a fine-grained igneous rock forms by choosing the appropriate terms (1. at great depth or on the surface, 2. slowly or rapidly, 3. extrusive or intrusive) When molten rock solidifies 1. ______________________________, it cools 2. ____________________, and produces 3. ___________________________igneous rock. (VII) Which of the sample(s) exhibit(s) vesicular texture? (VIII) Although samples A and C appear different, they have very similar mineral composition. Briefly explain what accounts for their different textures (appearances). Page 3 of 7 Q3. Use the color index shown at the bottom of Figure 2.10 (Lab Manual) as well as the photos of samples A-F in Figure 2.11 (Lab Manual) to complete the following statements. (I) Sample(s) _____________________ has/have a felsic (granitic) composition. (II) Sample(s) _____________________ has/have an intermediate (andesitic) composition. (III) Sample(s) _____________________ has/have a mafic (basaltic) composition. Q4. Carefully examine the common sedimentary rocks shown in Figure 2.13 (Lab Manual). Use these photos and the preceding discussion on pages 34 and 35 as well as Figure 2.14 to answer the following questions. (I) Name one physical feature that can be used to distinguish sample A (conglomerate) from sample B (breccia)? (II) Samples C, O, P, and Q are all primarily composed of quartz. What property of quartz would assist you in identifying these samples? Explain your answer. (III) Samples G-L all contain calcite. What property of calcite would assist you in identifying these samples? Explain your answer. (IV) Compare and contrast Sample I (Fossiliferous limestone) and sample J (coquina). (V) Sample G (crystalline limestone) in Figure 2.13 and sample F (granite) in Figure 2.11 are both crystalline. How do these samples differ in texture (appearance)? (VI) Sample H is a microcrystalline rock composed of calcite, and sample O is a microcrystalline rock composed of quartz. What test(s) could be used to figure out which is which? Explain your answer. Q5. Define the following terms (Hint: Read pages 40 and 41) (I) Foliation: __________________________________________________________________________ (II) Porphyroblast: _____________________________________________________________________ Q6. Examine the rock outcrops shown in Figure 2.23 and classify them as igneous, sedimentary, or metamorphic. (I) Outcrop A: ________________ (II) Outcrop B: ______________ (III) Outcrop C: _______________ Page 4 of 7 Q7. Match each of the metamorphic rocks listed in the table below with one (or more) possible parent rock/s from the following list: bituminous coal, shale, limestone, slate, granite, quartz sandstone. Refer to Fig 2.21, if needed. METAMORPHIC ROCK NAME OF PARENT ROCK MARBLE SLATE PHYLLITE GNEISS QUARTZITE ANTHRACITE Q8.
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