LAST NAME (ALL IN CAPS): ______FIRST NAME: ______
6. IGNEOUS ROCKS AND VOLCANIC HAZARDS
Instructions: Refer to Laboratory 5 in your lab book on pages 129-152 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. Some rocks that you would be working with may have sharp edges and corners, therefore, be careful when working with them! When you are done with your lab work, please clean the desk and leave all materials you worked with in the same way you found them!
INTRODUCTION MAFIC COLOR INDEX (MCI): Is an estimate of the % of mafic (dark) minerals by volume. Mafic minerals are dark colored minerals such as olivine, pyroxene, amphibole, and biotite; Felsic minerals are light colored minerals such as quartz, plagioclase feldspar, k-feldspar, and muscovite. Ultramafic rocks: CI > 85% (made up of more than 85% mafic minerals) Mafic rocks: CI 46-85% Intermediate rocks: CI 16-45% Felsic rocks: CI 0-15%
ORIGIN OF ROCK (INTRUSIVE/EXTRUSIVE) Intrusive (plutonic): formed from magma; made up of large minerals (coarse-grained) Extrusive (volcanic): formed from lava; made up of microscopic crystals (fine-grained)
TEXTURES OF IGNEOUS ROCKS Phaneritic: Coarse grain size; visible grains (1-10 mm); result of slow cooling Pegmatitic: Minerals are exceptionally large: > 1 cm long Porphyritic: Composed of crystals of 2 different sizes indicative of multistage cooling history (intrusive/extrusive) long period of slow partial cooling from a magma forms the large crystals sudden extrusion of the remaining magma and the already formed crystals Aphanitic: Fine grain size (< 1 mm); result of quick cooling Glassy: Lack crystals; superfast cooling Vesicular: Contains tiny holes called vesicles which formed due to gas bubbles in the lava. Very porous; may resemble a sponge. Commonly low density, pumice floats on water. Pyroclastic: Consist of mineral grains, volcanic glass, and rock fragments hurled into air by explosive volcanic eruptions. Grains that were hurled into the air will eventually settle to the ground or sea floor & form pyroclastic rocks.
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QUESTIONS Q1. Using the Mineral Database (Figure 3.21, pages 93-97), look up each mineral name listed below and identify its picture on page 144. Match each numbered picture with the mineral names listed on page 144 and determine whether each mineral is mafic or felsic. All samples are shown at their actual size (X1). Plagioclase, Augite, Olivine, Quartz, Hornblende, Biotite, Muscovite, K-feldspar NUMBER MINERAL NAME MAFIC OR FELSIC 1 2 3 4 5 6 7 8
Q2. The rock on the side figure has a “porphyritic texture”, which means that it contains two sizes of crystals. The large white plagioclase crystals are called phenocrysts and sit in a green-gray “groundmass” of more abundant, smaller (aphanitic) crystals. Explain how this texture may have formed.
Q3. What is the “Bowen’s Reaction Series”? Using one or two sentences, describe how it helps us understand igneous rock forming processes.
Q4. Based on figure 5.7 (page 139, Lab Manual) list in which tectonic and geologic settings the various igneous rocks form. Use the next format to answer the question.
TECTONIC/GEOLOGIC SETTING NAME OF IGNEOUS ROCK FORMED
Hot Spot
Divergent Plate Boundary
Transform Plate Boundary
Convergent Plate Boundary
Earth’s Mantle Page 2 of 5
Q5. Use figure 5.5, Igneous Rock Classification Chart (page 137) to fill out the blank boxes. MAFIC COLOR MINERALS PRESENT ORIGIN TEXTURE/S ROCK NAME INDEX (MCI) Musc., Qtz, K-Spar, Plag., Biot, Felsic Pegmatitic Hbd
Qtz, K-Spar, Plag., Biot, Hbd Extrusive Andesite
Mafic Phaneritic
Felsic Glassy
Intrusive & Mafic Extrusive
Felsic Volcanic tuff
Phenocrysts in Qtz, K-Spar, Plag., Biot, Hbd phaneritic groundmass
Felsic Intrusive Syenite
Mineral abbreviations used: Musc = Muscovite; Qtz = Quartz; K-Spar = K-feldspar; Plag = Plagioclase; Biot = Biotite; Hbd = Hornblende
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Q6. Analyze and classify each igneous rock below, then infer the origin of each rock based on its texture. Refer to Figures 5.4 and 5.5 as needed. All rocks shown are actual size (X1). To better estimate the Mafic Color Index (MCI), please see the color images in your Lab Manual on pages 143 & 148. Please use the following % ranges for the MCI: 0-15%; 1-45%; or 46-85%.
Mafic Color Index (% of mafic minerals Mafic Color Index (% of mafic Mafic Color Index (% of mafic Mafic Color Index (% of mafic present): minerals present): minerals present): minerals present):
Would you describe the rock as mafic, Would you describe the rock as Would you describe the rock as Would you describe the rock as intermediate, or felsic? mafic, intermediate, or felsic? mafic, intermediate, or felsic? mafic, intermediate, or felsic?
Texture(s) present: Texture(s) present: Texture(s) present: Texture(s) present:
The name of this rock is: The name of this rock is: The name of this rock is: The name of this rock is:
Based on its texture, how did this rock Based on its texture, how did this Based on its texture, how did this Based on its texture, how did this form? rock form? rock form? rock form?
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Q7. Obtain 6 igneous rocks from your instructor and for each rock fill in the information required on the Igneous Rocks Worksheet below.
IGNEOUS ROCKS WORKSHEET SAMPLE # TEXTURES PRESENT ORIGIN: MAFIC COLOR INDEX ROCK NAME (Fig 5.4 or 5.5) (INTRUSIVE OR EXTRUSIVE) (Fig 5.5) (Fig 5.4 or 5.5) (Pegmatitic, Phaneritic, Porphyritic, etc.) (FELSIC, INTERMEDIATE, OR MAFIC)
I-4
I-6
I-7
I-8 (Example)
I-13
I-14
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