Igneous Rocks Lab: Identification and Composition Guide

Laboratory 3: Igneous Rocks and Structures Cochise Community College Geology 101 Instructor: J. Deakin Igneous Rock Lab Identification (Submit Pages 6,7,12,13 AND 14 ONLY) Objective: Igneous rocks are the most abundant type of rock in the entire earth, when considered by volume. They all began as a liquid melt (magma) and cooled to form a solid rock. This lab has been designed to familiarize you with the basic types of igneous rocks that you are likely to encounter, and teach you how to identify the type of igneous rock by its chemical composition and its mineralogic texture. Introduction Like mineral identification, most igneous rocks can be identified quickly in the field by simple observational analysis. Most if not all igneous rocks can be identified by considering only two properties of the rock: 1) Composition (Chemistry) 2) Texture (Fabric) We will discuss the controls and the potential ranges for each of these two properties below. Part I Composition The composition of an igneous rock can be determined by identifying the minerals that make up the rock. The composition of the rock will be dictated by the chemical composition of the original melt from which the rock formed. Classification based on chemistry takes into account the amount of silica (SiO 2 ) and the composition of feldspar minerals (K, Na, Ca). Igneous rock chemistry is determined mainly by the source of the magma and any interactions between magma and the rocks through which it migrates. Chemical composition usually is indicated by the minerals or color of an igneous rock. Four main compositional categories result from this approach. 1. Felsic -- Rich in feldspars and silica. High in Silica means high in quartz minerals. Silica content ranges from about 55% to > 70%. Potassium feldspar makes up more than one-third of total feldspars; plagioclase (Na & Ca) feldspars are less than two-thirds of total feldspars. Typical of continental crust. Think granite countertop. 2. Intermediate -- Between felsic and mafic. Silica content ranges from about 55% to 65%. Plagioclase feldspars make up more than two-thirds of total feldspars. Na-rich plagioclase predominates over Ca-rich plagioclase. Found in association with subduction zones. Intermediate color –think salt and pepper. 3. Mafic -- Rich in magnesium and iron with less silica. Silica content is 45% to 50%. Ca-rich plagioclase is the dominant feldspar with little or no K- or Na-feldspars. Typical of oceanic crust. Dark colored 4. Ultramafic -- Still more magnesium and iron and even less silica. Silica content is less than 45%, and little or no feldspar is present. Derived from the mantle. Dark colored. These compositions correspond to distinct levels encountered along Bowens Reaction Series as represented in the diagrams that follow and as covered in the lecture. The general composition of an igneous rock can be estimated by simply looking at the color of the rock. Felsic rocks are generally lighter in color (white to pink to light grey) and less dense than mafic rocks, which tend to be darker in color and more dense. 1

Laboratory 3: Igneous Rocks and Structures Figure 1. Texture The texture of an igneous rock varies depending on whether it cooled above the ground, and thus rapidly, or below the ground, and thus slowly. Below are definitions for the different adjectives that describe texture. Definitions  Phaneritic -- Large crystals that are clearly visible to the eye with or without a 10-power hand lens. The entire rock is made up of large crystals, which are generally ½ mm to several cm in size; no fine matrix material is present. This texture forms by slow cooling of magma deep underground in the plutonic environment.  Aphanitic -- Small crystals that cannot be seen by the eye with or without a 10-power hand lens. The entire rock is made up of small crystals, which are generally less than ½ mm in size. This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments.  Porphyritic -- Texture in which an aphanitic matrix makes up part of the rock, and large crystals are present within the matrix. The large crystals are called phenocrysts . This texture implies two-stage cooling--an early stage of slow cooling in which the phenocrysts grow, followed by a later stage of rapid cooling that forms the matrix.  Pegmatitic -- Pegmatites are composed of quite large crystals--cm to tens of cm in size. They typically occupy veins or layers within a larger plutonic body. The large crystals form by slow cooling of magma.  Glassy -- Non-crystalline (glassy) structure of the rock, in which no minerals are present. Glass results from cooling that is so fast that minerals do not have a chance to crystallize. This may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface. Pure volcanic glass is known as obsidian.  Vesicular -- This term refers to vesicles (holes, pores, or cavities) within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks. Pumice is sold as is and is used for cleaning. Scoria are rough rocks like the ones found in many gas BBQ’s. The word scoria comes from the meaning to scour. ( to clean off) 2

Laboratory 3: Igneous Rocks and Structures  Breccia -- A rock composed of broken, angular fragments of mixed composition. Such texture forms in volcanoes, along fault zones, and in landslides. The magma rolls down slope picking up anything in its path, eventually it hardness an any object picked up that did not melt ends up as part of the substance.  Pyroclastic an igneous rock that is formed from the cemented rocky material that has been fragmented and/or ejected by explosive volcanic eruptions. These are projectiles that harden in the air. Volcanic bombs are this type of substance.  Volcanic Ash – pyroclastic ejecta with diameters less than 2 mm. Ash can be hardened or can be soft. Usually older ashes are lithified and hard unless weathered and then they can crumble easily. To distinguish with sedimentary rocks, like limestone use the HCl test.  Volcanic Cinders/Lapilli –pyroclastic ejecta with diameters of 2 – 64 mm. usually small projectiles. Sometimes found as “apache tears”. These objects usually harden while falling through the air. FIGURE 2 3

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Laboratory 3: Igneous Rocks and Structures Figure 3 4

Laboratory 3: Igneous Rocks and Structures From: http://faculty.chemeketa.edu/afrank1/rocks/igneous/igclass.htm PRE-LAB QUESTIONS 1. Which of the following are mafic minerals? (Select all that apply) 5

Laboratory 3: Igneous Rocks and Structures a. Quartz b. Pyroxene c. Plagioclase d. Olivine 2. Rocks that cool quickly have ___ crystals. 3. Rocks that cool slowly have ___ crystals. 4. True or False? Rocks formed from volcanic eruptions are an example of extrusive igneous rocks. 5. What textures are possible in intrusive rocks? (List all that apply.) 6. What textures are possible in extrusive rocks? (List all that apply.) Exercise 1. Utilize the charts in figure 2 above to determine the rock name for the following rocks based on the given percentages of minerals found in the rock and the described texture. Example 1: A phaneritic rock composed of 25% olivine, 50% pyroxene and 25 % plagioclase would be called a gabbro. Example 2: An aphanitic rock containing phenocrysts of amphibole in a light colored matrix, presumably plagioclase, would be classified as andesite porphyry. Example 3: An aphanitic rock containing phenocrysts of quartz with minor amounts of biotite in a plagioclase and potassium feldspar matrix would be called a rhyolite. Name the following rocks based on the given descriptions 1. Phaneritic texture 6

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Laboratory 3: Igneous Rocks and Structures 58% potassium feldspar 20% quartz 5% muscovite 7% amphibole 7% plagioclase feldspar 35 biotite Rock Name:__________________ 2. Phaneritic texture with equal amounts of light and dark minerals 60% plagioclase 25% amphibole 10% quartz 5% potassium feldspar Rock Name: _________________ 3. Light grey aphanitic matrix with phenocrysts of plagioclase 60% plagioclase Ca-Na feldspar 25% amphibole 15% pyroxene Rock Name: _________________ 4. Very light pink aphanitic matrix with phenocrysts of amphibole 5% plagioclase 4% biotite 8% amphibole 3% muscovite 23% quartz 57% potassium feldspar Rock Name: _________________ 5. Very dark rock with phaneritic texture 25% plagioclase 65% pyroxene 10% olivine Rock Name: _________________ Exercise 2 In this exercise, you will be examining pictures of different igneous rocks. Follow the steps below to determine the texture and color of the rocks. You will need to use the charts on pages 4 and 5 (figures 2 and 3 respectively).The chart on page 5 is especially helpful to use to compare to. You can also use the PowerPoint lecture slides for chapter 3. a. Examine the rock and determine the type of texture (Record this in the igneous rock data chart on the last page of this lab) Coarse = phaneritic Very coarse = pegmatitic Fine = aphanitic Fine grained matrix with phenocrysts = porphyritic Holes or bubbles = vesicular Very light with many holes = pumice Glassy – obsidian b. Determine the basic color of each rock, felsic, intermediate or dark. List the mineral present and record in the chart. c. Name the rock using one of the charts in figures 2 or 3. Record in the chart at the end of the lab. 7

Laboratory 3: Igneous Rocks and Structures Igneous Rock 1. Igneous Rock 2. 8

Laboratory 3: Igneous Rocks and Structures Igneous Rock 3 Igneous Rock 4. Igneous Rock 5. Igneous Rock 6. 9

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Laboratory 3: Igneous Rocks and Structures Igneous Rock 7. Igneous Rock 8. 10

Laboratory 3: Igneous Rocks and Structures Exercise 3 Igneous Rock Bodies Igneous rock bodies form below and on the Earth’s surface and are produced in a variety of shapes and sizes. Exam the Diagram below and become familiar with the different types of places volcanic rocks form and also the different type of features these rocks form. A description of each rock body follows the diagram. Figure 4 A Stock - a roughly circular structure usually less than 50 square miles forms below surface. B Batholith - large massive intrusion covering an area greater than 50 square miles – forms below the surface. C Laccolith - an intrusive body that has a flat floor but arches up the strata into which it was ejected to form a blister like structure – usually forms below surface. D Magma – molten rock from a magma chamber E Dike - a tabular intrusive rock body that cuts across the strata of the surrounding rock – usually forms below the surface. F Sill Pipe - a tabular body of igneous rock injected between layers of enclosing rock – can form above or below surface G Volcanic pipe - conduit for magma to a volcano. H Composite flows – layers of ash and lava that form a composite or strato volcano. I Lava Flow – lava that flows out of fissures or volcanic vents and spreads out. J Cinder Cone Volcano – small volcano form by layers of hot cinders that cool to form a rubble composed volcano. K Remnant Volcanic Neck - igneous rock body that originally formed the vent or neck of a volcano and subsequently is exposed by erosion Based on what you have learned in lectures and by reading this lab. Answer the following questions. 11

Laboratory 3: Igneous Rocks and Structures 1. In Hawaii, many basaltic flows are only 1 to 2 feet thick. On the Columbia plateau, similar flows are 20 to 40 feet thick. What main characteristic of lava do you think is important in determining the thickness of the basalt flows? 2. What rock type (felsic or mafic) would be most likely to form a sill? Explain your answer. 3. What rock type is most likely to form a stock? Explain your answer. 4. Why are igneous rocks classified on the basis of texture and composition instead of size, shape and color? Figure 5 is a picture of Shiprock, NM. 5. What type of igneous structure is this? Figure 5. 6. What are the igneous structures that radiate away from Shiprock? Figure 6 is a picture of Crater Lake, Oregon. What is the feature located inside the lake towards the left side of the lake? 12

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Laboratory 3: Igneous Rocks and Structures Figure 6 7. What do you think happened here to form the crater that the lake is now in? Igneous Rock Data Sheet Sample # Texture Minerals present Color index Rock name Intrusive or extrusive 1 2 3 4 5 6 7 8 13

Laboratory 3: Igneous Rocks and Structures 14

Lab 3 Igneous Rocks and Structures FOR ONLINE

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