Physical Geology Laboratory Manual (Fourth Edition)

PHYSICAL GEOLOGY LABORATORY MANUAL Fourth Edition

Karen M. Woods Lamar University Contributing Authors Margaret S. Stevens James B. Stevens Roger W. Cooper Donald E. Owen James Westgate Jim L. Jordan Bennetta Schmidt

KENDALL/HUNT 4050

Westmark

Drive

PUBLISHING Dubuque,

COMPANY Iowa

52002

Copyright © 1994, 1997, 2001, 2006 by Kendall/Hunt Publishing Company Revised Printing 2009

ISBN: 978-0-7575-6114-6 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of Kendall/Hunt Publishing Company. Printed in the United States of America 10 9 8 7 6 5 4

Preface v Chapter 1 Minerals 1 Introduction 1 Minerals 1 Identification of Mineral Unknowns 12 Mineral Property List 15 Mineral Uses 19 Chapter 2 Rocks 31 Igneous Rocks 31 Sedimentary Rocks 43 Metamorphic Rocks 53 Rock Property List 63 Uses for Common Rocks 67 Chapter 3 Tectonics, Structure, and Soils 69 The Earth (Zones and Characteristics) 69 Continental Drift 71 Plate Tectonics 71 Plate Boundaries 71 Structural Geology 79 Soils 97 Chapter 4 Topographic Maps 107 Elevation 107 Contours 107 Coordinate Systems and Map Locations 121

Chapter 5 Streams, Rivers, and Landscapes 129 Water Cycle 129 Streams and Rivers General Terminology 129 Rivers and Erosion: Development of Landscapes 131 Stream Drainage Patterns 134 Chapter 6 Groundwater, Karst Topography, and Subsidence 139 Groundwater 139 Caves and Karst Topography 141 Karst Topography 141 Subsidence 143 Chapter 7 Shorelines 149 General Shoreline Features 149 Sea-Level Changes: Eustatic, Local, and Regional 151 Emergent Shorelines, Causes and Characteristics 152 Submergent Shorelines, Causes and Characteristics 153 References 159

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Physical Geology L a b o r a t o r y Manual

Physical Geology is the first introductory course in the field of Geology. The faculty and staff of Lamar University, Department of Earth and Space Sciences have collaborated to produce a laboratory manual that is informative and easily understood. It has been customized to present the concepts and ideas the faculty feel are most important in Physical Geology. It is intended to supplement the main lecture course by exposing the student to conceptual exercises and hands-on experience of the subjects introduced in lecture.

,

INTRODUCTION Geology deals with the physical and historical aspects of the Earth. Physical geology is the study of the composition, behavior, and processes that affect the Earth's lithosphere. The science of geology also provides the means to discover and utilize the Earth's natural resources (coal, gas, petroleum, minerals, etc.). Geologists also study the Earth and its processes so that they can better understand and predict potentially dangerous geologic situations (earthquakes, volcanic eruptions, floods, etc.), which results in the saving of lives. Historical geology, the second introductory course, deals with geology as it relates to the Earth's history. This laboratory manual begins with the study of common Earth materials, minerals, and rocks that make up the lithosphere, and proceeds to the kinds of forces and situations that can alter (build up or tear down) the surface of the planet.

MINERALS Minerals are the basic building blocks of nearly all Earth materials for most geological purposes. A mineral is a naturally occurring, solid, inorganic combination (compound) of one or more elements, whose atoms are arranged in an orderly fashion (crystallinity), and has an established chemical composition that can vary slightly within specific limits. Minerals also have a set of physical properties (hardness, color, etc.) that distinguish them from each other. "Inorganic" means that the compound was not the result of organic processes. Natural compounds are not "pure" in the pharmaceutical sense, particularly if modern analytical methods are used. Most chemical elements can be shown to consist of several "isotopes," atoms of different atomic weights that have a closely similar set of chemical properties. Minerals as natural compounds are fairly complicated. They consist of one or more elements that consist of one or more isotopes, are not absolutely "pure" compounds, and show some variation, even within materials called by the same mineral name. The guideline geologists have agreed on to define a particular mineral is the nature of the internal geometric arrangement (the crystallinity) of the atoms. This arrangement is usually called the crystal structure (technically, the term "crystal structure" is redundant—the word "crystal" by itself is sufficient). Materials such as glass and opal have no particular geometric arrangement of their atoms, and are not true minerals because they lack crystallinity. The term "mineraloid" is used for these materials, and some mineraloids are simply called rocks (natural glass, obsidian, is a kind of volcanic rock). 1

SUMMARY: a material must be/have the following characteristics to be classified as a mineral: 1. be naturally occurring (not man-made). 2. be solid. 3. be inorganic (not compounds that can be produced only by living organisms). 4. have a geometric arrangement of its atoms—crystallinity. 5. have a chemical composition that can vary only according to specific limits. A substance that satisfies these requirements will have a characteristic set of physical properties that can be used for identification.

Common Minerals Many of the minerals studied in the laboratory (Table 1.1) are familiar to nongeologists. Some elemental materials (sulfur, graphite, and diamond) are classified as minerals when found in large, natural cohesive quantities. Quartz (Si0 2 , silicon dioxide) is the most commonly known mineral. Varieties of quartz include: rose quartz, milky quartz, chert (in many different colors), flint, agate, rock crystal (clear), amethyst (purple), aventurine (green), jasper (red), etc. Halite (NaCl, sodium chloride) is probably the most commonly used mineral and is found in most spice cabinets as table salt. Minerals have many unexpected uses and a list of some of these uses is found at the end of this chapter.

Physical Properties of Minerals All minerals have a set of distinctive physical properties that can be used to identify them. The goal of the student is to become familiar with geological terminology and apply the terms to unknown mineral specimens in order to correctly identify them. Students should note that the physical properties of each different mineral group are not absolutes. Hardness is one property that can vary from sample to sample of the same mineral. The mineral magnetite has a hardness of 6, but it can actually range between 5.5 and 6.5. Therefore, some specimens of magnetite will easily scratch a glass plate (hardness Ss 6) and some specimens may barely scratch glass or not scratch it at all. Color is another property of minerals that can vary widely and thus should not be the only criterion used for identification of an unknown mineral specimen. Quartz comes in many different colors and is easily confused with other minerals of similar color. Amethyst purple quartz is easily mistaken for purple fluorite, and vice versa. The student should not use any one property alone to identify unknown minerals. A group of physical properties leads to a more accurate identification.

Crystal Form Crystal form is the geometric arrangement of plane ("flat") surfaces on the outside of a mineral that reflect the internal crystallinity of the mineral (Fig. 1.1a and Fig. 1.1b). Crystal faces develop only when the crystal has enough room to grow without interference. The planar (flat) sides of a cube, for example, are called faces. A cube is a crystal form that has six faces (flat sides) (Fig. 1.1a). Halite and fluorite often have cubic ciystal form, while garnet and pyrite have more complicated crystal forms that are variations on the cube. Corundum, quartz, and calcite show different variations on the hexagonal (six-sided) ciystal form (Fig. 1.1b). The hexagonal form of calcite (Fig 1.1b) is the most difficult of these to see, but a calcite crystal will have one or two sharp points, and if one looks along the line between these two points, the visible outline is hexagonal. Minerals without an external crystal form are referred to as massive (chert, limonite, etc.).

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Physical Geology Laboratory Manual

TABLE 1.1

Chemical Groups of Selected Minerals

Chemical Class

Chemical Composition

Mineral/Mineraloid

Natives Only one kind of element present, "naturally pure"

Sulfur Graphite/diamond (not available)

Oxides

Quartz (quartz crystal, milky, rose, chert, smoky, agate, etc.) Oxides of Iron: Oolitic Hematite Specular Hematite Goethite Limonite (mineraloid) Magnetite Corundum Bauxite (mineraloid)

Si0 2

(Silicon dioxide)

Fe 2 0 3 Fe 2 0 3 FeO(OH) Fe 2 0 3 nH 2 0 Fe 3 0 4 A1203 Al 2 0 3 nH 2 0

(Iron oxide) (Iron oxide) (Hydrous iron oxide) (Hydrous iron oxide) (Iron oxide) (Aluminum oxide) (Hydrous Al oxide)

Sulphides (A metal bonds directly with sulfur as the nonmetal)

Pyrite Galena Sphalerite

FeS2 PbS ZnS

(Iron sulfide) (Lead sulfide) (Zinc sulfide)

Sulfates (A metal bonds with the S 0 4 complex ion acting as a nonmetal)

Gypsum (Selenite, Satin spar, Alabaster) Anhydrite

CaS0 4 2 H 2 0 (Hydrous calcium sulfate) (Calcium sulfate) CaS0 4

Carbonates (A metal bonds with the C 0 3 complex ion acting as a nonmetal)

Calcite Dolomite

GaC0 3 MgCaC0 3

(Calcium carbonate) (Calcium-magnesium carbonate)

Halides (A metal bonds with a halogen [CI, F, Br or I] as the nonmetal)

Halite Fluorite

NaCl CaF 2

(Sodium chloride) (Calcium fluoride)

(A metal bonds directly with oxygen as the nonmetal)

S

c

(Sulfur) (Carbon)

Silicates (A metal bonds with the Si0 4 complex ion as the nonmetal) Nesosilicates (island silicates)

Garnet

(Fe, Mg)Si0 4 (Iron magnesium silicate) Complex Ca, Mg, Fe, Al silicate

Inosilicates (chain silicates)

Hornblende Augite

Ca, Na, Fe, Mg, Al silicate (Ca,Na)(Mg,Fe,Al)(Si,Al)206

Phyllosilicates (sheet silicates)

Muscovite

Biotite Chlorite Talc Kaolinite

OH, K, Al silicate (Hydrous potassium-aluminum silicate) OH, K, Mg, Fe, Al silicate OH, Mg, Fe, Al silicate OH, Mg silicate OH, Al silicate

Orthoclase Plagioclase (Albite, Labradorite) Quartz

K, Al silicate Ca, Na, Al silicate SiQ2

Tectosilicates (3-D silicates)

Olivine

Chapter 1

Minerals

3

Crystal Systems Crystal systems are groups of crystals based on the symmetry of crystal faces. There are six crystal systems and within these systems there are the thirty-two classes of minerals. The six crystal systems are cubic (isometric), hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic (Fig. 1.1a and Fig. L i b ) . The cubic (isometric) crystal system consists of three equal-length axes intersecting at 90° angles from one another. The hexagonal crystal system consists of three equallength axes that intersect at 120° angles to one another and a fourth axis perpendicular the first three axes. The tetragonal crystal system consists of two equal-length axes and a third axes of a different length, all at 90° angles to one another. The orthorhombic crystal system consists of three axes of different lengths that intersect at 90° angles to one another. The monoclinic crystal system consists of two unequal-length axes that intersect at 90° angles and a third that intersects obliquely. The triclinic crystal system consists of three unequal-length axes that intersect obliquely. Crystal systems are studied in more detail in the upper-level Mineralogy course.

Crystals "grow" as "invisible atoms" of a solution bond together in a given geometric framework that is consistent with the atoms' electrical or size characteristics. As the geometric framework enlarges with continued "growth," that geometry becomes visible as smooth surfaces that are called crystal faces. The smooth crystal faces give crystals of various minerals their characteristic shape and beauty.

Galena

Isometric (Cubic) Ciystal System Three equal-length axes that intersect at 90" angles. Two of the axes intersect on the same plane, and the third is perpendicular. Typical Minerals Pyrite Malite Fluorite Galena Magnetite Tetragonal Ciystal System Two equal-length axes and a third, either longer or shorter, that intersect at 90" angles. Two of the axes intersect on the same plane, and the third is perpendicular. Typical Mineral Zircon Orthorhombic Ciystal System Three axes of different lengths that intersect at 90" angles. Two of the axes intersect on the same plane, and the third is perpendicular. Typical Minerals Topaz Staurolite

FIGURE 1.1a Crystal Systems, Crystal Forms, and Typical Minerals.

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Physical Geology Laboratory Manual

Monoclinic Crystal System Two unequal-length axes that intersect at 90° angles on the same plane, and a third that intersects obliquely. Typical Minerals Orthodase Gypsum

Triclinic Crystal System tersect obliquely. \ I v

Plagioclase Feldspar

Three unequal-length axes that in-

Typical Minerals Plagioclase Feldspar

Corundum Hexagonal Crystal System Three unequal-length axes that intersect at 120° angles on the same plane, and a fourth that is perpendicular to the other three.

Calcite

Apatite

Quartz

Typical Minerals Quartz Corundum Apatite Calcite

FIGURE 1.1b Crystal Systems, Crystal Forms, and Typical Minerals.

Chapter 1

Minerals

5

Cleavage Cleavage is the tendency of a mineral to break in a systematic (regular, ordered) way, along planes of weakness determined by the type and strength of the chemical bonds (see lecture book) between the atoms that make up the mineral (Fig. 1.2a and Fig. 1.2b). The cleavages (planes of weakness) represent layers between rows or sets of planar atoms where the atomic bonds are weaker. Some minerals (micas and gypsum) have one direction of cleavage (Fig. 1.2a) but most minerals have multiple cleavage directions. Not all specimens of a given mineral will have readily identifiable cleavage planes, although it is a useful identifying feature when present. Even when cleavage planes are not visible on a particular hand specimen, it does not mean that the mineral lacks cleavage. Look at other examples of the same mineral. Some cleavage surfaces are microscopic and therefore invisible to the naked eye. Since many minerals do not have cleavage or have microscopic cleavage (not visible to the naked eye), you can use the presence of visible cleavage to eliminate those minerals that do not have cleavage. Some minerals always demonstrate cleavage, such as muscovite and biotite, which have cleavage in one direction. Muscovite and biotite easily cleave (split) into flat, flexible sheets. Unfortunately, cleavage and crystal form are easily confused. They both result in flat planes, but for different reasons. Some minerals have both crystal form and cleavage (halite, fluorite, calcite, etc.), some only have cleavage (muscovite), and some only have crystal form (quartz). Minerals with cleavage will break in the same direction or set of directions every time and form flat planes or a stair-step pattern on the mineral face. A mineral with only crystal form will break in no particular direction and develop irregular (uneven) surfaces when broken.

Fracture Fracture is the nonsystematic and irregular way some minerals break. The fracture surface is rough or uneven, unlike cleavage planes, which are smooth and flat. Conchoidal fracture is a special kind of breakage that results in a curved parting surface. When a bullet passes through glass, a curved or listric surface is produced (conchoidal fracture). Conchoidal fracture is characteristic of homogenous materials that lack planes of weakness, thus the material is about equally strong in all directions (e.g., glass). Quartz commonly shows conchoidal fracture. NOTE: Some minerals display both fracture and cleavage. Albite, for example, has two directions of cleavage (two flat sides) and two opposing sides with fracture (rough sides).

Striations Striations are very fine, parallel lines visible on the cleavage planes or crystal faces of some minerals due to their crystal structure and growth patterns. Albite and labradorite, both plagioclase feldspars, commonly exhibit striations on one cleavage plane. The striations on plagioclase become increasingly obvious as the calcium content of the feldspar increases. Striations may also be visible on the crystal faces of other minerals such as pyrite, quartz, and garnet. Striations become more visible when the mineral is slightly rotated back and forth in the light. As the mineral is turned, the striations reflect the light.

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Physical Geology Laboratory Manual

Cleavage: Cleavage is the tendency of certain minerals to split (cleave) along planes of weakness, between layers of weak bonds that unite the atoms of which the mineral is made, when the mineral is broken. Some minerals cleave in only one direction, others have two, three, four, or even six directions of cleavage. Examples are shown below. CAUTION: Beginning geology students often confuse the smooth cleavages surfaces with the smooth crystal faces of minerals crystals, and thus often believe that cleavage "chunks" are crystals. Crystal faces are produced when minerals "grow" as invisible "atoms" of various elements within a solution and bond together in a given geometric framework called crystallinity. The cleavage surfaces of cleavage "chunks" form when the mineral breaks.

One Direction of Cleavage Certain minerals, when broken, break only along one plane. Typical Minerals Biotite Muscovite Chlorite Talc Selenite Gypsum

One dimensional cleavage sheet. Biotite or Muscovite

Two Directions of Cleavage Certain minerals, when broken, break along two plane surfaces that intersect at a 90° angle to each other. Typical Minerals Orthoclase Feldspar Plagioclase Feldspar Cleaved chunk removed

Three Directions of Cleavage Certain minerals, when broken, break along three plane surfaces that intersect at a 90° angle to each other. Typical Minerals Galena Halite

Halite

Galena

FIGURE 1.2a Cleavage

Chapter 1

Minerals

7

Cleaved chunk removed

Three Directions of Cleavage Certain minerals when broken, break along three planer surfaces that intersect obliquely to each other. Typical Minerals Calcite

Four Directions of Cleavage Certain minerals, when broken, break along four planar surfaces that intersect at different angles. Octahedral (8-sided) cleavage chunk, Fluorite

Typical Minerals Fluorite

Six Directions of Cleavage Certain minerals, when broken, break along six planar surfaces that intersect at different angles to each other. Dodecahedral (12-sided) cleavage chunk, Sphalerite

FIGURE 1.2b Cleavage.

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Physical Geology L a b o r a t o r y Manual

Typical Minerals Sphalerite

Tenacity Tenacity is the resistance of a mineral to breakage. Some minerals are very hard to break, whereas others are easily broken. Terms used to describe tenacity include brittle, elastic, and malleable. Gold, a soft mineral, is malleable and easily deformed when hit. Diamond, the hardest known mineral, is very brittle and will shatter when hit. Do not test the tenacity of mineral specimens unless instructed to do so. TABLE 1.2 Mohs' Scale of Hardness Btssititsisisssijsiaswswm^!^

10

Diamond

9

Corundum

8

Topaz

7

Quartz

6

Orthoclase Feldspar

5

Apatite

4

Fluorite

3

Calcite

2

Gypsum

1

Talc

.•,»:•,••5J \ / [No Cleavage) [Cleavage] Alb'ne (Plagioclase Feldspar) Labrador! te (Plagioclase

Corundum Alabaster Gypsum

Cheri Milky Quartz Rock Crystal Quartz.

Feldspar)

Rose Quartz

Orthoclase Feldspar

Augiie

[Hardness 5.5) or softer than the glass plate (

iL

~a) K^Jt

^ - - ^ ^ ^ Mineral Properties ~-~^[ame Chemical Formula Hardness Range Exact if on Mohs' Scale Luster ? if Metallic Describe if nonmetallic Streak (color) Diaphanaeity Transparent, Translucent or Opaque Magnetism ? if Magnetic X if Nonmagnetic Crystal Form Describe if Visible X if Not Visible Specific Gravity ? if Heavy, N if Normal, X if Light Cleavage # of Planes if Visible, X if None Visible Fracture Yes, No, ? if Conchoidal Reaction to HCI Acid Describe Reaction Color of Mineral Striations ? if Present, X if Not List one Use

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"* ^ ^ ^ Mineral Properties"""""-" Njime Chemical Formula Hardness Range Exact if on Mohs' Scale Luster ? if Metallic Describe if nonmetallic Streak (color) Diaphanaeity Transparent, Translucent or Opaque Magnetism ? if Magnetic X if Nonmagnetic Crystal Form Describe if Visible X if Not Visible Specific Gravity ? if Heavy, N if Normal, X if Light Cleavage # of Planes if Visible, X if None Visible Fracture Yes, No, ? if Conchoidal Reaction to HC1 Acid Describe Reaction Color of Mineral Striations ? if Present, X if Not List one Use

-w^

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sL^'

twin

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"^•^^.^^ Mineral Properties^^^^-^ 3 ™^ Chemical Formula Hardness Range Exact if on Mohs' Scale Luster ? if Metallic Describe if nonmetallic Streak (color) Diaphanaeity Transparent, Translucent or Opaque Magnetism ? if Magnetic X if Nonmagnetic Crystal Form Describe if Visible X if Not Visible Specific Gravity ? if Heavy, N if Normal, X if Light Cleavage # of Planes if Visible, X if None Visible Fracture Yes, No, ? if Conchoidal Reaction to HCI Acid Describe Reaction Color of Mineral Striations ? if Present, X if Not List one Use

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Chemical Formula Hardness Range Exact if on Mohs' Scale Luster ? if Metallic Describe if nonmetallic Streak (color) Diaphanaeity Transparent, Translucent or Opaque Magnetism ? if Magnetic X if Nonmagnetic Crystal Form Describe if Visible X if Not Visible Specific Gravity ? if Heavy, N if Normal, X if Light Cleavage # of Planes if Visible, X if None Visible Fracture Yes, No, ? if Conchoidal Reaction to HC1 Acid Describe Reaction Color of Mineral Striations ? if Present, X if Not List one Use

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Rocks

A rock is a natural aggregate (combination) of one or more minerals, mineraloids, glass, and/or organic material. There are three families of rocks distinguished from one another by the processes involved in their formation. The three rock families are: Igneous—originating from a molten silicate melt. Sedimentary—originating from the deposition of the by-products of weathering. Metamorphic—develop via the change in form or chemical composition of preexisting rocks and minerals by new conditions of temperature, pressure, and/or the addition of hot chemical fluids. Igneous, sedimentary, and metamorphic rocks are described and identified on the basis of their composition and texture. Composition, in general, refers to the chemical makeup, the particular elements that are present in the rock. Texture, in general, refers to the size, arrangement, and shape ("morphology") of the constituent minerals or materials in the rock. There are different sets of textural terms for each rock family that often denote the same or closely similar conditions.

IGNEOUS ROCKS Igneous rocks are the solids produced by the cooling and crystallization of molten silicate material initially formed beneath the Earth's surface. Crystallization occurs when cooling allows for the growth of mineral crystal grains. The cooling rate and space available determine the size of the crystals that form. Large crystals form when magma, molten silicate material below ground, is insulated by the surrounding country rock (rock that has been intruded by the magma), and therefore cools very slowly. When magma solidifies underground, it forms intrusive (plutonic) igneous rocks. The shape and position of emplacement differentiate plutonic igneous rock bodies. A dike is a pluton that cuts across pre-existing rock (strata). (Principle of Cross-Cutting Relationships: a rock body must already exist in order for it to be cut by another). A sill is a two-dimensional pluton that is placed parallel to and between layers (strata) of existing rock. Batholiths are very large, three-dimensional plutons, usually the result of multiple intrusions of magma, hundreds of miles in length and width, which cool and crystallize very slowly beneath the Earth's surface. A laccolith is a smaller three-dimensional pluton with a convex roof and a flat floor. Volcanic (extrusive) igneous rocks form on or above the surface of the Earth by the cooling of lava (molten silicate flows on the surface), or by the deposition of violently ejected pyroclastic (pyro = fire, dast = fragment) material such as volcanic ash. Lava cools

31

faster than magma because it is exposed to environments that allow for the rapid dissipation of heat and therefore prevent the formation of large crystals. In general, most extrusive igneous rocks develop crystals that are too small to be seen without the aid of a microscope. There are different types of basaltic lava. Aa is blocky, sharp-edged lava that moves very slowly and pahoehoe is ropy, "smooth" lava. Volcanic glass (obsidian) forms when lava is cooled too rapidly for crystals to develop.

Bowen's Reaction Series Igneous rocks, with few exceptions, are made of silicate minerals. An understanding of igneous rock formation can be gained by considering Bowen's Reaction Series. Bowen's Reaction Series (Figure 2.1) is the result of experiments conducted by N. L. Bowen and first published in 1928. Bowen's Reaction Series is an organization of the silicate minerals according to the conditions required to crystallize them, as the temperature of a melt lowers. Bowen discovered that in addition to the availability of needed chemical elements, temperature and pressure determine when and where given silicate minerals form. He observed that some minerals form as a continuous series belonging to a single silicate family (tectosilicates) but with progressive change (substitution) of chemical composition, whereas others form as a discontinuous series of different silicate crystal families as their crystal structures readjust. The discontinuous series of readjustments proceeds from what could be thought of as 0 (zero) dimensional arrangements (highest temperatures and pressures) through 1-D, 2-D, to 3-D arrangements (low temperature/pressure) if nil of the necessary elements to build a particular mineral are available. The continuous series involves the plagioclase feldspar group. These minerals have a three-dimensional covalently bonded structure that includes metal ions. The structure is

Bowen's Reaction Series (Near Surface Conditions) Low Temperature Low Pressure

Matiy textbooks show Bowen's Reaction Series in fliis orientation

Quartz Muscovite Sheet Silicate Microcline Orthoclase #

3-D Silicates

9 ^

©

#

.c

4?

Biotite /\ Sheet Silicate A , b » t e Amphlbole (Hornblende) Ohgoclase Double Chain Silicate Andesinc

Pyroxene (Augite) Single Chain Silicate Olivine y Island Silicate FIGURE 2.1 Bowen's Reaction Series.

32

a

Physical Geology L a b o r a t o r y M a n u a l

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r

Qj r

t-

<S> \

Labrador! te x High Temperature Bytowmle High Pressure > ,; ., Deeper AnorthUe ... \ or Deep Underground

continuously modified as ions are exchanged with the magma during cooling. Calcium-rich (Ca) plagioclase crystals (anorthite, CaAl2Si208) first begin to form when the magma has cooled to 1400 to 1200°C. As cooling continues (1200 to 1000°C), the crystals exchange Ca and aluminum (Al) ions for sodium (Na) and silicon (Si) ions from the magma, to form crystals that are more sodium and siliconrich. Calcium-rich plagioclase crystals also form directly from the magma at this temperature range. If the temperature of the magma continues to decrease very slowly so that equilibrium is approximately maintained, plagioclase feldspars will continue to exchange ions in this manner until the magma solidifies. If there is sufficient sodium, Ca plagioclases disappear completely, but in many magmas all of the Na and Al becomes bonded early and is lost from the system. Thus this process— which can proceed successively from anorthite (Ca-rich), to bytownite, labradorite, andesine, oligoclase, and albite (Na-rich)—in practice produces a variety of different minerals, depending on the original composition of the magma and the rate of cooling. Silicate minerals of the discontinuous series have a variety of different structures of increasing complication that appear and disappear successively and predictably, as conditions (mainly temperature) in magmas change. The following FIGURE 2.2 Silicate Structures. discussion is primarily concerned with decreasing temperature, but the effects of pressure are generally similar. Olivine (1400-1200°C) is the first mineral (stable silicate or structure) to appear. The olivine crystal consists of individual tetrahedra (plural of "tetrahedron;" four oxygen and a much smaller silicon hidden in the center; Fig. 2.2a) tied together by bivalent iron [Fe++] and magnesium [Mg++] ions in a three-dimensional network. Olivine crystals become unstable when the melt cools to about 1200 to 1000°C, the temperature range in which pyroxene becomes stable. Augite is an example of a common mineral in the pyroxene family. Olivine crystals suspended in the magma react to form the more complex single chain (pyroxene, augite, Fig. 2.2b) silicate structure. Amphibole (another family of silicate minerals, of which hornblende is a common example) becomes stable at approximately 1000 to 800°C. Again the earlier-formed (pyroxene, augite) crystals react with the melt and form double chain (Fig. 2.2c) amphibole (hornblende) crystal structures. If sufficient magma and silica (Si0 2 ) are still available, the hornblende will react with it and will begin to change to biotite, a sheet silicate (Fig. 2.2d). Orthoclase and microcline (both three-dimensional covalently bonded structures with metal ions), muscovite (sheet structure), and quartz (three-dimensional structure) will form last if enough magma is left.

Igneous Rocks: Composition The composition of igneous rocks can be determined, in a general way, in hand specimens by the relative abundance and color intensity (pale versus dark or strong color) of the minerals that make up the rock (Figure 2.3). Chapter 2

Rocks

33

FIGURE 2.3 Common Igneous Rocks (mineralogy and composition).

1. Felsic (sialic) igneous rocks are composed mainly of potassic and sodic feldspars (light-colored minerals) that formed under low-temperature and low-pressure conditions (Bowen's Reaction Series). Felsic rocks include syenite, trachyte, granite, rhyolite, granodiorite, dacite, and some obsidian. 2. Intermediate igneous rocks have subequal amounts of light and dark minerals. Intermediate rocks include andesite (named for the Andes Mountains), and diorite. 3. Mafic igneous rocks have a large percentage of darker and strongly colored minerals rich in ferromagnesian components and calcic plagioclase feldspars. These are minerals that form under high-temperature and high-pressure conditions (Bowens Reaction Series). Mafic rocks include basalt and gabbro, and some obsidian. 4. Ultramafic igneous rocks often contain 70 to 90 percent olivine, other dark and strongly colored ferromagnesian minerals, the most calcic plagioclases, and very minor, if any, percentages of silica. These minerals form under very high-temperature and high-pressure conditions (Bowen's Reaction Series). Ultramafic rocks include peridotite and komatiite. Ultramafic rocks are not common at or near the Earth's surface, but form in the asthenosphere and mantle.

Igneous Rock Texture The texture of igneous rocks refers to the physical appearance ("visual feel") and arrangement of minerals within the rock. Texture may include the absence of crystals in a rock, the presence and/or relative size of the crystals that make up the rock, any contrast in crystal sizes within a given rock specimen, the arrangement of minerals in a rock, and/or the presence of bubbles (vesicles) in the rock. 1. Glassy texture is applied to igneous rocks that have cooled so rapidly that crystals didn't have time to develop and grow. Igneous rocks that have a combination of glass and visible crystals are referred to as vitrophyres. Vitrophyres often form during rapid intrusion when magma comes in contact with much cooler, surrounding,

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Physical Geology Laboratory Manual

country rock. A chilled (rapidly cooled) margin is a thin zone of rapidly cooled igneous rock that forms a rind on the pluton, and can be aphanitic, glassy, or vitrophyric. 2. Aphanitic is the textural term used to describe igneous rocks that have crystals that are approximately uniform and small in size. "Small" means that the crystals are not distinguishable by the unaided eye ( « 1 millimeter); both microcrystalline (crystals visible only under the microscope) and cryptocrystalline (crystals too small to be clearly distinguished with the ordinary microscope) textures are included. 3. Phaneritic is the textural term used to describe igneous rocks that have crystals large enough to be seen without magnification ( ~1 mm) and less than 1 inch (2.54 cm, medium size). The rock is "megascopically crystalline." 4. Pegmatitic is the textural term applied to igneous rocks in which the crystals are large or very large. "Large" means very coarsely crystalline; crystals in a pegmatite may be a few centimeters (1 inch = 2.54 cm) or several meters (100 cm, or =»39 and 1/3 in/m) in length. 5. Porphyritic texture is the term used when the crystals in a rock fall into two distinct size groups (small versus large). When a rock has this combination of crystal sizes, porphyritic is usually appended to the rock name. Porphyritic basalt is an example. A vitrophyre is a special kind of porphyritic rock. The smaller aphanitic crystals and glass, if present, form what is called the matrix or groundmass matrix of the rock. The matrix, if aphanitic or partly glassy, contains minerals that formed at low temperatures and pressures. The larger crystals are called phenocrysts. Phenocrysts are commonly early-formed, slow-growing minerals that crystallize at higher temperatures and pressures. As a result, phenocrysts are more likely to have better formed crystal faces (be euhedral) than crystals that form later. Crystals that form late, quartz in a granite for instance, do not develop crystal faces (are anhedral). The rock is probably volcanic or part of a very shallow intrusion (plumbing of a volcano) if the groundmass is aphanitic. 6. Vesicular texture describes volcanic igneous rock with bubbles (holes). The bubbles form when pressure is released during eruption and volatile components of a magma exsolve (come out of solution). Water (H 2 0) and carbon dioxide (C0 2 ) are the two most abundant volatile components. Bubbles are most commonly found in volcanic rocks, but sometimes occur in the uppermost parts of dikes that were part of the plumbing for an eruptive center. Expansion of the gasses formed the bubbles, and the expansion helps to cool the magma/lava. Bubbles range in size from very small (small fractions of a mm) to more than a meter, although very large vesicles are uncommon. The rate of cooling and the viscosity of the magma/lava control the size of the bubbles. Most of the terms arising trom vesicular textures are associated with abundance of vesicles as well as size. Rocks with widely spaced and clearly visible vesicles are referred to as vesicular. Vesicular basalt is a common volcanic igneous rock. Rocks with closely spaced bubbles that are on the order of 1 to 2 mm in diameter or larger may be referred to as scoria. Scoriaceous basalt identifies the tops of basalt flows. Contact with oxygen in the air oxidizes iron in the glass to produce tiny crystals of hematite, and a reddish color in many scorias. Scoria has a very low density for a rock, but usually does not float on water. Most scoriaceous rocks are mafic in composition. Rocks with microscopic bubbles (less than a mm, usually) may be referred to as pumice. Most pumiceous rocks are felsic. Pumice often has a density so low that the rock will float on water. Scoria and pumice both have vesicular texture, yet they are both forms of obsidian, volcanic glass. The release of gases creates a frothy or vesicular texture in the obsidian. Vesicles later filled with solid material (secondary minerals) are called amygdules. Amygdaloidal basalt is basalt with filled vesicles. Common vesicle-filling minerals include quartz crystals, chalcedony, agate, and calcite. Large chalcedony or agate-filled

Chapter 2

Rocks

35

amygdules can be handsomely colored and have some value to collectors. Large, partially hollow amygdules are sometimes referred to as "geodes," though technically geodes form in sedimentary rocks.

Arrangements of Crystals and Bubbles The arrangement of crystals and bubbles is also an important aspect of texture in igneous rocks. For the purposes of this discussion, an "arrangement" of textural elements is a situation where the occurrence or orientation of the feature is not random. Arrangements are a product of local variations in the chemical and physical conditions within a magma or lava and gravity. Understanding of such nonrandom organizations of textural features, and particularly specific kinds of crystals (minerals) is a major area of study in igneous petrology and geochemistry. The beginning of crystallization of any mineral variety requires just the right balance of ion availability, temperature, and pressure. A simple way for all of these to vary at once is for the magma to be in motion, flowing. Shearing stresses induced by flow can align existing acicular (needle-shaped) crystals producing an arrangement called flow lineation. Surfaces of low pressure sub-parallel to the boundaries of flow develop when flow expands (cross-sectional area of the flow increases). These can localize precipitation of sheet-like mineral masses, flow foliation. Early-formed crystals have densities greater than that of the magma, and collect near the bottom of magma chambers under the influence of gravity. The natural form of bubbles and vesicles is spherical, or, when many bubbles are closely packed (scoria), compact. Under the influence of gravity, bubbles rise and expand to collect at the top of a lava flow, if the viscosity is low. Flow will deform vesicles into ellipsoids all aligned in the direction of flow. Basaltic lava can move fast enough to achieve very complicated (turbulent) flow, somewhat like that of water in a brook. In this situation, vesicles can take complicated shapes.

Identification of Igneous Unknowns Use the "Key to the Identification of Some Common Igneous Rocks" as you identify your igneous rock unknowns, then use the study guide for review. Igneous rock pictures can be found on the Earth & Space Sciences website (http://ess.lamar.edu/) Click on People, Staff, Woods, Karen M., Teaching, Physical Geology Lab, Igneous Rocks

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Physical Geology Laboratoiy Manual

i Mill I I III ll

III

U

I

Key to the Identification of Igneous Rocks Felsic (Sialic) Composition, Light Colored Minerals TEXTURE

NAME If with substantial quartz

Phaneritic Aphanitic

•• RHYOLITE

Glassy

"*~ OBSIDIAN

Vesicular

If with small bubbles, finely puffed

-; Volcanic (Extrusive)

•*• PUMICE PORPHYRITIC •*" RHYOLITE •>- LLANITE

If with phenocrysts, primary angular crystals. Porphyritic

Plutonic (Intrusive)

-• GRANITE

If with blue quartz crystals

Dual Origin

Intermediate Composition, Subequal amounts of Light & Dark Colored Minerals Phaneritic D

.

Porphyritic

• DIORITE ^

T

o

.+.

.

t

.

.

• If with phenocrysts, primary angular crystals

__

•

•

+ - PORPHYRITIC PORPHYRITICDIORITE DIORITE**\ .

Plutonic (Intrusive) _

"^~~~--~-~w PORPHYRITIC ANDESITE -

Aphanitic

, _ . .

Dual On gin Volcanic (Extrusive)

•*- ANDESITE

Mafic Composition, Dark Colored Minerals Phaneritic If with large bubbles, coarsely puffed Aphanitic —•BASALT \

If vesicles filled with secondary mineral such as quartz or calcite If with medium sized bubbles

Glassy Porphyritic

_^ AMYGDALOIDAL BASALT SCORIA

^r

Phaneritic

•

Volcanic (Extrusive)

BASALTIC OBSIDIANTf with phenocrysts, primary angular crystals

PORPHOR1TIC BASALT

Ultramafic Composition, 70-90% Olivine UJ

Plutonic (Intrusive)

GABBRO -• VESICULAR BASALT.

•• PERIDOTITE

Dual Origin Plutonic (Intrusive)

I

H

1.1

CO

Guide to the Identification of Common Igneous Rocks

PUMICE

^

v

^