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Geology • Geology – the study of the the composition, structure, physical/chemical processes, and history of the Earth
Chapter 22: Minerals, Rocks, and Volcanoes
– Geology is also related to the study of planets and moons of our solar system
• Homework: All questions on the “Multiple-Choice” and the oddnumbered questions on “Exercises” sections at the end of the chapter.
• The basic concepts of geology are introduced and briefly discussed in the next four chapter so that we may better understand the physical nature of the world that we live in.
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Intro
Minerals
Minerals
• Mineral – a naturally occurring, inorganic, crystalline (solid) substance consisting of one or more chemical elements in fairly specific proportions with a distinctive set of physical properties • Minerals are around us everywhere – some are quite valuable (diamond, sapphires, emeralds) other minerals are very common (calcite, quartz.) • Mineralogy – the study of minerals
• We also speak of certain rock-types (ores) as being rich in minerals.
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Section 21.1
– For example an iron ore, gold ore, or copper ore – Rocks classified as ores have a commercially valuable amount of a some type of element or mineral.
• In many cases, mineral names have some type of historical connotation and reflect the name of a geographic locality or a person’s name. 21 | 3
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Relative Abundance by Mass of Elements in the Earth’s Crust
Mineral Composition • Eight chemical elements are the major constituents of most of the minerals. • These elements are the most abundant elements in the Earth’s crust and include O, Si, Al, Fe, Ca, Na, K, and Mg. • Over 2000 minerals have been identified in the Earth’s crust. • Only about 20 of these minerals are common. • Less than 10 minerals account for more than 90% of the Earth’s crust by mass. Copyright © Houghton Mifflin Company. All rights reserved.
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Section 21.1
• Only two elements, O & Si, account for 74% of the elements (by mass) in the Earth’s crust. Section 21.1 21 | 5
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Mineral Groups – the Silicates
Silicates • Silicate structure is based on a network of SiO4-4 tetrahedra. • These basic building blocks of the silicates are called the silicon-oxygen tetrahedra.
• Most minerals that form crustal rocks have significant amounts of oxygen and silicon. • The mineral quartz (silicon dioxide or silica) is entirely composed of O & Si, and has the chemical formula, SiO2. • Quartz and many other minerals that contain O & Si are called silicates. • Silicates are the most abundant family of rock-forming minerals in the crust. Copyright © Houghton Mifflin Company. All rights reserved.
Section 21.1
– Covalent bonding occurs within the tetrahedron.
• A significant amount of variation occurs in the structural arrangement of these tetrahedra. • The silicon-oxygen tetrahedra may exist independently (no sharing of O) or they may share oxygens in various ways. 21 | 7
The SiliconOxygen Tetrahedron
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• In addition to O & Si, most silicate minerals also contain Al and one or more other elements. • The various silicate minerals result from the different structural arrangement of the siliconoxygen tetrahedron building blocks and the different metal ions within these structures. • The silicon-oxygen tetrahedra may be arranged independently, in single chains, in double chains, as continuous sheets, and as a 3-dimensional network. Section 21.1
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Molecular Structures of Several Common Silicate Minerals
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Section 21.1
Silicates
• Basic Building Block of the Silicate Mineral Family
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Feldspars – Most Common Silicate Minerals • Feldspar group - a group of structurallyrelated (3-dimension network) minerals, that as a set, are the most abundant minerals in the Earth’s crust • There are two basic types of feldspars: • Plagioclase feldspars – O, Si, Al, and Ca or Na • Potassium feldspars – O, Si, Al, and K • All silicates have Si and O in their formulas. Section 21.1
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Nonsilicate Minerals
Carbonates
• All of the other minerals in the crust are considered “nonsilicate.” • Nonsilicate minerals comprise less than 10% of the minerals in the crust. • Carbonates, oxides, and sulfides are the most common nonsilicate minerals. • Pure elements such as gold and silver, and some gemstones (diamond, ruby, sapphire) are also nonsilicates.
• Carbonate minerals form when the carbonate ion (CO32-) bonds with certain metal ions.
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Section 21.1
– Ca, Mg, Mn, Fe, and others may bond with CO32-
• CaCO3, calcite, is a very common carbonate mineral. – CaCO3 readily reacts (dissolves) in acidic water.
• Calcite is a very common mineral component of the the rock limestone. – Limestones will dissolve over time in slightly acidic ground waters, contributing to cave formation. 21 | 13
Oxides & Sulfides
Section 21.1
• Mineral classification is based on both physical and chemical properties. • This is advantageous because there are some minerals that have the same chemical formula but different molecular structures. • For example the two minerals graphite and diamond are both made of pure C but they are dramatically different minerals.
– Fe2O3, SnO2, and UO2 are all oxide ores for their respective metallic ion.
• Sulfides minerals form through the bonding of S ions with metallic ions. – Fe, Pb, Cu, Zn, and others may bond with S2-. – FeS2 is pyrite, commonly called “fool’s gold.” – CuFeS2, PbS, ZnS are all sulfide ores for their respective metallic ion.
– Pure diamond is very hard, clear, and crystalline. – Pure graphite is soft and black (dry lubricant and pencil lead.) Section 21.1
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Section 21.1
Identification of Minerals
Physical Properties – Crystal Form
• Certainly minerals can be identified by careful and costly chemical analysis. • Many minerals can be easily identified by taking advantage of the distinctive physical properties that each exhibits. • These physical properties are easily learned and well known by any serious rock and mineral collector.
• Crystal form – the outward form of a crystal is a visible representation of its internal molecular arrangement
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– Many minerals have characteristic crystal forms. – If crystallization occurs in unrestricted space, with just the right components, and just the right P & T conditions a perfect crystal forms. This is very rare.
• Usually an aggregate of crystals form with none exhibiting a perfect crystal form.
– A particular physical property that helps ID one mineral may not help with another, so the key is to find which property ID’s which mineral. Copyright © Houghton Mifflin Company. All rights reserved.
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Identification of Minerals
• Oxide minerals form through the bonding of O ions with metallic ions.
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Each of these minerals has an orderly internal atomic structure that forms a cube. Galena (PbS)
Pyrite (FeS2)
Physical Properties – Hardness & Cleavage • Hardness – a mineral’s resistance to scratching or abrasion – Hardness is relative to other minerals or substances.
• Cleavage – the tendency of some minerals to break along distinct planes
Halite (NaCl)
– These planes represent planes in the crystal’s structure where the bonds are weakest.
Fluorite (CaF2) Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Physical Properties – Fracture & Color
Mohs Hardness Scale
• Fracture – irregular or random breakage of a mineral
Friedrich Mohs (1773-1839)
– Minerals that do not cleave have no planes of weakness in their structure, therefore do not break in a uniform manner.
• Color – the property of reflecting particular light wavelength – Although this is probably the first property noticed, it is usually not reliable. – Quartz, for example, occurs in many colors, including clear, milky, smoky, yellow, purple, red, orange, pink. Copyright © Houghton Mifflin Company. All rights reserved.
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Physical Properties – Streak & Luster
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Hematite – red brown streak
• Streak – the color of the mineral in powder form – Although an intact mineral sample my exhibit several colors, the streak color is always the same.
• Luster – how a mineral’s surface reflects light – Minerals may exhibit either metallic or nonmetallic luster. – Metallic lusters appear like polished metal. – Nonmetallic lusters can be vitreous (glassy), adamantine (diamond), pearly (opal), greasy (talc), or earthy/dull (clay.) Source: Breck P. Kent Copyright © Houghton Mifflin Company. All rights reserved.
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Physical Properties – Specific Gravity
Rocks
• Specific gravity – ratio of a mineral’s weight to the weight of an equal volume of pure water • Many minerals have specific gravities between 3-4 (in other words, 3 to 4 times as heavy as water.) • Some minerals have a significantly higher specific gravity.
• Rock – a natural, solid, cohesive aggregate of one or more minerals • Different types of rocks comprise the vast majority of the Earth’s crust. • In general, when we look at a rock we do not see the individual minerals. • Rocks can be divided into three major types as a result of the way they originated.
– Galena (PbS) has a specific gravity of 7.6. – Pure gold (Au) has a specific gravity of 19.3.
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Igneous Rocks
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Enchanted Rock, TX – Solidified Magma
• Igneous rock – a type of rock formed from a molten material that has cooled and solidified • Magma – molten rock material that originates deep within the Earth – Rocks that solidify from a magma, beneath the Earth’s surface, are called “intrusive” igneous rocks.
• Lava – molten rock material that reaches the Earth’s surface due to a volcanic eruption – Rocks that solidify from lava, at the surface, are called “extrusive” igneous rocks. Copyright © Houghton Mifflin Company. All rights reserved.
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Kalapana Flow, HI – Solidified Lava
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Sedimentary Rocks •
•
Sedimentary rock – rocks that form at or very near the surface of the Earth due to compaction and cementation of sediments The sediments that comprise sedimentary rocks come from three general sources: 1) Rock fragments due to the erosion of preexisting (older) rocks 2) Minerals chemically precipitated from solution 3) Plants or animal remains (fossils)
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Grand Canyon, AZ – Sedimentary Rock Layers
Metamorphic Rocks • Metamorphic rock – forms by the alteration of a preexisting rock due to the effects of pressure, high temperature, and/or a chemical change • Metamorphism generally occurs well below the surface of the Earth but at shallower depths and temperatures than would cause the rock to melt.
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El Paso, TX Contorted Metamorphic Rocks
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Uniformitarianism • Beginning with the Scottish physician/scientist James Hutton, scientists started to realize that ancient rocks were formed the same way as modern rocks. – Since they were formed the same way, they can be interpreted similarly.
• Unifomitarianism – geologic processes occurring today operated similarly in the past and can therefore be used to explain past geologic events – “The present is the key to the past”
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Hutton and the Rock Cycle
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The Rock Cycle
• James Hutton recognized that rocks in the Earth’s crust were continuously being formed, broken down, and then re-formed. – These are the same processes that are responsible for the three types of rocks – igneous, sedimentary, and metamorphic.
• This model of the continuous cycling of the crustal rock material is called the rock cycle. Copyright © Houghton Mifflin Company. All rights reserved.
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Formation of Igneous Rocks
Hutton and Geologic Time
• Igneous rocks form from the solidification of molten material that generally originates far beneath the surface of the Earth. • Magma – molten material beneath the Earth’s surface • Lava – molten material a the Earth’s surface
• Hutton also recognized that the processes that form and breakdown the different rocktypes take enormous amounts of time. – Thus he hypothesized the Earth to be very old.
• Central to the science of geology are the following three concepts: – The principle of uniformitarianism – The rock cycle – The recognition that the Earth is very old
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– The term “lava” is also used to describe the resulting and cooled igneous rock. Section 21.2
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Igneous Rocks
Section 21.3
• Igneous rocks can be divided into two basic categories:
Section 21.3
– Extrusive igneous – igneous rocks that cool at the surface of the Earth, generally due to some type of volcanic eruption – Intrusive igneous – igneous rocks that cooled somewhere beneath the surface of the Earth • Intrusive igneous rocks only appear at the Earth’s surface due to erosion of the overlying rocks or due to some type of uplift. 21 | 39
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Section 21.3
Plate Tectonics
Plate Tectonics
• The prediction of individual volcanic eruptions is generally not possible. • However we are aware of specific trends where volcanic eruptions typically do occur. • Most active volcanoes are located along linear zones, particularly along the margins of the Pacific Ocean.
• According to the Theory of Plate Tectonics the solid outermost shell of the Earth is called the lithosphere. • The rigid lithosphere rests or “floats” on a semimolten layer called the asthenosphere. • We will study plate tectonics in greater detail in the next chapter. • According to plate tectonics the lithospheric plates slowly move over the semimolten asthenosphere.
• The theory of plate tectonics can explain why volcanoes occur where they do. Section 21.3
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– The lithosphere is separated into several large and small fragments, called plates.
– The so-called “ring of fire”
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Types of Igneous Rocks
• Most geologists think that the first rocks on Earth were formed as the outer crust slowly solidified billions of years ago. • Therefore, the first rocks formed were igneous. • Since these initial rocks were formed, geologic processes have modified, covered, or eroded most of these ancient materials. • It is estimated that approximately 80% of the Earth’s crust is comprised of igneous rocks. Copyright © Houghton Mifflin Company. All rights reserved.
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Most Volcanoes and Earthquakes occur along Plate Boundaries in thePacific Ocean – “Ring of Fire”
Plate Boundaries • The lithospheric plates interact with each other in three basic ways: • Convergent boundary – two plates move towards each other • Divergent boundary – two plates move away from each other • Transform boundary – two plates slide past each other • The vast majority of the volcanoes on Earth occur at convergent boundaries.
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Subduction forms a Volcanic Island Arc
Convergent Boundaries
The islands of Japan are a result of subduction.
• When both converging plates consist of oceanic crust one plate will bend and slide slowly underneath the other – a process called subduction. • During this process of subduction, an enormous amount of friction is generated, resulting in the melting of rocks close to the subduction zone. • The new magma rises to the surface and forms a volcanic island arc. Copyright © Houghton Mifflin Company. All rights reserved.
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Convergent Boundaries
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Andes Mountains, South America
• Another type of convergent boundary exists when an oceanic plate converges with a continental plate. • The oceanic plate is more dense and therefore is subducted beneath the continental plate. – Once again friction between the two plates will melt nearby rocks.
• The Andes and Cascade Mountains are both volcanic mountain ranges formed by the convergence of an oceanic & continental plate. Copyright © Houghton Mifflin Company. All rights reserved.
Section 21.3
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Igneous Rock Texture and Composition
Texture and Cooling Rate
• Recall, there are two types of igneous rocks:
• The texture (grain or crystal size) of an igneous rock is determined primarily by the cooling rate of the magma or lava. • If a magma cools very slowly, deep within the Earth, large crystals can form. • If lava cools rapidly at the surface, large crystals do not have sufficient time to form. • If lava cools extremely rapidly (in water or ice), no crystals form, resulting in volcanic glass.
– Intrusive – cool slowly below Earth’s surface – Extrusive – cool quickly at the Earth’s surface
• Igneous rocks are classified according to two criteria: – Texture – generally refers to the size of the mineral grains (crystals) in the igneous rock – Mineral Composition – refers to the specific type of mineral crystals in the igneous rock
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Mount Rushmore, South Dakota – Granite Visible Crystals – Slow Cooling
Mt. Rushmore, South Dakota This rock originally cooled slowly at great depth.
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Composition and the Color of Igneous Rocks
Effects of Cooling on the Textures of Igneous Rocks
• In general, the color of an igneous rock gives a clue as to its composition. • The amount of silica (SiO2) within an igneous rock can be used to classify it according to composition. • If the magma/lava is high in silica, minerals form with abundant Si, Na, & K. These minerals are usually light in color. • When the magma/lava is low is silica, minerals rich in Fe, Mg, and Ca form that are dark in color. Copyright © Houghton Mifflin Company. All rights reserved.
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Stone Mountain, Georgia. Light Colored Granite – High in Silica
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Dark Volcanic Basalt – Low in Silica
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Common Igneous Rocks Organized by Composition
Density and Composition • The dark low-silica rocks are more dense than the light colored high-silica rocks. • Continental plates are composed of less dense high-silica rocks and therefore stand high about sea level. • Oceanic plates are composed of more dense low-silica rocks and therefore rest lower, usually below sea level. • Andesite is the name given to many rocks of medium-silica content. Copyright © Houghton Mifflin Company. All rights reserved.
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Plutons
Pluton • A pluton is said to be discordant if it cuts across the grain or layering of the surrounding rock.
• Pluton – a large body of intrusive igneous rock formed below the Earth’s surface by solidification of magma • Plutons are classified according to two criteria:
– Batholiths and dikes are discordant igneous bodies.
• A pluton is said to be concordant if it is parallel to the grain or layering of the surrounding rock.
– Size (exposed aerial extent) and shape of the intrusive rock body – Relationship of the intrusive rock body to the surrounding rock that they penetrate Copyright © Houghton Mifflin Company. All rights reserved.
Section 21.4
– Sills and laccoliths are concordant igneous bodies. 21 | 59
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Plutonic Bodies
Batholiths • Batholith – large exposed discordant intrusive igneous body – By definition, a batholith must have an exposed area of at least 103 km2.
• They originally crystallize slowly at great depths below the surface of the Earth. • Batholiths only become exposed at the Earth’s surface when powerful forces push them up or when a very deep canyon is carved by erosion. Copyright © Houghton Mifflin Company. All rights reserved.
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Yosemite National Park, CA Part of the Sierra Nevada Batholith
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Dikes • Dike – a discordant intrusive igneous body formed from magma that filled a vertical or near-vertical fracture • Since most fractures are narrow, the infilling intrusive igneous material hardens into a thin vertical sheet of igneous rock. • The size and extent of individual dikes is quite variable, depending on the dimensions of the fractures they fill.
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Volcanic Dike Exposed in Colorado
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Sills & Laccoliths
The dike is more resistant to weathering.
• Sills are similar to dikes except they are concordant to existing layering. – Sills are injected between layers of preexisting rock.
• Laccoliths are also concordant, and are injected between layers of preexisting rock. • Laccoliths cause a noticeable blistering of the overlying rock layers. Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Green Mountain, WY – Laccolith
Volcanoes
The preexisting layers of rock are blistered up.
• Volcano – a hill or mountain formed by the accumulation of lava and volcanic rock fragments ejected through a vent in the Earth’s surface • Three basic products are ejected from active volcanoes: – Gas – Lava – Solid rocks Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Tephra Emissions
Products of Volcanic Eruptions
Active Volcano near Huehuetenango, Guatamala
• Gases are generally expelled from a volcano during its entire life cycle . – Mainly H2O with lesser amounts of CO2 and H2S
• Lava is extruded from volcanoes in varying amounts and varying viscosities. • Some volcanoes also emit large volumes of solid material, collectively known as tephra. – Tephra is expelled in almost any size. – Tephra includes material that is initially ejected molten and hits the ground as a solid. Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Volcano – Eruptive Style
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Magma Viscosity
• Volcanoes display two basic eruptive styles: explosive and peaceful (non-explosive) • The viscosity of the magma largely determines the eruptive style of a particular volcano. • Low viscosity magma can flow easily and therefore is characterized by peaceful eruptions. • High viscosity magma has difficulty flowing and only moves when subjected to great pressure. Copyright © Houghton Mifflin Company. All rights reserved.
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Section 21.4
• Magma viscosity, in turn, is dependent on two factors: temperature and silica content • The higher the temperature the lower the viscosity. (flows easier) – In general magmas that originate deep within the Earth have higher temperatures.
• The higher the silica content the higher the viscosity. (more difficult to flow) – In general magmas that originate at shallow depths have a higher silica content.
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Peaceful Eruptive Style
Basaltic Hawaiian Lava – Peaceful Eruption
• Peaceful eruptions are involve basaltic magmas. • Basaltic magmas originate deep within the Earth and therefore are very hot. • Basaltic magmas also have a relatively low silica content. • Due to the combination of high temperature and low silica content, basaltic magmas flow easily, resulting in peaceful eruptions. Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Hawaiian Islands & Emperor Seamount Chain
Basaltic Eruptions
• Rising magma from a stationary “hot spot” in the asthenosphere forms volcanoes as the Pacific plate moves slowly over the hot spot.
• Most basaltic eruptions occur in two distinct settings: – Along the length of a divergent plate boundary – In isolated areas, as a lithospheric plate moves over an unusually hot and stationary zone in the upper mantle, called a “hot spot” or mantle plume
• Hot spots appear at the surface as a line of active, dormant, and extinct volcanoes. – The Hawaiian Islands and Emperor Seamounts are a continuous series of volcanoes formed over the past 70 million years. Copyright © Houghton Mifflin Company. All rights reserved.
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Volcanic Features High in the Andes Mountains Due to Subduction Beneath South America
Explosive Eruptive Style • Explosive eruptions generally occur along subduction zones and involve silica-rich magma. • Magmas that originate within a subduction zone are not deep, and are therefore cooler in temperature. • These magmas are also higher in silica content. • Due to the combination of ‘low’ temperature and high silica content, these magmas are very viscous, resulting in explosive eruptions.
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Explosive Eruption - Mt. Saint Helens
Volcanic Structures
Due to Subduction Beneath North America
• Molten volcanic material that reaches the surface of the Earth creates a number of dramatic surface expressions. • The type of volcanic feature formed is largely dependent on the composition and temperature of the lava extruded. • Specific and common volcanic structures include flood basalts, shield volcanoes, stratovolcanoes, cinder cones, and calderas. Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Fissure Eruptions and Flood Basalts
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Columbia Plateau Flood Basalts, Oregon
• Fissure eruptions take place when large volumes of lava are extruded from long fractures along the surface of the Earth. • Significant portions of Washington, Oregon, and Idaho are covered with a series of ancient flood basalts called the Columbia Plateau. • The basaltic lava that formed these layers was extremely fluid (high temperature & low silica content), eventually covering an area of 576,000 km2. – Average thickness of 150 meters Copyright © Houghton Mifflin Company. All rights reserved.
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Kilauea Crater, Hawaii- Shield Volcano
Shield Volcanoes
Note the very gentle slope away from the crater.
• Shield volcanoes form from lava that is not quite as fluid as the lava that formed flood basalts. • The Hawaiian volcanoes are the classic examples of shield volcanoes. • Shield volcanoes have very gentle slopes and were formed by repeated basaltic flows. • Eruptions are generally frequent but not particularly violent. Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Stratovolcanoes
Mt. Shasta, CA – Stratovolcano
• Stratovolcanoes are composed of alternating layers of medium- and high-silica content lava and tephra. • These volcanoes generally have violent eruptions, although they usually erupt much less frequently than shield volcanoes. • Most of the majestic solitary mountains of the world are stratovolcanoes. – For example, Mount Fuji, Mount Shasta, Mt. St. Helens, and Mount Hood are all stratovolcanoes Copyright © Bobby H. Bammel. All rights reserved
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Recent Volcanic Cinder Cone to the right of an older ‘grown-over’ and eroded Cinder Cone
Cinder Cones • Some volcanic features are relatively small. • Cinder cones are built when a volcanic eruption is particularly high in gas content, and thus emits primarily tephra. • Cinder cones rarely exceed 300 m in height.
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Crater Lake occupies a Caldera at the top of Mt. Mazama
Caldera • Caldera – a large, roughly circular, steep-walled depression at the volcano’s summit, formed when the roof of the magma chamber collapses after all the volcanic material within has been emitted • Crater Lake in southern Oregon occupies a caldera formed by the collapse of the magma chamber of a once active stratovolcano.
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Devastation Due to 1980 Mt. St. Helen Eruption
Historic Eruptions
Over 8 km from the volcano’s summit
• Mount Saint Helens 1980 – a stratovolcano located in southern Washington state that had been dormant since 1857 – This eruption devastated more than 400 km2 of area and resulted in the death of more than 60 people.
• Mount Pinatubo 1991 – located in the Philippines – Propelled ash into the atmosphere to a height of over 24 km – Destroyed thousands of acres of cropland and forced the abandonment of Clark Air Force base Copyright © Bobby H. Bammel. All rights reserved Copyright © Houghton Mifflin Company. All rights reserved.
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Section 21.4
Why do People Continue to live in the Shadow of a Dangerous Volcano?
Sediments & Sedimentary Rocks
• An eruption by Mt. Vesuvius in A.D. 79 buried 2000 residents of Pompeii. • Yet the large and growing Italian city of Naples lies only a few kilometers from Mt. Vesuvius. • Soils derived from volcanic ash are rich and fertile, resulting in an extremely productive agricultural area.
• Particles that are eroded from one place are later deposited as sediments somewhere else, usually along rivers, in lakes, at the shore, or on the seafloor. • Hutton also observed layered rocks in the highlands that were composed of cemented sand and other rock fragments. He called these sedimentary rocks.
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Sedimentary Rocks
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Importance of Sedimentary Rocks • Although sedimentary rocks only comprise 5% of the Earth’s crust, they are exposed over approximately 75% of the Earth’s surface.
• Hutton also realized that most sediments were deposited in horizontal layers, or strata. • Older layers are converted to sedimentary rock as they are compacted by the weight of the overlying and successively younger layers. • Later, some of these sedimentary layers are pushed up by powerful forces within the Earth to form mountain ranges. • The process continues as the newly uplifted areas are affected by weathering processes. Copyright © Houghton Mifflin Company. All rights reserved.
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Section 21.5
– The Earth’s crust is dominated by igneous and metamorphic rocks with sedimentary rocks serving as a thin veneer of only a few kilometers in thickness.
• Many of the necessities of modern life come directly from sedimentary rocks. – Petroleum, coal, some metal deposits, and many materials used in the construction industry come from sedimentary rocks. 21 | 95
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The Grand Canyon – Erosion of Sedimentary Rocks
Sedimentary Rocks - Landforms • Some of the most picturesque spots on Earth are composed of sedimentary rocks. • Sedimentary rocks occur in a number of varieties and colors. • The Colorado Plateau of the southwestern U.S. offers a particularly outstanding display of sedimentary rocks and their erosional products.
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Section 21.5
Origins of Sedimentary Rocks
Sedimentary Rock Classification
• Lithification – the process of transforming sediments into sedimentary rocks • Compaction and cementation of the loose sediments are the dominantly responsible for lithification. • Calcium carbonate (CaCO3), silica (SiO2), and iron oxides that are dissolved in groudwater serve as the dominant cementing agents.
• Sedimentary rocks can be classified according to the origin of their constituents. • There are two main types of sediments: detrital and chemical. • Detrital sediments originate from the solid fragments (detritus) that erode from preexisting rocks. • Chemical sediments originate as dissolved minerals in solution.
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Detrital Sedimentary Rocks
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Classification of Detrital Sedimentary Rocks
• Detrital sedimentary rocks are classified according to the grain size of its components. • Shale – composed of very fine-grained particles that were initially mud • Sandstone – composed of sand-sized particles • Conglomerate – composed of rounded pebbles • Breccia – composed of angular pebbles • Detrital rock particles are generally held together by CaCO3 or SiO2. Copyright © Houghton Mifflin Company. All rights reserved.
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Chemical Sedimentary Rocks
Types of Chemical Sedimentary Rocks
• Chemical sedimentary rocks are divided into two basic types: organic and inorganic. • In both cases the chemical sediments are composed of minerals that were transported in solution to their eventual deposition site.
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Organic Chemical Sedimentary Rocks
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Organic Limestone, full of marine fossils, Everglades, Florida
• Organic sediments are formed largely by the action of organisms that remove the minerals out of solution. • For example, many large and microscopic organisms in the ocean extract CaCO3 out of seawater to construct skeletal and shell material. • As these organisms die, their hard parts come to rest on the bottom and eventually lithify into organic limestone. Copyright © Bobby H. Bammel. All rights reserved
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Coal
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Inorganic Chemical Sedimentary Rocks
• Although coal is not formed from minerals carried in solution, it is still classified as an organic sedimentary rock. • Coal is formed from the lithified remains of ancient plants.
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• Inorganic chemical sedimentary rocks are commonly formed by the evaporation of water. • As the water evaporates, dissolved minerals in solution will precipitate out. • Examples of inorganic chemical sedimentary rocks include halite (HCl rock salt), gypsum (CaSO4), and various cave deposits (CaCO3). Copyright © Houghton Mifflin Company. All rights reserved.
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Sedimentary Rock Characteristics
Rock Color Can be Quite Varied
• Many inherent characteristics of sedimentary rocks are used by geologists to gain a better understanding of the conditions under which these rocks were deposited. • Characteristics such as rock color, grain rounding, grain sorting, bedding, fossil content, ripple marks, mud cracks, footprints, and even raindrop prints within the rock reveal the rock’s origin and history.
• Gray shades - common in sedimentary rocks – This color indicates that the rock was deposited in shallow, well aerated marine waters.
• Shades of yellow, brown, and red indicate that the rock was deposited on land, above sea level in the presence of abundant oxygen. – These bright shades indicate an iron oxide.
• Dark gray to black rocks contain abundant amounts of organic carbon. – Accumulated in stagnant, oxygen-poor areas
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Bright Colors Indicate Terrestrial Deposition
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Grain Rounding and Size • Angular grains indicate that these sediments were not transported very far. • Rounded grains indicate great transport distances and vigorous water/wind action that wore away the sharp edges. • Large grains indicate that strong currents deposited the sediments. • Small grains indicate that a very quiet environment existed when the sediments were deposited.
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Glacial Moraine, WY
Sorting
Glacial deposits are typically poorly-sorted
• Sorting is the degree to which sediments are separated according to size. • Well-sorted sediments are comprised of grains all about the same size. – Well-sorted sediments have been transported great distances over long periods of time.
• Poorly-sorted sediments are comprised of grains a vastly different sizes. – Poorly-sorted sediments indicate a short transportation distance and/or time. Copyright © Bobby H. Bammel. All rights reserved
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Bedding/Stratification • Bedding – layering of the rock that develops as the sediment is deposited • Bedding may be very obvious or subtle. It may be very thick (massive) or it may be thin. • Most bedding is horizontal, since most sediments come to rest due to gravity. • In some instances, where strong water/wind currents are present, bedding is tilted. This type of bedding is called cross-bedding.
Cross-bedding within a Sandstone Zion National Park, Utah
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Dinosaur tracks and fossilized dinosaur bones are both considered fossils.
Fossils • Perhaps the most distinctive and interesting feature of many sedimentary rocks is the fossils it may contain. • Fossil – the remains, an imprint, or any type of trace of an ancient organism, preserved in the rock record • Later in Chapter 24 we will discuss in detail the use of fossils in the determination of geologic age. Copyright © Houghton Mifflin Company. All rights reserved.
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Metamorphic Rocks
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Metamorphism
• As we have already learned, igneous rocks form by the crystallization of magma that generally forms deep within the Earth. • Sedimentary rocks are deposited at or near the surface of the Earth. • Metamorphic rocks form in conditions below where sedimentary rocks form and above where igneous rocks form. – Metamorphic rocks form where pressure and temperature conditions lie between the other two rock types. Copyright © Houghton Mifflin Company. All rights reserved.
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• Metamorphism – conditions within the Earth that result in the changing of the structure and mineral content of a solid rock without melting it – Any type of rock may be metamorphosed igneous, sedimentary, or even metamorphic.
• Heat, pressure, and chemically active fluids are the ‘agents’ of metamorphism. – Heat and pressure break some of the mineral bonds in the original mineral, with the fluids serving to allow movement of the ions. Copyright © Houghton Mifflin Company. All rights reserved.
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Classification of Some Common Metamorphic Rocks
Parent Rocks • Parent rock – the original rock before metamorphism takes place • In addition to heat, pressure, and fluid activity, the composition of the parent rock helps determine what metamorphic rock forms. • The overall process of metamorphism can change both the texture (crystal size) and the mineral composition of the parent rock. – Or metamorphism could only change one of these characteristics of the parent rock Copyright © Houghton Mifflin Company. All rights reserved.
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Parent Rock Metamorphic Rock
– Both of these rocks are composed of CaCO3.
• When limestone is metamorphosed into marble, the ‘structure’ of it is changed but not the chemical composition.
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Fossiliferous Limestone & Marble As the limestone recrystallizes the fossils are destroyed Copyright © Bobby H. Bammel. All rights reserved
• In general, if the parent rock only contains one mineral. The metamorphic rock will also be composed of that same mineral. • For example limestone (parent rock) will metamorphose into marble.
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Fossiliferous Limestone Marble Copyright © Houghton Mifflin Company. All rights reserved.
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Parent Rock Metamorphic Rock • The change from sandstone to quartzite is another example of the metamorphism of a rock containing only one mineral. – Both sandstone and quartzite are composed of SiO2.
• When sufficient heat and pressure are applied to a sandstone, the grains fuse together to form the very resistant rock quartzite. Copyright © Houghton Mifflin Company. All rights reserved.
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Parent Rock Metamorphic Rock • If the parent rock is composed of several minerals, the process of metamorphism will create a new suite of different minerals. – These new minerals are formed in response to the escalated heat and pressure conditions.
• Shale is a very fine-grained mixture of several minerals: quartz, clay, mica, and chlorite. • Depending on the degree of metamorphism, shale will be progressively changed into slate, schist, or gneiss. Copyright © Houghton Mifflin Company. All rights reserved.
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Effects of Increasing Temperature and Pressure on the Metamorphism of Shale
Types of Metamorphism • Three basics types of metamorphism are recognized: contact, shear, and regional. • Contact metamorphism occurs when magma comes into direct contact with the parent rock – Changes in the parent rock are primarily due to very high temperatures but not increased pressures.
• Immediately next to the magma, intense metamorphism results in the formation of coarse-grained crystals. – Moving away from the magma, the resulting rock becomes progressively finer-grained.
• A contact metamorphic zone occurs when a parent rock is intruded by molten magma. Copyright © Houghton Mifflin Company. All rights reserved.
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Types of Metamorphism
Types of Metamorphism
• Shear metamorphism results from the intense pressures that exists along active fault zones, where rock units slide past each other. • Mechanical deformation and recrystallization of the minerals result from the heat, pressure, and movement of fluids as rock units slide or shear past one another.
• Regional metamorphism affects extremely large areas and is caused by a combination of both high temperature and high pressure. • Most areas that are affected by regional metamorphism are areas undergoing intense deformation due to mountain building processes.
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– Convergent plate boundaries are common zones of regional metamorphism. Copyright © Houghton Mifflin Company. All rights reserved.
Foliation
Foliation
• The enormous pressures that accompany regional metamorphism cause the newly formed mineral grains to align themselves in a distinctly parallel arrangement. • Foliation – the prominent layering in a metamorphic rock • Only rocks that are metamorphosed under intense pressure will exhibit foliation. – Metamorphic rocks that form almost entirely due to high temperatures will not have foliation. Copyright © Houghton Mifflin Company. All rights reserved.
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• The progressive patterns of foliation that develop in a metamorphosed shale serve as a good example of foliation types. • Slaty cleavage develops when a shale undergoes mild metamorphism. • Schists are a foliated metamorphic rock with larger, visible grains (crystals.) • Gneisses form under intense regional metamorphism and display banding. – The more intense the metamorphism, the larger the crystal but less distinct the foliation. Copyright © Houghton Mifflin Company. All rights reserved.
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