Essentials of Geology F.K. Lutgens E.J. Tarbuck D.G. Tasa Eleventh Edition

Transcription

1 Essentials of Geology F.K. Lutgens E.J. Tarbuck D.G. Tasa Eleventh Edition

2 Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: Pearson Education Limited 2014 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 either the prior written permission of the publisher or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6 10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. ISBN 10: ISBN 13: British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Printed in the United States of America

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4 THE PRECEDING CHAPTER PROVIDED THE BACKGROUND YOU needed to understand the origin of sedimentary rocks. Recall that weathering of existing rocks begins the process. Next, gravity and erosional agents remove the products of weathering and carry them to a new location where they are deposited. Usually the particles are broken down further during this transport phase. Following deposition, this material, which is now called sediment, becomes lithified (turned to rock). Sandstone in Arizona s Vermillion Cliffs Wilderness Area. (Photo by James Kay/DanitaDelimont.com) To assist you in learning the important concepts in this chapter, focus on the following questions: What is a sedimentary rock? About what percentage of Earth s crust is sedimentary? How is sediment turned into sedimentary rock? What are the two general categories of sedimentary rocks, and how does each type form? What is the primary basis for distinguishing among various types of detrital sedimentary rocks? What criterion is used to subdivide the chemical rocks? What are sedimentary structures? Why are these structures useful to geologists? What are the two broad groups of nonmetallic mineral resources? Which energy resources are associated with sedimentary rocks? FOCUS ON CONCEPTS 173

5 The Importance of Most of solid Earth consists of igneous and metamorphic rocks. Geologists estimate these two categories represent 90 to 95 percent of the outer 16 kilometers (10 miles) of the crust. Nevertheless, most of Earth s solid surface consists of either sediment or sedimentary rock! Across the ocean floor, which represents about 70 percent of Earth s solid surface, virtually everything is covered by sediment. Igneous rocks are exposed only at the crest of mid-ocean ridges and at some volcanic areas. Thus, while sediment and sedimentary rocks make up only a small percentage of Earth s crust, they are concentrated at or near the surface the interface among the geosphere, hydrosphere, atmosphere, and biosphere. Because of this unique position, sediments and the rock layers that they eventually form contain evidence of past conditions and events at the surface (FIGURE 1). Furthermore, it is sedimentary rocks that contain fossils, which are vital tools in the study of the geologic past. This group of rocks provides geologists with much of the basic information they need to reconstruct the details of Earth history. Such study is not only of interest for its own sake but has practical value as well. Coal, which provides a significant portion of our electrical energy, is classified as a sedimentary rock. Moreover, other major FIGURE 1 Sedimentary rocks are exposed at Earth s surface more than igneous and metamorphic rocks. Because they contain fossils and other clues about the geologic past, sedimentary rocks are important in the study of Earth history. (Photo by Scott T. Smith/ DanitaDelimont.com) energy sources oil, natural gas, and uranium are derived from sedimentary rocks. So are major sources of iron, aluminum, manganese, and phosphate fertilizer, plus numerous materials essential to the construction industry, such as cement and aggregate. Sediments and sedimentary rocks are also the primary reservoir of groundwater. Thus, an understanding of this group of rocks and the processes that form and modify them is basic to locating additional supplies of many important resources. 1 2 CONCEPT CHECK 1 How does the volume of sedimentary rocks in Earth s crust compare to igneous and metamorphic rocks? Why are sedimentary rocks important? Origins of Sedimentary Rock GEODe ESSENTIALS OF GEOLOGY Introduction FIGURE 2 illustrates the portion of the rock cycle that occurs near Earth s surface the part that pertains to sediments and sedimentary rocks. A brief overview of these processes provides a useful perspective. Weathering begins the process. It involves the physical disintegration and chemical decomposition of preexisting igneous, metamorphic, and sedimentary rocks. Weathering generates a variety of products, including various solid particles and ions in DID YOU KNOW? Coal is responsible for slightly more than half the electricity generated in the United States. solution. These are the raw materials for sedimentary rocks. Soluble constituents are carried away by runoff and groundwater. Solid particles are frequently moved downslope by gravity, a process termed mass wasting, before running water, groundwater, wind, and glacial ice remove them. Transportation moves these materials from the sites where they originated to locations where they accumulate. The transport of sediment is usually intermittent. For example, during a flood, a rapidly moving river moves large quantities of sand and gravel. As the flood waters recede, particles are temporarily deposited, only to be moved again by a subsequent flood. Deposition of solid particles occurs when wind and water currents slow down and as glacial ice melts. The word sedimentary actually refers to this process. It is derived from the Latin sedimentum, 174

6 Wind Glacier Dunes River Lake Ocean Chemical and mechanical weathering break down rock Erosional agents transport sediment Reef Deposition occurs when solid particles settle out or when ions precipitate Diagenesis burial and transformation into sedimentary rock FIGURE 2 This diagram outlines the portion of the rock cycle that pertains to the formation of sedimentary rocks. Weathering, transportation, deposition, and diagenesis represent the basic processes involved. (Left and center photos by E. J. Tarbuck; right photo by Jenny Elia Pfeiffer/Corbis) which means to settle, a reference to solid material settling out of a fluid (water or air). The mud on the floor of a lake, a delta at the mouth of a river, a gravel bar in a stream bed, the particles in a desert sand dune, and even household dust are examples. The deposition of material dissolved in water is not related to the strength of wind or water currents. Rather, ions in solution are removed when chemical or temperature changes cause material to crystallize and precipitate or when organisms remove dissolved material to build shells. As deposition continues, older sediments are buried beneath younger layers and gradually converted to sedimentary rock (lithified) by compaction and cementation. This and other changes are referred to as diagenesis ( dia = change ; genesis = origin), a collective term for all of the changes (short of metamorphism) that take place in texture, composition, and other physical properties after sediments are deposited. Because there are a variety of ways that the products of weathering are transported, deposited, and transformed into solid rock, three categories of sedimentary rocks are recognized. As the overview reminded us, sediment has two principal sources. First, it may be an accumulation of material that originates and is transported as solid particles derived from both mechanical and chemical weathering. Deposits of this type are termed detrital, and the sedimentary rocks that they form are called detrital sedimentary rocks. The second major source of sediment is soluble material produced largely by chemical weathering. When these ions in solution are precipitated by either inorganic or biologic processes, the material is known as chemical sediment, and the rocks formed from it are called chemical sedimentary rocks. The third category is organic sedimentary rocks. The primary example is coal. This black combustible rock consists of organic carbon from the remains of plants that died and accumulated on the floor of a swamp. The bits and pieces of undecayed plant material that constitute the sediments in coal are quite unlike the weathering products that make up detrital and chemical sedimentary rocks. 175

7 1 2 CONCEPT CHECK 2 Outline the steps that would transform an exposure of granite in the mountains into various sedimentary rocks. List and briefly distinguish among the three basic sedimentary rock categories. Detrital GEODe ESSENTIALS Types of OF GEOLOGY Though a wide variety of minerals and rock fragments (clasts) may be found in detrital rocks, clay minerals and quartz are the chief constituents of most sedimentary rocks in this category. Clay minerals are the most abundant product of the chemical weathering of silicate minerals, especially the feldspars. Clays are fine-grained minerals with sheetlike crystalline structures similar to the micas. The other common mineral, quartz, is abundant because it is extremely durable and very resistant to chemical weathering. Thus, when igneous rocks such as granite are attacked by weathering processes, individual quartz grains are freed. Other common minerals in detrital rocks are feldspars and micas. Because chemical weathering rapidly transforms these minerals into new substances, their presence in sedimentary rocks indicates that erosion and deposition were fast enough to preserve some of the primary minerals from the source rock before they could be decomposed. Particle size is the primary basis for distinguishing among various detrital sedimentary rocks. FIGURE 3 presents the size categories for particles making up detrital rocks. Particle size is not only a convenient method of dividing detrital rocks, but the sizes of the component grains also provide useful information about environments of deposition. Currents of water or air sort the particles by size the stronger the current, the larger the particle size carried. Gravels, for example, are moved by swiftly flowing rivers as well as by landslides and glaciers. Less energy is required to transport sand; thus, it is common to such features as windblown dunes and some river deposits and beaches. Very little energy is needed to transport clay, so it settles very slowly. Accumulations of these tiny particles are generally associated with the quiet waters of a lake, lagoon, swamp, or certain marine environments. Common detrital sedimentary rocks, in order of increasing particle size, are shale, sandstone, and conglomerate or breccia. We will now look at each type and how it forms. Shale Shale is a sedimentary rock consisting of silt- and clay-size particles (see Figure 3). These fine-grained detrital rocks account for well over half of all sedimentary rocks. The particles in these rocks are so small that they cannot be readily identified without great FIGURE 3 Particle size classification for detrital sedimentary rocks. Particle size is the primary basis for distinguishing among various detrital sedimentary rocks. (Photos by E. J. Tarbuck) magnification and for this reason make shale more difficult to study and analyze than most other sedimentary rocks. Much of what can be learned is based on particle size. The tiny grains in shale indicate that deposition occurs as the result of gradual settling from relatively quiet, nonturbulent currents. Such environments include lakes, river floodplains, lagoons, and portions of the deep-ocean basins. Even in Size Range (millimeters) > /16 2 1/256 1/16 <1/256 Particle Name Boulder Cobble Pebble Granule Sand Silt Clay Common Name Gravel Sand Mud Conglomerate or Breccia Sandstone Shale, Mudstone or Siltstone DID YOU KNOW? The term clay may be confusing because it has more than one meaning. In the context of detrital particle size, the term clay refers to those grains less than 1/256 millimeter, thus being microscopic (see Figure 3). However, the term clay is also used as the name of a group of silicate minerals. Although most clay minerals are of clay size, not all clay-size sediment consists of clay minerals! these quiet environments, there is usually enough turbulence to keep clay-size particles suspended almost indefinitely. Consequently, much of the clay is deposited only after the individual particles coalesce to form larger aggregates. Sometimes the chemical composition of the rock provides additional information. One example is black shale, which is black because it contains abundant organic matter (carbon). When such a rock is found, it strongly implies that deposition occurred in an Detrital Rock mm 176