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Library and Archives Canada Cataloguing in Publication Library of Congress Cataloging-in-Publication Data Saunders, Craig, 1972- Saunders, Craig, 1972- What is the theory of plate tectonics? [electronic resource] / Craig What is the theory of plate tectonics? / Craig Saunders. Saunders. p. cm. -- (Shaping modern science) Includes index. (Shaping modern science) ISBN 978-0-7787-7202-6 (reinforced lib. bdg. : alk. paper) -- Includes index. ISBN 978-0-7787-7209-5 (pbk. : alk. paper) -- Electronic monograph in PDF format. ISBN 978-1-4271-9531-9 (electronic (PDF) Issued also in print format. 1. Plate tectonics--Juvenile literature. 2. Geology, Structural-- ISBN 978-1-4271-9531-9 Juvenile literature. I. Title. II. Series. 1. Plate tectonics--Juvenile literature. 2. Geology, Structural--Juvenile QE511.4.S288 2011 literature. I. Title. II. Series: Shaping modern science (Online) 551.1'36--dc22 2010052622 QE511.4.S29 2011 j551.1'36 C2011-900173-X Crabtree Publishing Company www.crabtreebooks.com 1-800-387-7650 Copyright © 2011 CRABTREE PUBLISHING COMPANY. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or be transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of Crabtree Publishing Company. In Canada: We acknowledge the financial support of the Government of Canada through the Canada Book Fund for our publishing activities. Published in Published in the Published in the Published in Canada United States United Kingdom Australia Crabtree Publishing Crabtree Publishing Crabtree Publishing Crabtree Publishing 616 Welland Ave. PMB 59051 Maritime House 386 Mt. Alexander Rd. St. Catharines, ON 350 Fifth Avenue, 59th Floor Basin Road North, Hove Ascot Vale (Melbourne) L2M 5V6 New York, New York 10118 BN41 1WR VIC 3032 Contents What Is the Theory of Plate Tectonics? Introduction 4 A Flat Earth? Chapter One . . . . . . . . . . . . . . . . . . . . . . . . . . 8 In the Beginning Chapter Two . . . . . . . . . . . . . . . . . . . . . . . 16 From the Crust to the Core Chapter Three . . . . . . . . . . . 24 On the Move Chapter Four . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Making Mountains Chapter Five . . . . . . . . . . . . . . . . . . . . 40 Explosive Earth Chapter Six . . . . . . . . . . . . . . . . . . . . . . . 46 The Cutting Edge Chapter Seven . . . . . . . . . . . . . . . . . . . . . . 52 Timeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 For More Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 INTRODUCTION What Is the Theory of Plate Tectonics? The Earth and Its Structure The ground beneath your feet is constantly moving. The Earth’s surface is made up of solid sections of rock The Earth is not completely solid. called plates, which float on a more Much of it is actually made up of liquid layer beneath them. You can’t molten (liquid) rock. see them move, but sometimes you can feel earthquakes that result from Earth has four layers. In the solid inner the plates crashing into each other. core, rock is very dense and contains large amounts of the metals nickel and In studying the shapes of the iron. The liquid outer core is about continents, the makeup of rocks, and 1,400 miles (2,260 km) thick. Further fossil records, scientists gradually out is the mantle, a layer of hot, mostly developed the Theory of Plate solid material about 1,770 miles Tectonics. More recently, scientists (2,850 km) thick. The solid crust is very have been able to regularly monitor thin compared to the other layers. It is at the movement of the plates. most 25 miles (40 km) thick, and usually much thinner. If your body were the The Theory of Plate Tectonics explains Earth, the crust would be your skin. how the lighter, solid plates that form the Earth’s crust (or surface) float on Quick fact denser material just below the crust. The plates move apart, opening up Canada is home to oceans or valleys; crash into each the oldest known rocks other, thrusting up mountains; or push in the world. A patch of under one another, creating volcanoes. rock on Hudson Bay formed 4.28 billion years ago. 4 Rocks are made up of many elements, particularly iron, tend to sink, which is why primarily iron, oxygen, silicon, and the core is so dense. Lighter ones, such as magnesium. When rocks melt, the silicon and aluminum, tend to float, which elements separate. Heavy ones, is why they are abundant in the crust. hThis map of Earth shows earthquake and volcanic activity for the past one million years. Scientific Theory or Law? In science, a theory is a well-tested set of ideas that explains how something occurs. For example, many kinds of evidence together support the Theory of Plate Tectonics. A scientific law describes how something consistently happens under certain conditions. For example, the law of gravity describes how objects fall to Earth’s surface. 5 Is Earth Unique? Our solar system began as a vast leaving these planets dense and cloud of gas and dust. About 4.56 iron-rich. The gravity of the larger billion years ago, gravity caused the outer planets—Jupiter, Saturn, dust and gas to form clumps. Most of Uranus, and Neptune—captured the material gathered in the center. This much of the lighter material. clump became denser and hotter, and a nuclear reaction began. The result was Extinct volcanoes, mountains, and other the huge, radiating Sun we know today. signs of a dynamic crust have been identified on Mercury, Venus, and Mars. Material left over from the Sun’s formation Our Moon also looks like it had plates became the planets. The inner planets— that moved, before it cooled and most Mercury, Venus, Earth, and Mars—were activity stopped about three billion years very hot. Many lighter elements boiled ago. Earth appears to be the only planet and were swept away by solar winds, that is still tectonically active. hOur solar system includes the Sun, eight planets, several dwarf planets, asteroids, comets, rocks, and dust. 6 Shifting Plates and Life The processes at work in plate Today, the same heat still drives molten tectonics probably also helped rock toward the surface. The heat create Earth’s atmosphere. The air causes the mantle and crust to move. we breathe and the hydrogen and So, heat movement not only makes oxygen that make water may once mountains and ocean basins, but it have been contained in rocks. As also carries important minerals to the heat from the early Earth’s core surface. Deep in the ocean, where melted those rocks, it would have sunlight cannot reach, scientists have released those elements and forced found abundant life where hot, mineral- them to the surface. rich gases push through the crust. How Old Is the Theory of Plate Tectonics? While the process of plate tectonics In this book, we explore early is as old as the Earth itself, the theory theories about the Earth’s surface only became established in the 1960s. and investigate how those ideas Before that, people thought the changed as scientists learned more continents were fixed in place. about our planet. “All earth sciences must contribute evidence towards unveiling the state of Quick fact our planet in earlier times, and…the Our universeformed about truth of the matter can only be reached 12–14 billion years ago. Our own solar system is by combining all this evidence.” thought to be much younger, about 4.5 billion years old. —Alfred Wegener, interdisciplinary German scientist, in The Origins of Continents and Oceans, 1915 7 CHAPTER ONE A Flat Earth? People have always tried to explain how Anaxagoras said the Earth was so oceans and mountains form. As people heavy that it stayed where it was and have learned more about the Earth, our did not spin. He also thought Earth ideas have changed. floated on air. Because Earth is flat, he believed, water could spread evenly Strange as it may seem, people's over its surface, with rivers being caused understanding of how the Earth by evaporation and rain. Sometimes the works began with the idea that the air holding Earth up would get into planet is flat and floats on a pillow cracks in the ground. When it became of air. About 2,500 years ago trapped, the moving air forced its way in Greece, the philosopher out, causing earthquakes. Anaxagoras tried to explain the universe. He thought the universe iAnaxagoras was a began as one big mass made Greek philosopher in the fifth century B.C. of all the same material. According to Anaxagoras, a powerful mind started everything rotating. That spinning made the material break up and then re-form, creating planets and stars. Heavier material pushed to the center and lighter material, such as fire, was thrown to the edges, he said. Earth was at the center of it all. That explained why the Earth was made of heavy materials, such as iron. He thought the Sun was a lump of fiery metal and the Moon was made of reflective earth. 8