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Chapter 2 Plate Tectonics

Chapter 2 Plate Tectonics - Plate Tectonics PART 1(Chapter...

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1 CHAPTER 2 THE WAY THE EARTH WORKS: EXAMINING PLATE TECTONICS PURPOSE To examine the evidence that led to development of the plate tectonic theory To use major Earth features to understand the processes involved in plate tectonics To learn how geologists estimate the direction and rate of plate motion MATERIALS NEEDED Tracing paper Colored pencils A ruler with divisions in tenths of an inch or millimeters A protractor A calculator or pencil and paper for simple arithmetic 2.1 INTRODUCTION Earthquakes and volcanic eruptions show that Earth is a dynamic planet with internal processes strong enough to affect the surface. In the 1960s and 70s, geologists developed the theory of plate tectonics (or just plate tectonics ) that recognizes these events as parts of a global process that splits continents and moves the pieces across the globe, opens and closes oceans, and creates mountains. Plate tectonics is a unifying theory for Geology because it explains many problems that had puzzled geologists for years. This chapter explores the observations and reasoning that led geologists to propose the plate tectonics theory. Before going farther, it is important to understand what scientists mean by the word “theory”. A theory is an idea that successfully explains many observations, can be used to make predictions that can be tested by experiments and which, after many years of testing, has not been proven wrong by conflicting observations. After nearly 50 years of rigorous testing, plate tectonics continues to meet all these criteria and therefore joins the ranks of other important theories like the germ theory of disease, the theory of relativity, and the theory of evolution. 2.2 INTRODUCTION TO THE PLATE TECTONICS THEORY Plate tectonics is based on many aspects of the Earth that you will examine throughout this course. The basic concepts of the theory include: © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company
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2 • Continental lithosphere is thicker than oceanic lithosphere because continental crust alone (without the mantle component) is 25- 70 km thick whereas the entire oceanic lithosphere is only ~ 7 km thick. • The plates rest on the asthenosphere , a zone in the upper mantle (Figure 2.2) that, although solid, has such low rigidity that it can flow like soft plastic. The asthenosphere acts as a lubricant, permitting the plates above it to move. Plates move relative to one another at 1 to 15 cm/year, roughly the rate at which fingernails grow. • A contact between plates is called a plate boundary and in some places three plates come together at a triple junction . There are three different kinds of plate boundaries based on the relative motions of the plates on opposite sides (Figures 2.1, 2.2). 1.
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Chapter 2 Plate Tectonics - Plate Tectonics PART 1(Chapter...

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