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Chapter 7 Metamorphic Rocks

Chapter 7 Metamorphic Rocks - 2009 Allan Ludman and Stephen...

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CHAPTER 7 © 2009 Allan Ludman and Stephen Marshak USING METAMORPHIC ROCKS TO INTERPRET EARTH HISTORY PURPOSE • To become familiar with metamorphic rock textures and mineral assemblages • To use metamorphic rocks to interpret conditions in metamorphic environments. MATERIALS NEEDED • A set of metamorphic rocks. • A magnifying glass or hand lens and, ideally, a microscope and thin sections of metamorphic rocks. • Standard supplies for identifying minerals (streak plate, glass plate, etc.) 7.1 INTRODUCTION Metamorphism occurs within the Earth when rocks exposed to physical and chemical conditions significantly different from those under which they first formed change in response to the new conditions in the solid state without melting or disaggregating . Everything about a rock may change: its color, mineralogy, texture, even its chemical composition. Minerals may react with one another to produce new ones (in the solid state), the texture may change as grains become larger or smaller (in the solid state!), or both textural and mineralogy may change. Metamorphic rocks provide information about conditions between the surface and depths where melting takes place, filling a gap between the areas from which we get information from sedimentary and igneous rocks. Erosion of kilometers of overlying rock has revealed metamorphic rocks at the core of every continent and the cores of mountain systems. The stories that metamorphic rocks tell are fascinating – once we know how to read them. For example, metamorphic rocks help us understand conditions when continental plates collide . Erosion has removed the upper parts of the Appalachians, Rockies, Alps, Urals, and other ancient mountain systems, revealing their intensely metamorphosed roots. Metamorphic minerals in these mountains help us estimate the T and P conditions beneath the modern Himalayas and Andes, mountains that are still growing today. the effect of enormous amounts of heat released when billions of tons of granitic magma formed the Sierra Nevada batholiths. Metamorphic changes near the granite contact help estimate conditions during intrusion and field mapping shows how far metamorphism extended into the host rock. Mapping also shows how far hydrothermal fluids carried potentially valuable mineral deposits from the cooling magma. the effects of meteorite impacts Studies of rocks around Meteor Crater in Arizona led geologists to recognize that meteorite impacts produce unique metamorphic minerals and textures. Supported by studies of 1
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rocks from lunar craters, these help identify ancient meteorite impact sites on Earth where erosion has removed all signs of the original crater. 7.2 Agents of Metamorphism Changes in the physical and chemical environment – heat, pressure , and chemically active (hydrothermal) fluids – cause metamorphism and are called agents of metamorphism . Added heat breaks bonds in minerals, freeing ions to migrate to seeds of new minerals more stable at the new temperature. Two kinds of pressure affect rocks differently.
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Chapter 7 Metamorphic Rocks - 2009 Allan Ludman and Stephen...

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