Chapter 8 METAMORPHIC ROCKS THE THREE ROCK GROUPS Igneous rocks Formed by cooling and consolidation of magma Sedimentary rocks Formed by chemical precipitation of material from water at the Earth s surface or by deposition and cementation of particles and debris transported by water wind or ice Metamorphic rocks Igneous or sedimentary rocks that have been changed under high temperatures and or high pressures INTRODUCTION Metamorphic Changed from an original parent Meta Change Morph Form or shape Parent rocks are called protoliths Metamorphism can occur to any protolith METAMORPHIC PROCESS Heating recrystallization not melting Pressure tighter arrangement of atoms Hot groundwater Change in chemical composition ions dissolved or deposited Often initiated by burial METAMORPHISM Metamorphism occurs in the solid state It doesn t include weathering melting Metamorphics often look totally unlike protoliths EXAMPLES Red mudstone Garnet gneiss Fossiliferous limestone Marble METAMORPHIC PROCESSES Recrystallization Minerals change size shape o Quartz siltstone Large quartz crystals Plastic Deformation o Quartz sandstone Wavy pancakes CAUSES The agents of metamorphism are Heat Pressure Differential stress Hydrothermal uids Not all agents are required they often co occur Rocks may be overprinted by multiple events METAMORPHIC ROCK TYPES Foliated Has a through going planar fabric Subjected to differential stress METAMORPHIC ROCK TYPES Non foliated No planar fabric evident Crystallized without differential stress METAMORPHIC ROCKS Slate Low grade metamorphic shale Has a distinct foliation called slaty cleavage Slaty cleavage oriented perpendicular to compression Slate breaks along this foliation creating at sheets METAMORPHIC INTENSITY Different minerals are stable as temperature and pressure change Grade is a measure of metamorphic intensity Low grade Slight High grade Intense Occurs when rocks are heated and chemically changed by the intrusion of a body of CONTACT METAMORPHISM hot magma SUBDUCTION METAMORPHISM Subduction creates the unique blueschist facies Trenches and accretionary prisms have a low geothermal gradient low temperature but high pressures