Chapter 7 Metamorphism and Metamorphic Rocks v Metamorphic

Chapter 7 Metamorphism and Metamorphic Rocks v. Metamorphic rock is formed when existing rocks change due to subjection to pressure and or temperature u. Any rock can undergo metamorphism v. Metamorphism is the process by which heat, pressure, and chemical reactions deep within the earth alter the mineral content and or structure of existing rock without melting it down

METAMORPHISM & METAMORPHIC ROCKS NORMAL DEPTHS FOR METAMORPHISM: >10 -KM FROM SURFACE (BELOW SEDIMENTARY DEPTHS & ABOVE IGNEOUS DEPTHS) METAMORPHISM: MINERAL CONTENT & STRUCTURE OF SOLID ROCK ALTERED BY HEAT, PRESSURE & CHEMICALLY ACTIVE FLUIDS – DOESN’T MELT CONDITIONS PROMOTING METAMORPHISM: ROCK STABILITY IS RELATED TO FORMATION ENVIRONMENT METAMORPHISM BREAKS SOME BONDS, BUT DOES NOT INCLUDE MELTING

Sedi. , meta. , igne. rocks

Metamorphic Rocks in North America

CONDITIONS-CONTD 1) HEAT – MOST IMPORTANT FACTOR GEOTHERMAL GRADIENT (20 -30 DEG C/KM-200 DEG C TO METAMORPHOSE ROCKS) l SOURCES OF HEAT FOR METAMORPHISM: – – CONDUCTION FROM DEEP INTERIOR RISING MAGMA RADIOACTIVE ISOTOPES FRICTION BETWEEN ROCK BODIES PRESSURE

Pressure-contd. l Lithostatic: Confining pressure in all directions, rock becomes compressed l Directed Pressure: Acts in one principal plane l Results of Pressure: – Compaction – Smaller & Denser – Ion migration of unbonded ions (recrystallization) – Foliation – Preferred alignment of minerals - Subjected to direct pressure – Units of Pressure: 1 Bar = 1 Atm. Pressure = 1. 02 kg/cm 2 = 14. 7 lbs/inch 2 = 105 Pascal

Circulating Fluids Often necessary for chemical reactions (Facilitates movement of ions and atoms) l Sources of Water l – Surface Infiltration – Pores in Sedimentary Rocks – Cracks on Subducting Plates – Magmas – Decomposing Minerals ALL METAMORPHIC ROCKS ARE CATEGORIZED AS ‘FOLIATED’ OR ‘NON FOLIATED’

PARALLEL CLEAVAGE

MINERAL GRAINS

MINERAL GRAINS-CONTD.

Deep burial of rocks

What Drives Metamorphism l Heat u Accelerate pace of chemical reactions l Pressure u Lithostatic (confining)- rock becomes smaller and denser u Directed- minerals become aligned- Foliation l Circulating Fluids u Ions in water- change mineral composition l Parent Rocks u Original rock’s composition will affect the outcome of metamorphism l

CONFINING PRESSURE

DIRECTED PRESSURE

TYPES OF METAMORPHISM l Contact u Heat is the dominant factor u Area affected generally smaller than regional metarmorphism l Regional are two types with extensive coverage t t l Burial- occurs in deep sedimentary basins- no plate tectonics involved Dynamothermal- occurs where converging plates squeeze a rock caught between them Others u Hydrothermal- involves hot water from magma u Fault-zone- rocks grinding past one another u Shock- meteorites strike u Pyrometamorphism- lightning

CONTACT METAMORPHISM

DYNAMOTHERMAL METAM.

HYDROTHER. METAM-CONTD

Hydrothermal Metamorphism

METAMORPHIC ROCK TYPES l Foliated- based on type of foliation u Slate- fine grain u Phyllite- fine grain with sheen u Schist- has ‘split’ appearance u Gneiss- layers/bands of minerals l Non-foliated- based on mineral composition u Marble u Quartzite u Hornsfel l Mixed Rock u Migmatite- indicates partial melting

Quartz-Quartzite

Quartz-Sandstone

Metamorphism Temperature & Pressure Information about degree to which a metamorphic rock differs from its parent material l Metamorphic Gradeu low (200 -400) slate u high (500 -800) gneiss l Index minerals/metamorphic Zones are used to determine metamorphic condition of temperature and pressure u Chlorite, muscovite-low grade (low P/T) u Garnet, staurolite- intermediate u Sillianite- high grade (high P/T)

Common Metamorphic Rocks l FOLIATED ROCKS DERIVED FROM SHALE OR MUDSTONES: – SLATE (FINE GRAINED) SLATY CLEVAGE l SLATE COLORS (RED: IRON OXIDE; GREEN: CHLORITE; PURPLE: MANGANESE OXIDES; BLACK: CARBON) l PHYLLITE: FINE-GRAINED l SCHIST: COARSE GRAINED l GNEISS: COARSE GRAINED l

Relationships that produce crystallization

Three main metamorphic environments

Relationships that produce metamorphic rocks

Metamorphic Surface Rocks

Rock Slide of 1959

St. Francis Dam