Rock Cycle. Metamorphism, Metamorphic Rocks, Hydrothermal Rocks. Metamorphism. Equilibrium. Interrelationships between:

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1 Rock Cycle Equilibrium Interrelationships between: igneous rocks sediment sedimentary rocks metamorphic rocks weathering and erosion Metamorphism, Metamorphic Rocks, Hydrothermal Rocks Metamorphism solid state change in texture, mineralogy How do we know this?

2 Metamorphism, Metamorphic Rocks, Hydrothermal Rocks Metamorphic rock important clues to deep processes Parent rock Equilibrium Adjustment to new T & P conditions Plastic bending, flowing under stress Factors Controlling the Characteristics of Metamorphic Rocks Composition of Parent Rock Temperature Geothermal gradient Stability of minerals Pressure Confining pressure Lithostatic pressure

3 Factors Controlling the Characteristics of Metamorphic Rocks Composition of Parent Rock Temperature Geothermal gradient Stability of minerals Pressure Confining pressure Lithostatic pressure Geothermal gradient varies with tectonic setting

4 Factors Controlling the Characteristics of Metamorphic Rocks Composition of Parent Rock Temperature Pressure Confining pressure in Earth s crust = Lithostatic pressure Favors more dense minerals Roughly 1000 atm or 1 kbar per 3 km depth What is the pressure at base of the crust? P = ρ * g * h P = 2800 kg/m 3 * 10 m/s 2 * 35,000 m P = 9.80 x 10 8 kg/m*s 2 Pa = 1 kg/m*s 2 (SI unit for pressure) 10 5 Pa = 1 bar (~1 atm), so P = 9.80 kbar Factors Controlling the Characteristics of Metamorphic Rocks Stress Differential Stress Compressive stress Shearing Foliation (influence of stress on texture) Slaty cleavage Schistose Gneissic texture

5 Factors Controlling the Characteristics of Metamorphic Rocks Fluids Hot water (vapor) Trigger metamorphic chemical reactions Time minerals grow, re-crystallize slowly

6 Classification of Metamorphic Rocks Nonfoliated Named on the basis of its composition Example - quartzite, marble Foliated Determine type of foliation Example -schist, gneiss

7 Baraboo quartzite 1 mm Classification of Metamorphic Rocks Nonfoliated Named on the basis of its composition Example = quartzite, marble Foliated Determine type of foliation slate- splits neatly along parallel planes schist- alignment of platy, or needle shaped minerals gneiss- separation into distinct light and dark layers

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9 Strongly foliated schist in thin section 1 mm

10 Types of Metamorphism Contact Metamorphism (thermal metamorphism) Regional Metamorphism (most common) Progressive Metamorphism slate > schist > gneiss > migmatite Types of Metamorphism Contact Metamorphism (thermal metamorphism) Narrow zone of contact Shale => hornfels Limestone => MARBLE Sandstone => QUARTZITE Quartz grains welded together

11 Types of Metamorphism Regional Metamorphism (dynamothermal metamorphism) Takes place at considerable depth and high temperatures degrees C > 5 km depth Basalt => greenschist- actinolite, and sodium-rich plagioclase Basalt => amphibole schist - hornblende, plagioclase feldspar, garnet

12 Types of Metamorphism Progressive Metamorphism Progressively greater temperature and pressure Shale => Slate => Phyllite => Schist => Gneiss => Migmatite (partial melting & crystallization) Progressive Metamorphism

13 Plate Tectonics and Metamorphism Gravitational collapse and spreading flow, foliation at deep levels Geothermal gradient Isotherms vary in subduction zone different metamorphic rocks may occupy various levels at different tectonic positions e.g., sites A, B, C in Figure 7.16 Geothermal gradient varies with tectonic setting

14 Hydrothermal Processes Hydrothermal Minerals => Hydrothermal Rock Veins Hydrothermal Activity at Diverging Plate Boundaries Sulphide ore deposits black smokers

15 Hydrothermal Processes Hydrothermal Rocks and Minerals disseminated ore deposits Sources of Water Groundwater Water Trapped in Sediments Hydrous Minerals Metasomatism introduction of ions from fluid Sources of Water Groundwater Water vapor from cooling, solidifying magma

16 Tectonic scale sources of water Water Trapped in Sediments Hydrous minerals in basalt or granite Solidifying magma