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?
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
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
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/m3 * 10 m/s2 * 35,000 m
P = 9.80 x 108 kg/m*s2
Pa = 1 kg/m*s2 (SI unit for pressure)
105 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
Factors Controlling the Characteristics of
Metamorphic Rocks
• Fluids
– Hot water (vapor)
– Trigger metamorphic chemical reactions
• Time
– minerals grow, re-crystallize slowly
Classification of Metamorphic Rocks
• Nonfoliated
– Named on the basis of its composition
• Example - quartzite, marble
• Foliated
– Determine type of foliation
• Example -schist, gneiss
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
Strongly foliated schist in thin section
1 mm
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
Types of Metamorphism
• Regional Metamorphism
• (dynamothermal metamorphism)
– Takes place at considerable depth and high temperatures
• 300-800 degrees C
• > 5 km depth
• Basalt => greenschist- actinolite, and sodium-rich plagioclase
• Basalt => amphibole schist - hornblende, plagioclase feldspar,
garnet
Types of Metamorphism
• Progressive Metamorphism
– Progressively greater temperature and pressure
– Shale =>
Slate =>
Phyllite =>
Schist =>
Gneiss =>
Migmatite
(partial melting & crystallization)
Progressive Metamorphism
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
Hydrothermal Processes
• Hydrothermal Minerals =>
– Hydrothermal Rock
• Veins
• Hydrothermal Activity at Diverging Plate Boundaries
– Sulphide ore deposits
• black smokers
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
• Tectonic scale sources of water
– Water Trapped in Sediments
– Hydrous minerals in basalt or granite
– Solidifying magma