Metamorphic Rocks Metamorphism refers to solid-state changes to rocks in Earth’s interior –Produced by increased heat, pressure, or the action of hot, reactive fluids –Old minerals, unstable under new conditions, recrystallize into stable ones Rocks produced from pre-existing or parent rocks in this way are called metamorphic rocks breadtoast
Metamorphic rocks common in the old, stable cores of continents, known as cratons Metamorphic Rocks *Oldest stable continental core *Seismically fast *Geothermally cold *Buoyant – low density (chemically depleted)
Factors Controlling Metamorphic Rock Characteristics Temperature during metamorphism –Heat for metamorphism comes from Earth’s deep interior –If temperature gets high enough, melting will occur Pressure during metamorphism –Confining pressure applied equally in all directions –Pressure proportional to depth within the Earth increases ~1 kilobar per 3.3 km of burial within the crust 1 kb = 14,500 psi and VW bug = 2000 lbs Pressure at 100 km depth = 239 VW Bugs on 1 square inch area (A bigger area of 1 sq foot would require 34,452 VW bugs!)
Temperature and pressure conditions for metamorphism Figure 10.2 Low grade metamorphism: Rocks that are metamorphosed under temperature and pressures upto 400C and 400 MPa High grade metamorphism: Rocks that are metamorphosed under temperature and pressures higher than 400C and 400 MPa
Pore Fluids and Metamorphic Veins The presence of pore space and aqueous fulids can facilitate metamorphic reactions. Porous fluid flow transports mineral chemicals and speeds up mineral growth and recrystallization.
Metamorphic Rocks Under Stress Granite consists of quartz, feldspar, biotite which recrystallized from magma under conditions of uniform stress. Grains are randomly oriented. This gneiss, a high-grade metamorphic rock, contains the same minerals as the rock on the left but deformed in solid state under differential stress. Biotite grains are aligned in a layered texture. Foliation is the planar arrangement of mineral grains giving a layered texture
Slaty cleavage Figure 10.7 Horizontal slaty cleavage Compression Vertical slaty cleavage Bedding Planes Bedding planes are perpendicular to the maximum stress direction. Bedding and cleavage planes change if applied stress changes.
Types of Metamorphism -Contact metamorphism: Occurs when rocks are heated and chemically changed adjacent to an introduced body of hot magma -Burial: Occurs as a result of burial of sediments in deep sedimentary basins -Regional: Extensive area of crust associated with plate convergence, collision, and subduction.