Geology 101 - Lecture 13

(Brief Outline Only)

I. Definition

II. Agents (Causes) of Metamorphism

• A. Heat - temperatures range from about 200 degrees C to the melting temperature of rocks
  Geothermal gradient - temperature increases with depth in the Earth on average 25 degrees C per kilometer
  
  Rocks that form at relatively low temperatures are referred to as low grade metamorphic rocks, and those that form at high termperatures are referred to as high grade metamorphic rocks
  
  
• B. Pressure
  • 1. Confining pressure - due to the weight of overlying rock. Doesn't result in a change of shape or in a foliated texture
  • 2. Directed pressure/Differential stresses - pressure that is greater in one direction than another. Produces a change in shape and causes a foliation
  
  
• C. Fluids - hot water acts as catalyst for metamorphic reactions
  

III. Changes that Occur During Metamorphism

• Textural changes
  
  1. Size of minerals changes
  
  2. Shape of minerals changes
  a. Flattening from directed pressure
  b. Develop foliation (slaty, schistose, gneissose)
• Mineralogical Changes
  e.g. Clay changes to mica, graphite changes to diamond
  

III. Types of Metamorphism

• A. Contact metamorphism - generally non-foliated rocks
• B. Regional metamorphism - foliated and deformed rocks
• C. Shock metamorphism - meteorite impacts
• D. Hydrothermal metamorphism - hot fluids, i.e., at mid ocean ridges

IV. Metamorphism Associated with Convergent Plate boundaries

A. low P, hi T in upper part of volcanic arc
B. high P, hi T in lower part of volcanic arc
C. high P, low T in downgoing lithosphere - blueschist, e.g. Mt. Shuksan

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