Compare and contrast confining pressure and compressional stress ...

[charchew]

Compare and contrast confining pressure and compressional stress relating to metamorphic rocks. What will happen to the mineral crystals in each situation?
 
  Also, provide examples of geologic environments where such pressure would be the main agent of metamorphism.
  What will be an ideal response?

Question 2

What is gneissic banding, how does it form, and how does it compare to the color banding that is often found in marble?
 
  What will be an ideal response?

[onowka]

Answer to Question 1

Answer: Confining pressure on metamorphic rocks by applying pressure equally in all directions will collapse any open spaces between mineral grains to produce a more dense rock. An environmental example would be younger overlying materials burying sedimentary rocks and forcing them deeper into the interior. Compressional stress metamorphoses rocks by applying pressure that is greater in one direction and weaker in another. (For example, the horizontal pressure is greater than the vertical pressure.) This will cause the mineral crystals to elongate or reorient so that the crystal's long axis is perpendicular to the direction of highest pressure. An environment that would create such a scenario can include a convergent plate boundary.

Answer to Question 2

Answer: Gneissic banding is the highest degree of foliation that can form before the rock begins to melt. It forms in high-grade metamorphic conditions and when minerals segregate into separate layers, which often gives the appearance of alternating light and dark layers. This effect differs from color banding, which has alternating light and dark layers but little difference in mineral crystals between the layers.

[charchew]

Thanks for the timely response, appreciate it

[shailee]

Gracias!