Question

Which of the following indicate(s) the presence of directed stress in a rock?

• A. Porphyritic texture
• B. Schistosity
• C. Gneissosity
• D. Mylonitic texture

Hint:

Schistosity is the most direct indicator of directed stress in rocks, where minerals align parallel to the direction of maximum stress.

Answer 1

The Correct Option is B, C, D

To determine which of the given textures indicate the presence of directed stress in a rock, we need to understand the nature of each texture:

• Porphyritic Texture: This texture is characterized by large crystals (phenocrysts) embedded in a finer-grained groundmass. It typically forms during the cooling of molten rock, which is not necessarily related to directed stress, but rather to the cooling rate of the magma or lava. Hence, it is not associated with directed stress.
• Schistosity: This is a type of foliation that results from the parallel arrangement of platy minerals, such as micas, due to directed pressure. Schistosity is a common feature in metamorphic rocks that have undergone significant recrystallization under directed stress, making it a clear indicator of directed stress.
• Gneissosity: Similar to schistosity, gneissosity refers to the alternating bands of differing mineral composition found in gneiss. The formation of these bands is usually due to high-grade metamorphic conditions involving substantial directed stress, which causes mineral segregation and alignment. Therefore, gneissosity indicates directed stress.
• Mylonitic Texture: Mylonites are rocks formed due to intense shearing under directed stress, typically in fault zones. The texture is caused by the plastic deformation of minerals, indicating substantial directed stress. Thus, mylonitic texture is a strong indicator of directed stress.

Based on this explanation, the textures that indicate the presence of directed stress in a rock are:

• Schistosity
• Gneissosity
• Mylonitic texture