Question

Is granite or mica schist more likely to fold or flow rather than fracture when subjected to differential stress? Explain.

Short answer

Mica schist is more likely to fold or flow than granite under differential stress.

Step-by-step solution

Assess Material Behavior

Understand that granite is an igneous rock and is generally more rigid and less likely to fold or flow. Mica schist, on the other hand, is a metamorphic rock with minerals aligned in layers, which can allow it to fold more easily under stress.

Determine the Effect of Differential Stress

Differential stress tends to cause deformation in rocks. Rocks like mica schist that have a planar, layered structure can accommodate stress by folding or deforming due to their mineral alignment. Granite, which lacks this structure, is more prone to fracturing.

Consider the Role of Temperature and Pressure

Higher temperatures and pressures make rocks more ductile. Mica schist, under such conditions, is more likely to fold or flow due to its mineralogical composition. Granite may remain brittle unless subjected to extremely high temperatures and pressures.

Conclude Based on Rock Properties

Considering the properties of both rocks and their responses to differential stress, mica schist is more likely to fold or flow compared to granite because of its layered mineral composition and ability to deform ductilely.

Key concepts

Differential Stress

Differential stress occurs when force is applied unevenly in different directions on a rock. This stress can cause rocks to change shape or volume, leading to deformation. In geological terms, deformation refers to the alteration of the original shape or structure of the rock.

Under differential stress, rocks can respond in a few distinct ways:

• Elastic Deformation: Temporary shape change that is recovered when the stress is removed.
• Plastic Deformation: Permanent change in shape without fracturing, typically occurring under conditions of high pressure and temperature.
• Fracturing: Occurs when rocks break due to stress, common in brittle materials like granite.

Mica schist, being a metamorphic rock, can often accommodate differential stress through plastic deformation due to its mineral alignment, allowing it to fold or flow rather than fracture, unlike the more rigid granite.

Metamorphic Rocks

Metamorphic rocks are formed through the transformation of existing rocks under conditions of high temperature and pressure. This process, known as metamorphism, alters the mineral composition and structure of the initial rock.

Key characteristics of metamorphic rocks include:

• Foliation: A layered appearance caused by the reorganization of mineral grains under stress. This trait enables rocks like mica schist to exhibit greater flexibility and folding abilities compared to non-foliated rocks.
• Recrystallization: The process by which minerals change shape and size without melting, enhancing the durability and strength of the rock.
• Mineral Alignment: Minerals are typically aligned in a parallel fashion, enhancing their ability to deform easily.

Because of these features, metamorphic rocks like mica schist are more capable of folding and flowing under differential stress, unlike igneous rocks that may fracture under similar conditions.

Igneous Rocks

Igneous rocks form from the solidification of molten magma or lava. These rocks can be formed either below the earth's surface (intrusive) or on the surface (extrusive).

Characteristics of igneous rocks include:

• Crystal Size: Generally, the slower the cooling rate, the larger the crystals. Granite, an intrusive igneous rock, typically has larger crystals due to its slow cooling process.
• Composition: Composed mainly of silicate minerals, igneous rocks have varied chemical makeup ranging from felsic, like granite, to mafic rocks, which contain more iron and magnesium.
• Texture: The texture can range from coarse to fine-grained, influenced by the cooling rate of the magma or lava.

Due to their inherent rigidity and lack of mineral alignment, igneous rocks like granite are less likely to fold or flow under differential stress. Instead, they are prone to fracturing, making them less ductile compared to their metamorphic counterparts.