Question

Evidence of metamorphism at this location could be from the discovery of: a. gneiss b. quartzite c. tektites d. mylonite

Short answer

Evidence of metamorphism includes gneiss, quartzite, and mylonite (options a, b, and d).

Step-by-step solution

Understand Metamorphism

Metamorphism refers to the process by which rocks undergo physical and chemical changes due to extreme heat, pressure, or chemically active fluids, typically without the rock melting. This process often results in the formation of new minerals and textures.

Identify Metamorphic Rocks

Metamorphic rocks are those which have undergone such changes as described in Step 1. Common examples of metamorphic rocks include gneiss, schist, marble, quartize, and mylonite. Recognizing these helps in identifying evidence of metamorphism.

Analyze the Options

Look at each option to determine if it represents a type of metamorphic rock: - **Gneiss (a)**: A high-grade metamorphic rock with distinctive banding patterns. - **Quartzite (b)**: A hard metamorphic rock formed from quartz-rich sandstone. - **Tektites (c)**: Naturally occurring glass objects formed from molten debris after meteorite impacts—not a metamorphic rock. - **Mylonite (d)**: A fine-grained, foliated rock formed by dynamic recrystallization during shearing.

Confirm Evidence of Metamorphism

From the options in Step 3, gneiss, quartzite, and mylonite are all metamorphic rocks. These would provide evidence of metamorphism as they are results of the metamorphic process. Tektites, however, are not involved in traditional metamorphism.

Key concepts

Gneiss

Gneiss is a fascinating type of high-grade metamorphic rock known for its distinctive banding patterns. These bands, or stripes, result from the segregation of mineral grains into layers during the metamorphic process. This rock primarily forms under conditions of extreme pressure and temperature, often deep within the Earth's crust.

Gneiss originates from rocks like granite or sedimentary rock such as shale. The transformation process, known as metamorphism, reorganizes a rock's mineral structure without melting it. This results in the unique appearance of gneiss, making it easily identifiable in the field.

Key characteristics of gneiss include:

• Texture: Coarse-grained with a banded texture.
• Mineral Composition: Commonly contains quartz, feldspar, and mica.
• Uses: Often used as a dimension stone in buildings and monuments due to its impressive strength and visual appeal.

Understanding gneiss helps geologists determine the geological history of an area and recognize the intense conditions that were present during its formation.

Quartzite

Quartzite is a durable and robust metamorphic rock that originates from sandstone. Predominantly composed of quartz, quartzite forms when sandstone is subjected to high temperatures and pressures usually associated with tectonic compressions.

The process that creates quartzite is fascinating. Intense heat and pressure cause the quartz grains in sandstone to fuse together, forming an interlocking crystalline structure. This transformation does not change the mineral composition significantly but tightens the bonds between quartz grains, increasing its hardness.

Critical traits of quartzite include:

• Texture: Non-foliated or slightly foliated, often resembling the texture of sugar.
• Color: Typically white to gray, but it can exhibit shades of pink and red due to iron oxide impurities.
• Durability: Extremely hard and resistant to weathering and erosion, making it useful for constructing roads, railway ballast, and as decorative stone.

Recognizing quartzite is crucial when identifying areas that have undergone metamorphism, as it points to former conditions of extreme heat and pressure.

Mylonite

Mylonite is a unique type of metamorphic rock formed in zones of intense deformation, typically associated with faults or shear zones. It forms during dynamic recrystallization of minerals under conditions of high strain, frequently at low temperatures relative to other metamorphic environments.

This rock is distinct because of its fine-grained texture and foliated appearance, which results from the plastic deformation of minerals. Unlike other metamorphic rocks, mylonites form primarily through mechanical processes rather than chemical alterations, which gives them their characteristic structure.

Key features of mylonite include:

• Texture: Generally fine-grained with a foliated or banded appearance.
• Formation Environment: Occurs in high-strain zones, often indicative of major tectonic movement.
• Recognition: Often found alongside fault lines and tectonic boundaries, providing critical insights into past tectonic activities.

Recognizing mylonite can help geologists piece together the history of tectonic processes and understand the mechanical conditions that have shaped the earth's crust.