Question

During metamorphism of granite into gneiss, what happens to minerals? A) They partly melt. B) They become new sediments. C) They grow smaller. D) They align into layers.

Short answer

D) They align into layers.

Step-by-step solution

Identify the Question

The question asks about the changes that occur to minerals during the metamorphism of granite into gneiss. We need to focus on how the minerals are transformed or restructured in this process.

Review Metamorphism Process

Metamorphism is the process by which a rock's structure or mineral composition is altered in a solid state because of heat, pressure, and chemically active fluids. It does not involve melting but rather the recrystallization of minerals.

Analyze Granite to Gneiss Metamorphism

When granite is subjected to high pressure and temperature, it transforms into gneiss. This metamorphic process leads to the alignment of minerals into bands or layers, a characteristic feature of gneiss.

Evaluate the Options

- Option A ('They partly melt') is incorrect because metamorphism occurs without melting. - Option B ('They become new sediments') is incorrect, as sediments form through weathering and erosion, not by metamorphism. - Option C ('They grow smaller') does not reflect the typical mineral changes during metamorphism. - Option D ('They align into layers') accurately describes the mineral alignment into bands or layers, which is a key characteristic of gneiss.

Key concepts

Granite

Granite is an igneous rock, which means it is formed from the cooling and solidification of magma. Granite is primarily made up of three key minerals: quartz, feldspar, and mica. Each of these minerals contributes to the overall strength and appearance of the granite.

• Quartz: This mineral gives granite its hardness. It is generally transparent and is the most durable component.
• Feldspar: Often pink, white, or gray, feldspar adds to the rock's toughness and provides bulk.
• Mica: This mineral, which can be biotite or muscovite, adds sparkle and reflective qualities, often seen as shining flakes.

Granite's structure is interlocking and granular, which is why it is used in various architectural applications such as countertops and monuments. However, while it seems stable, granite undergoes significant changes under the right conditions of heat and pressure, leading to its metamorphosis into gneiss.

Gneiss

Gneiss is a type of metamorphic rock that originates from granite. Through the process of metamorphism, granite is transformed into gneiss. This transformation occurs when the rock is exposed to high temperatures and pressures over time.
Unlike granite, gneiss is known for its distinct banded appearance. The bands in gneiss are created by the alignment of mineral grains, which is a significant characteristic. These bands often display alternating layers of light and dark minerals, giving gneiss a striped look. This rock is often found in regions where tectonic forces have been at work, such as mountain ranges. The banded structure of gneiss not only makes it an interesting and attractive rock for geological studies but also for use in construction and decorative purposes.

Mineral Alignment

Mineral alignment is a crucial concept in understanding the metamorphic transformation of granite into gneiss. During metamorphism, the minerals within the granite do not grow smaller or turn into sediment; instead, they undergo recrystallization. This process causes the minerals to realign due to differential stress.
The minerals, as they recrystallize, align perpendicularly to the direction of applied pressure. This is a result of the minerals adjusting to the intense pressures they are exposed to. For example:

• Quartz and feldspar: These minerals form elongated grains that align in response to stress.
• Mica: Typically grows in sheet-like structures, which rotate and flatten to align with the pressure.

This alignment results in the distinctive layered or banded appearance seen in gneiss. The process reflects a significant structural change that emphasizes the intense conditions these rocks have withstood.

Heat and Pressure Effects

The effects of heat and pressure are pivotal in the metamorphism process, leading to the transformation of granite into gneiss. These forces do not melt the rock but change its structure and composition solidly.

• Heat: Heat plays a critical role by providing the energy needed for the recrystallization of minerals. It can cause atoms in the minerals to migrate, forming new mineral grains.
• Pressure: Pressure, particularly from tectonic forces, compresses the rock, which influences the alignment and recrystallization of minerals. Extreme pressure forces minerals to form in preferred orientations.

Together, heat and pressure facilitate the realignment of mineral grains and the eventual formation of a banded structure typical of gneiss. Understanding these effects helps to explain why the transformation process results in such a dramatic change in the rock's characteristics.