Here's a breakdown of the process:
1. Starting Material: Igneous rocks like granite, basalt, or sedimentary rocks like sandstone and limestone are the common starting materials for gneiss.
2. Heat and Pressure: These rocks are subjected to intense heat and pressure deep within the Earth's crust, often associated with tectonic activity like mountain building.
3. Recrystallization: The heat and pressure cause the existing minerals to recrystallize and align themselves in a foliated structure. This means the minerals arrange in distinct bands or layers, giving gneiss its characteristic banded appearance.
4. Mineral Transformation: During the process, the minerals can also transform into new minerals, depending on the specific conditions. For example, feldspar in the original rock can transform into mica, which is a common mineral in gneiss.
Therefore, gneiss is not a single, unique rock type. Instead, it's a metamorphic rock group, characterized by its foliation and banded structure.
Key Points:
* Foliation: The distinct banding in gneiss is called foliation, a characteristic feature of metamorphic rocks formed under high pressures.
* Mineral Composition: The specific minerals present in gneiss will vary depending on the original rock type and the intensity of metamorphism.
* Types of Gneiss: There are various types of gneiss, classified based on their mineral composition and origin. Some common types include granitic gneiss, migmatitic gneiss, and augen gneiss.
Understanding the origin of gneiss helps us to appreciate its unique characteristics and how it reflects the powerful forces that operate within the Earth's crust.