* Igneous rocks: These are formed from the cooling and solidification of magma or lava. Examples include granite, basalt, and gabbro.
* Sedimentary rocks: These are formed from the accumulation and cementation of sediments. Examples include sandstone, limestone, and shale.
* Other metamorphic rocks: Yes, even metamorphic rocks can be transformed into gneiss under high pressure and temperature.
How Gneiss Changes from its Protolith:
The key to understanding how gneiss differs is the process of metamorphism. This involves intense heat and pressure, often combined with chemically active fluids, that cause significant changes in the original rock:
1. Mineralogical Changes: The minerals in the protolith are rearranged and recrystallized. For example, if the protolith was granite (rich in quartz, feldspar, and mica), the gneiss will have the same minerals but they will be aligned in distinct bands or layers.
2. Texture: Gneiss has a characteristic foliation, meaning it has a layered or banded appearance. This banding is due to the alignment of minerals, often with lighter and darker bands. The protolith might not have had this distinct layering.
3. Structure: The original structure of the protolith is often obliterated during metamorphism. The gneiss may show evidence of folding, shearing, or other structural changes.
Examples:
* Granite to Gneiss: Granite, when subjected to metamorphism, becomes a granitic gneiss. The original minerals (quartz, feldspar, and mica) are still present, but they are recrystallized and aligned in bands.
* Shale to Gneiss: Shale, a fine-grained sedimentary rock, can transform into a mica schist and then into a gneissic schist with further metamorphism. The mica minerals are aligned, forming distinct foliation.
In short: Gneiss is a metamorphic rock with a distinct banded texture and mineralogical changes, resulting from the intense heat and pressure that transformed its protolith (original rock).
