What is the Difference Between Gneiss and Granite

The main difference between gneiss and granite is that gneiss is a type of metamorphic rock, whereas granite is a type of igneous rock.

Rocks are naturally occurring solid masses or aggregates of minerals. There are three major types of rocks as sedimentary rocks, igneous rocks, and metamorphic rocks. These three types have differences in their appearance and chemical composition.

Key Areas Covered

1. What is Gneiss 
     – Definition, Features, Formation
2. What is Granite
     – Definition, Features, Formation
3. What is the Difference Between Gneiss and Granite
     – Comparison of Key Differences

Key Terms

Gneiss, Granite, Metamorphic, Ingenious

What is Gneiss

Gneiss is a type of metamorphic rock. It is, in fact, the most common and widely distributed type of metamorphic rock. These rocks form via high temperature and high-pressure metamorphic processes acting on sedimentary or igneous rocks. For example, gneiss rocks formed from igneous rocks are orthogneiss rocks while gneiss formed from sedimentary rocks are paragneiss rocks. Gneiss always shows a banded texture. There are alternating darker and lighter bands without any distinction foliation.

Figure 1: Gneiss

Generally, gneiss rocks are medium to coarse-foliated materials. This means these rocks are largely recrystallized, but they do not carry large quantities of mica and some other minerals. The parallel bands in the gneiss rock are called gneissic banding. These bands develop due to the effect of high temperature and high pressure during the formation of the rock. Moreover, the minerals in these rocks are also arranged in layers, which will appear in a cross-section. This is called compositional banding. These layers occur because they have different compositions of minerals. However, all gneiss rocks do not have distinguishable banding patterns. Some rocks appear as random clumps.

What is Granite

Granite is a type of igneous rock. It falls under the category of felsic intrusive igneous rocks. These rocks have a granular and phaneritic texture. Predominantly, these rocks appear in white, pink or grey colour. The name granite is derived from the meaning “grain-containing”. This is due to the coarse grain structure. These are completely crystalline rocks. Granite contains about 20-60% of quartz (by volume) and about 35% of feldspar.

Generally, granite occurs as massive masses which are hard and tough. Therefore, granite has been very important as a construction material since ancient times. It has a comparatively high density and a good compressive strength. Moreover, this material has a poor permeability when not cracked and a strong permeability when cracked.

Figure 2: Appearance of Granite

When considering the occurrence of granite, it occurs mainly in the continental crust. Sometimes we can find this material in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite is easily subjected to weathering; it undergoes physical weathering via exfoliation joints (this type of weathering occurs on a large scale). We can also observe chemical weathering in this material; carbonic acid, rain and saltwater can cause chemical weathering of the rock.

Difference Between Gneiss and Granite

Definition

Gneiss is a type of metamorphic rock while granite is a type of igneous rock.

Formation

High temperature and high pressure processes that act on igneous and sedimentary rocks cause the formation of gneiss while the slow crystallization of magma forms granite.

Appearance

Gneiss has alternating dark and light bands while granite has a coarse grain structure.

Hardness

Moreover, gneiss is very hard compared to granite. 

Conclusion

There are three types of rocks as sedimentary rocks, igneous rocks, and metamorphic rocks. The main difference between gneiss and granite is that gneiss is a type of metamorphic rock, whereas granite is a type of igneous rock.

Reference:

1. “Granite.” ScienceDirect Topics, Available here.
2. “Granite.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 24 Jan. 2020, Available here.

Image Courtesy:

1. “Gneiss” By Siim assumed – No machine-readable source provided. Own work assumed (based on copyright claims) (CC BY-SA 3.0) via Commons Wikimedia
2. “Granite” By I, Friman (CC BY-SA 3.0) via Commons Wikimedia