Mafic and felsic are not commonly used terms, as one may observe. The few that know the existence of these words are mainly geologists or ones related to that field of study. So, what is it?
Igneous rocks on chemical analysis were found to be abundant in silicate oxides and therefore it was decided that these igneous rocks would be differentiated based on their silica content.
Mafic and felsic are the terms used to describe the silica content of igneous rocks formed on the cooling of lava. Sometimes, they are also used to differentiate the characteristics of the lava (or magma).
Mafic vs Felsic
The main difference between Mafic and Felsic rocks are the silica content present. Since igneous rocks are characterised by silica content in them, mafic is the one with less silica content (approx. 45-55%) while felsic is the one with a greater silica content (approx.70-90%).
There is a contrasting difference even in the appearance of the two igneous rocks; one being dark shaded and the other having light shade.
Comparison Table Between Mafic and Felsic
| Parameters of Comparison | Mafic | Felsic |
|---|---|---|
| Viscosity of Lava | Low viscosity due to the high temperature of lava | High viscosity |
| Density | High density and heavy | Low density and lightweight |
| Colour | Dark shades of green, or black | Light-coloured |
| Mineral Content | Rich in magnesium and iron | Rich in aluminium and silica |
| Silica Level | 45-55% | 70-85% |
What is Mafic?
Mafic is the acronym for magnesium and ferric (Latin for Iron) because this igneous rock is rich in these two minerals. The mafic lava is usually at 950 and often leads to a weak volcanic eruption.
Mafic rocks are generally found in dark shades of green or greenish-black in colour. Their characteristics are medium to coarse-grained.ย
The low density of these rocks is due to the low content of silica present in them. This is since their origin is mostly the sea bed, that is not abundant in silica.
Due to the low viscosity of the lava, the lava erupted are usually very runny. Basalt is a well-known example of a mafic rock.
What is Felsic?
Felsic is the term used for the rocks rich in the mental feldspar, that contains aluminium and silica at high levels. The felsic lava is usually found at 650-750ห C and is can cause explosive volcanic eruptions.
The high silica content (70-85%) is because of the geographical location the felsic rocks are found at. They are mostly found in specific geographic areas known as convergent zones, where the geologic plates collide.
Graphite is a well-known example of felsic rocks.
Main Differences Between Mafic and Felsic
- Felsic is also rich in low-density minerals like oxygen, sodium and potassium. Thus, these rocks have low density unlike mafic rock, which have high density are heavy.
- The minerals present in the two is also the reason due to which there is a contrasting difference in their characteristic appearance.
Conclusion
Mafic and felsic, are both igneous rocks formed on the cooling of lava from the volcanic eruptions. Igneous rocks, characterized by their silica content can be put in two categories: mafic and felsic (other two categories being ultramafic and intermediate).
The silica level of mafic rocks is 45-55%, which is because they are originated in volcanoes present at the sea bed. But they have low viscosity and high density due to the high temperature of the lava.
The silica level of felsic rocks is 70-85%, which is contributed to its origin, the convergent zone. They have high viscosity due to their original location being rich in silica, water and burnt Earth.
Mafic rocks are rich in the heavy minerals, iron and magnesium. This gives these rocks their characteristic dark green or greenish-black colour.
References
- https://www.sciencedirect.com/science/article/pii/S0024493704002014
- https://mingtang.me/publications/Tang%20et%20al_Science%202016.pdf
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