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. Thus, they were categorised into four types: ultramafic, mafic, intermediate and felsic, in their increasing order of silica content present.
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). They are also contrastingly different in colour: Mafic being dark shades of green while Felsic is usually lightly shaded rocks.
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. The difference in the level of silica present in the two arises from the origin of the two rocks.
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. That is not explosive or violent.
Mafic rocks are generally found in dark shades of green or greenish-black in colour. Their characteristics are medium to coarse-grained.
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 650-750˚ C and is can cause explosive volcanic eruptions. These rocks are also rich in other minerals like oxygen, potassium and sodium. This is the main reason for the low density of felsic rocks.
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. The location has high levels of silica along with water and burnt earth (due to the magma), thus making the lava sticky.
Graphite is a well-known example of felsic rocks.
Main Differences Between Mafic and Felsic
- The characteristic difference between mafic and felsic is the silica content present in the two rocks. Mafic has low levels of silica. Approximately only 45-55%. Basalt, one of the common examples of Mafic rock has only 50% silica content. On the other hand, Felsic has much higher levels of silica. Approximately, 70-85%. Graphite is found to have 70% of silica.
- The lavas that are cooled to form these rocks are also different, in terms of their viscosity. The mafic lava usually has a low viscosity and is running. This is per the fact that these lavas erupt at a high temperature. Whereas, felsic rocks have high viscosity. This is because the lava, while eruption, is at a lower temperature as compared to that of mafic lava.
- Although, the two are igneous rocks, which are defined by their levels of silica, the difference in the silica content in the two rocks, is its origin. Mafic rocks are obtained from mostly the sea bed volcanoes whereas felsic rocks are obtained from the cooling of lavas from the convergent zones.
- Due to their origin, the minerals found in the two rocks are also different. Mafic is rich in heavy minerals like magnesium and ferric, or iron. But Felsic is rich in silica and aluminium.
- 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. Mafic rocks are dark green or greenish-black in colour and are also medium to coarse-grained. But felsic rocks are light-coloured and have a smooth surface.
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. The low density and light-weight, ascribable to the presence of light-weight minerals such as oxygen, sodium and potassium.
Mafic rocks are rich in the heavy minerals, iron and magnesium. This gives these rocks their characteristic dark green or greenish-black colour. Whereas, felsic rocks are rich in silica and aluminium along with oxygen, sodium and potassium that gives the light shade for these rocks.
References
- https://www.sciencedirect.com/science/article/pii/S0024493704002014
- https://mingtang.me/publications/Tang%20et%20al_Science%202016.pdf