[Geography Notes] on Felsic and Mafic Pdf for Exam

Mafic and Felsic Both are made-up terms used to indicate the chemical composition of silicate minerals, magmas, and igneous rocks.

Mafic refers to silicate crystals, magmas, and rocks with a high proportion of heavier elements. The name “mafic magma” comes from the combination of the letters MA and FIC, which stand for magnesium and iron in Latin. Mafic magmas are also high in calcium and sodium. Mafic minerals are usually dark in color with high specific gravities (greater than 3.0). Olivine, pyroxene, amphibole, biotite mica, and plagioclase feldspars are common rock-forming mafic minerals. Mafic magmas are typically formed at spreading centers and represent newly differentiated upper mantle material. Basalt and gabbro are examples of mafic rocks. (It should be noted that certain geologists with dubious intentions reverse the magnesium and iron order and coin the name “femag.”) This is not to be confused with Femag, the Diabolical Dr. Saprolite’s dimwitted henchman.)

Felsic rock like the one shown above, on the other hand, refers to silicate crystals, magmas, and rocks with a smaller proportion of heavier elements and a higher concentration of lighter elements including silicon and oxygen, aluminum, and potassium. The word is derived from FEL, which stands for feldspar (in this case, the potassium-rich variety), and SIC, which stands for silica content. Felsic minerals have a light color and a basic gravity of less than 3.0. Quartz, muscovite mica, and orthoclase feldspars are common felsic minerals. Granite, the refined result of the earth’s internal separation process, is the most common felsic rock.

Felsic and mafic rocks are igneous rocks classified according to their silica content. Chemical analysis of the most abundant components in rocks are commonly presented as oxides of the elements; igneous rocks generally contain about 12 major oxides, which account for over 99 percent of the rock. Silica (SiO₂) is normally the most prevalent of the oxides. Due to this abundance and the fact that silicates make up the majority of igneous minerals, silica content was used as a basis for early classifications, and it is still generally accepted today. Rocks are classified as felsic, intermediate, mafic, or ultramafic according to this scheme (in order of decreasing silica content).

Rocks with more than 65 percent silica are felsic; those with 55 to 65 percent silica are intermediate; those with 45 to 55 percent silica are mafic; and those with less than 45 percent are ultramafic, according to a generally recognized silica-content classification scheme. Rhyolite and granite are felsic rocks, with an average silica content of 72 percent; syenite, diorite, and monzonite are intermediate rocks, with an average silica content of 59 percent; gabbro and basalt are mafic rocks, with an average silica content of 48 percent; and peridotite is an ultramafic rock, with an average silica content of 41 percent. Despite the fact that the averages have full gradations, rocks appear to cluster around them. The transition from felsic to mafic is generally accompanied by a rise in the color index (dark-mineral percentage).

Classification of Felsic Rocks

A rock must contain >75 percent felsic minerals, such as quartz, orthoclase, and plagioclase, in order to be categorized as felsic. Rocks that contain more than 90% felsic minerals are known as leucocratic, which means ‘light-colored.’ 

Felsite is a petrologic word for fine-grained or aphanitic light-colored volcanic rocks that may be reclassified after a more thorough microscopic or chemical examination.

Some felsic volcanic rocks contain phenocrysts of mafic minerals, such as hornblende, pyroxene, or a feldspar mineral, and must be named after the phenocryst mineral, such as ‘hornblende-bearing felsite.’ 

The TAS diagram of Le Maitre is used to determine the chemical name of a felsic rock (1975). This, however, is only true of volcanic rocks. If the rock is felsic yet metamorphic and lacks a definite volcanic protolith, it may be appropriate to simply refer to it as a ‘felsic schist.’ Extremely sheared granites that can be mistaken for rhyolites have been discovered.

The QAPF diagram should be used for phaneritic felsic rocks, and a name should be given according to granite nomenclature. Since the term granite already assumes feldspar and quartz, species of mafic minerals are often included in the name, such as hornblende-bearing granite, pyroxene tonalite, or augite megacrystic monzonite.

Mafic vs Felsic 

Two terms are widely used to describe the characteristics of rocks and lava in the concept of mineralogy, or geology in a wider context. The terms mafic and felsic are used to describe these types of rocks. 

1. Sticky or Runny 

Mafic lava is runnier or more viscous than felsic lava when used to characterize its characteristics.

The amount of silica in the lava is the explanation for this. Mafic lavas have less silica than felsic lavas. The volcanic eruption would most likely be less violent than the Hawaiian Island volcanic eruptions due to the runnier lava.

Summary: Mafic vs Felsic 

1. Mafic lava is viscous compared to felsic lava. 

2. Mafic lava dominates mid-ocean ridges, while felsic lava is found mostly at convergent areas. 

3. Mafic lava flows more easily than felsic lava, and the former has a lower risk of exploding. Since felsics tend to cap steam and other gases, explosive eruptions are more likely. 

4. Basalt is formed by mafic lava, while andesitic and rhyolite are formed by felsic lava. 

5. When discussing rocks or lava, mafic means the lava or rock contains less silica, while felsic means the lava or rock contains the most silica. 

6. Mafic rocks are darker than felsic rocks in color.