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Mafic Vs. Felsic Rocks: Know the Difference
Classification of rocks, minerals, and magma based on the silica content present in them defines them as either mafic or felsic. Learn more about the Mafic vs. Felsic geological classification through this ScienceStruck article.
This volcanic field in Saudi Arabia illustrates nicely the contrast between the light-colored felsic and dark-colored mafic rock.
Volcanoes are not mere lava mountains. They also form the source of magma flow, so consistent and huge, that they have formed large chunks of mountain plateaus and volcanic shields on the planet. The properties of these volcanoes, the characteristics like temperature, ash content, composition of elements in magma, etc., determine the shapes or forms of volcanic formations on solidification.
Volcanic rocks are the primary formations studied in order to reveal new geological facts. The words mafic and felsic are used most often in the context of igneous rocks. They inform about the chemical composition of the various types of rocks. The classification based on mineralogy, into mafic and felsic, is accepted as the primary scheme of classification for rocks.
Difference Between Mafic and Felsic Rocks
Igneous rocks consist of primarily 12 types of oxides. The classification is done based on their silica content, as silica is usually the most abundant of all the oxides present. Therefore, rocks are divided into ultramafic, mafic, intermediate, and felsic. This is an order showing increased silica content. Mafic rocks have between 45 and 55% of silica, whereas felsic rocks have over 65% of silica, the highest of all types. Due to the color of the minerals forming them, the rocks are either light or dark. As iron-rich silicates are normally dark in color, mafic rocks are dark-colored.
|It is relating to, denoting, or containing a group of dark-colored, mainly ferromagnetic minerals such as pyroxene and olivine.
The word Mafic comes from (ma) for magnesium and (f) for ferrum or the Latin for iron. Mafic rocks are dark in color. Common mafic rocks include basalt, dolerite, and gabbro.
|It is of, relating to, or denoting a group of light-colored minerals, including feldspar, feldspathoids, quartz, and muscovite.
It is also known to be a mnemonic, combining (fe) for feldspar, (l) for lenad or feldspathoid, and (s) for silica. Felsic rocks are light in color. Common felsic rocks include granite, rhyolite, and dacite.
|Specific Gravity (The ratio of the density of a substance to the density of a standard is specific gravity. It is usually water for a liquid or solid, and air for a gas.)
|Specific gravity is less than 3.||Specific gravity is greater than 3.|
|List of Mafic and Felsic minerals|
|Olivine, amphibole, pyroxine, and biotite.||Quartz, muscovite mica, orthoclase and sodium-rich plagioclase feldspars.|
|Lava Flow (Viscosity is one of the basic characteristics of these types of lava. It is basically a measure of the substance’s consistency. It is defined as the ability of a substance to resist flow.)|
|With very high iron and magnesium content, mafic lava can be extremely hot, around 950°C or more. Because of these high temperatures, the viscosity is usually low as compared with felsic lava. Mafic lava leads to weaker volcanic eruptions. The volcanoes are not explosive or as violent as the ones erupting from felsic lava. Basalt lava is associated with lowly shield-volcanoes.||Most felsic lavas have high viscosity. It has a higher composition of silica, aluminum, potassium, sodium, and calcium. The temperatures are around 650°C to 750°C. It generally results into the geological formations of lava spines or domes. Felsic magma, on cooling, forms feldspar and quartz.|
Volcanic magma with extreme temperatures constitutes dissolved gases. When these gases are released, bubbles are formed. The process of bubble formation is known as vesiculation. The bubbles frozen in the rocks are called vesicles. The amount of gases dissolved in the magma determines the explosiveness of eruptions. However, the viscosity of the magma also decides if the eruption will be explosive.
Basalt, with low-viscosity magma, provides an easy escape of the gases to the surface. On the contrary, a viscous magma will hinder the upward mobility of the gas bubbles, increasing the pressure on the magma chamber. This, in turn, would lead to rapid explosive ejection of the gas bubbles.
Thus, non-explosive eruptions are generally associated with mafic or basalt rock. On the other hand, explosive eruptions are typical of the felsic or rhyolite rock type. Nevertheless, there are exceptions to this rule; when degassed felsic lava erupts at the surface, it is non-explosive, and some hydro-volcanic eruptions with mafic magma are explosive.