Felsic and mafic are field terms we use to classify igneous rocks, their magma, or lava. This classification is based on their mineral composition, i.e., silicates they have.
Silicates are the most common and important rock-forming minerals. They make up over 90% of the Earth’s crust.
What are silicates? Silicates are minerals with negatively charged tetrahedron units of silicon and oxygen, i.e., SiO44- and positively charged metal ions (cations).
Note: The words mafic and felsic are not formal or official IUGS terms for igneous rock classification. They will only infer on which kind of minerals a rock has, i.e., dark or light. We use these words mainly in the field. Also, they don't refer to specific igneous rock textures like aphanitic, phaneritic, pagmatitic, porphyritic, etc.
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What are felsic and mafic minerals?
Felsic minerals are light-colored rock-forming silicates. They have mostly light-colored elements like potassium, silicon, sodium, and oxygen.
Examples are alkali and plagioclase feldspars, quartz, feldspathoids, and corundum. Others are silica forms like tridymite and cristobalite.
On the other hand, mafic minerals are dark-colored rock-forming silicates. These minerals are rich in iron and magnesium.
Common examples are pyroxene, olivine, hornblende, and biotite. Less common ones include some garnets, humite, and chloritoids.
What are felsic and mafic rocks?
Felsic rocks are light-colored rocks high in silica and felsic minerals and low in mafic minerals. Due to their high silica content, which is > 63 wt. %, these rocks are sometimes known as silicic or acidic.
Examples of felsic rocks are granite, obsidian, rhyolite, dacite, and rhyodacite.
On the other hand, mafic rocks are silica-poor, dark-colored rocks high in mafic minerals and lower in felsic minerals. These rocks are often called basic since they are poor in silica, usually 45-52 wt.%.
Common mafic minerals are basalt and gabbro. Others are diabase (dolerite), norite, basanite, and basaltic volcanic glasses like sideromelane, Pele’s tears, and hair.
Felsic vs. mafic differences
Under differences, we will look at color/appearance, density, viscosity, and eruption of felsic and mafic rocks. Also, we will discuss their crystallization temperature, where they occur, and their stability on the surface.
1. Colors and appearance
Mafic rocks, minerals, or magmas are darker in color, usually dark green, brown, or black. In contrast, felsic ones are light-colored, usually off-white, gray, buff, yellowish, and sometimes tan, reddish, bluish, or pinkish.
However, texture, inclusions, and alteration may change these colors. Also, light reflection by tiny crystals (microlites) can influence color. For instance, obsidian is black or jet-black, yet it is a felsic natural volcanic glass.
2. Density
Mafic rocks are denser, with typical densities of 2.75-3.3g/cm3. They have high density because they have iron and magnesium minerals that are also denser.
On the other hand, felsic rocks are less dense with densities, usually less than 2.7g/cm3, i.e., 2.4-2.7 g/cm3. These rocks are less dense because their minerals, like quartz, feldspars, and muscovite, are less dense.
2. Viscosity
Felsic magma and lavas are more viscous (have high viscosity or resist flow more) than mafic. This happens because they are high in silica that links to form longer units. The formation of the cross-linked units makes it thick and resists flow.
However, the exact viscosity values will depend on composition, temperature, and the amount of volatiles in each magma sample.
3. Volatile content
Felsic magmas are high in volatiles. Their high viscosity prevents the escape of these volatiles.
On the other hand, mafic minerals are lower in volatiles. This happens as their lower viscosity allows some gases to escape.
4. Eruption
Felsic magmas or lavas mostly erupt explosively since they trap more volatiles. In contrast, mafic magmas or lavas erupt often effusively since they are low in volatiles.
Explosive eruptions will eject a column of gases, ash, lapilli, and other debris seen in Plinian and Vulcanian eruption styles.
On the other hand, effusive eruptions will form lava flows like pahoehoes and less often aa. More viscous magma will form blocky lava flows.
However, some felsic or mafic magmas can erupt effusively or explosively. It depends on their volatile contents.
5. Crystallization temperatures
Felsic rocks crystallize at lower temperatures of 600-750°C (1112°F-1382°F), while mafic rocks crystallize at higher temperatures of 1000°C (1832°F) to 1200 °C (2192°F).
Therefore, in magmas, mafic minerals tend to crystallize earlier and mafic later.
6. Stability of the surface
Since they form at lower temperatures from magmas high in water or volatile, felsic rocks tend to be more stable near or on the Earth’s surface than mafic. Thus, they will undergo alteration or chemical weathering slowly.
In contrast, mafic rocks are less stable on or near the Earth’s surface. This is because they form from drier, hotter magmas. Thus, they are prone to chemical weathering to more stable products.
7. Where they occur or are found
Felsic rocks occur mostly in the continental crust, especially on active mount-building-related (orogenic) volcanic margins.
On the other hand, mafic rocks occur mostly in the oceanic crust. Those on oceanic crust form at mid-ocean ridges (divergent boundaries)
However, these rocks can occur in other tectonic settings, including hot spots and continental rifts.
Frequently Asked Questions (FAQs)
Both. Pegmatite is a textural term for rocks with abnormally large crystals in a finer-grained matrix. It doesn’t infer to composition. Therefore, mafic, felsic, or intermediate rocks can be pegmatitic. Examples are granite, gabbro, diorite, syenite, and granodiorite pegmatites.
Colors, density, and minerals will help you identify these rocks. Felsic will be less dense, light-colored, and will have mostly lighter minerals like feldspar or quartz. In contrast, mafic will be denser and dark-colored. Also, they will have mostly dark, iron, and magnesium-rich minerals.