Felsic describes rocks, magma, or lava high in silica and felsic minerals. Felsic minerals are rock-forming silicates high in silicon, oxygen, aluminum, potassium, and sodium.
You are probably wondering what silicates are. These are minerals with oxygen and silicon forming a tetrahedron SiO44- units. They represent the most abundant mineral in the Earth’s crust. This makes them important.
The common felsic minerals include feldspars (alkali and plagioclase), quartz, muscovite, and feldspathoids. Others are corundum, tridymite and cristobalite. The last two are forms of silica.
These minerals are usually light-colored when compared to mafic, which are dark-colored. Mafic minerals are silicates rich in magnesium and iron.
Lastly, felsic comes from the first three letters of “feldspar” and two letters of “silica” plus c.
What are felsic rocks?
Felsic rocks are silica-rich rocks high in minerals like quartz, feldspars, or muscovite and relatively low iron and magnesium minerals.
Usually, rocks classified as felsic have at least 75% felsic minerals. These minerals are orthoclase, quartz, and plagioclase. Those with more than 90% are leucocratic.
Felsic rocks are usually light-colored. Typical colors are off-white, pink, gray, reddish, yellowish, or bluish. Some may be brown, buff, reddish, tan, greenish, dark gray, or black. Mineral presents and textures may affect colors.
The usual color index M of most felsic rock is less than 30. However, some rocks may have a lower color index. For instance, granite is between 5 and 20.
Composition
Felsic rocks are high in silica, feldspars, and quartz or feldspathoids like nepheline or leucite. These rocks commonly have alkali feldspar and lesser plagioclase.
Also, it may have muscovite or conundrum and small amounts of mafic minerals like biotite, hornblende, pyroxene, or olivine and accessory minerals.
However, the exact minerals will depend on which rock you have.
Usually, felsic rocks have silica content greater than 63 wt. %. However, some authors place this value at 65 wt. % silica. Others classify rocks with h 63-69% silica as intermediate-felsic and those with > 69% as felsic.
Examples of felsic rocks
The common felsic rocks are granite, rhyolite, obsidian, dacite, and rhyodacite. Others like tonalite, syenite, nepheline syenite, and trachyte are intermediate to felsic, depending on silica content.
These rocks may be aphanitic (fine-grained), coarse-grained, or glassy. Also, they can have other textures, including pegmatitic, poikilitic, or porphyritic.
For instance, granite is an intrusive rock with a coarse-grain texture. It forms deep inside the Earth’s crust.
On the other hand, rhyolite and dacite are aphanitic or fine-grained. They form close to or on the Earth’s surface.
Lastly, obsidian is a volcanic glass. It forms from rapid quenching that doesn’t allow crystals to form. Thus, it has an amorphous solid structure.
Characteristics or facts
Felsic rocks are light-colored, less dense, and crystallize at lower temperatures. Their magmas are more viscous and erupt explosively compared to other rock types.
1. Are highly viscous
Felsic rocks are highly viscous because they are high in silica. Silica units link to form longer chains (polymerize), making it more viscous – thick and sticky. This makes it flow slowly, i.e., resist flow.
2. The erupt explosively
Felsic volcanoes will erupt explosively. Most will be Plinian or Vulcanian eruptions. Such will produce a high and dense column of ash.
These magmas erupt explosively since they will have more volatiles. They are high volatiles because their high viscosity makes it hard for volatiles to escape.
However, felsic magmas can also erupt effusively. Such will ooze like toothpaste and form thick, slow-moving lava flows known as blocky lavas. Block lavas have large angular to slightly smooth debris or blocks on their surface.
However, most will have both explosive at the start and effusive towards the end of an eruption.
3. Low density
Felsic rocks are lower in density (less than 3), making them more buoyant. Therefore, they can occur at higher elevations from the Earth’s mantle. This makes the continental crust thicker.
They are less dense because the various minerals that make these rocks are low density.
4. Form at lower temperatures
Unlike mafic rocks, felsic rocks form at lower temperatures on the Bowen reaction series. This is true, considering constituent minerals have a lower melting point.
How does felsic magma form?
Felsic magmas are highly evolved and rich in silica. These magmas form from partial melting of crustal material or fractional crystallization.
In partially melting crustal, elements lower in melting point will first melt. These include silica, sodium, potassium, etc. Thus, you will have magma rich in these elements.
On the other hand, crystal fractionation occurs when earlier, higher-density, and melting points minerals, mostly mafic, settle down and are separated from the rest of the magma. This progressively leaves more magma rich in silica, potassium, sodium, and other lower-melting minerals.
Where are felsic rocks found?
Felsic rocks are common on continental Earth’s crust. They mostly form active margin arcs, especially those associated with mountain building. However, these rocks may also form on continental hotspots and rifts.
Usually, extrusive varieties like rhyolite are common on volcanism above subduction zones. Here, intrusive rocks may form, too.
Additionally, continental collisions may form intrusive rocks like granite. These form from tectonic thickening and crustal heat that causes partial melting of crustal rocks.
Frequently Asked Questions (FAQs)
Obsidian is a naturally occurring felsic rock or glass. It forms from rhyolitic magmas. Those not from rhyolitic will bear the name of the magma, such as dacitic obsidian.
It depends. Pumice is a textural term for highly frothy, glassy rock. Most pumice rocks are felsic, i.e., rhyolitic or dacitic. However, some can be intermediate to felsic, like trachyte or phonolite, and on rare occasions, mafic (basaltic).
Felsite is a field term that refers to any aphanitic or very fine-grained rock yet to be identified and classified after a chemical or microscopic analysis.
It refers to any metamorphic rock formed from an unknown but chemically confirmed felsic protolith (parent rock).