Felsic magmas and lavas are proportionately higher in lighter elements like oxygen, silicon, aluminum, sodium, or potassium and lower in iron, calcium, and magnesium.
Also, they have higher viscosity and are at or erupt at lower temperatures than intermediate or mafic magmas. Viscosity is a measure of how fluids resist flow.
Lava describes hot, liquid, or semi-liquid rocks that arrive at the surface, while magma is if it is inside the Earth’s crust.
The term felsic comes from two names, felspar and silicic, which are significant components. Also, since they are high in silica, they are known as silicic magma or lava. The modifier silicic only tells you they are silica-rich.
Lastly, these magmas are acidic since they have more than 63 wt.% silica. Those with 52-62 wt. % are intermediate and 45-52 wt.% basic. Ultrabasic has less than 45 wt.% silica.
Chemical and mineral composition
Chemically, felsic magmas or lavas are high in silica (SiO2), with silica weight percent content exceeding 63%. Also, they are relatively high in alkali oxides (sodium and potassium oxide) and aluminum and low in iron and magnesium oxides.
Mineralogically, they are higher in feldspars, feldspathoids, and quartz, and their forms (polymorphs) like tridymite and cristobalite. Common feldspathoids include leucite, sodalite, and nepheline.
On the other hand, these felsic magmas are, however, low in mafic minerals like biotite, olivine, amphibole, or mica.
Lastly, they often have volatiles and may also contain other solids, such as the ones they pick as they move in the Earth’s crust.
Characteristics of felsic magmas and lavas
Some of the characteristics they possess include the following:
1. Have lower temperatures
Felsic magmas are at and erupt at lower temperatures. These lower temperatures indicate they don’t originate from very deep inside the Earth’s crust compared to mafic magma.
Typical eruption temperatures are 650°C to 800°C (1202°F to 1472°F). However, some can erupt at higher temperatures above 950°C (1742°F). Such will be less viscous and flow longer distances.
2. Are highly viscous
Felsic magmas and lavas are highly viscous or thick. Typical values are 105 Pa⋅s (108 cP) to 108 Pa⋅s (1011 cP) at 1200°C(2192°F) and 800°C (1472F°), respectively.
Remember, viscosity is dependent on temperature.
Why are they more viscous? These magmas are more viscous because 1) they are cooler and 2) they are high in silica.
Cooling and the high silica content favor silica joining to form chains and cross-linking, i.e., polymerization, making them resist flow.
3. High in volatiles
Most of these magmas are high in volatiles because their high viscosity prevents escape. These high gases are what cause explosive eruptions.
4. Are less dense
These silica-rich magmas are less dense, with typical densities of 2.18 to 2.25g/cm3.
They are less dense because they have light-weight components. This includes silica, sodium, and potassium oxides compared to mafic, which has denser components like iron, calcium, and magnesium.
However, as they rise near the Earth’s surface, their densities will drop as volatiles form bubbles, i.e., exsolve.
How do felsic magmas erupt?
Felsic magmas erupt explosively since they often are high in gases and are viscous. These two facts make eruptions explosive.
Explosive eruptions include Vulcanian, Strombolian, and Plinian. Such will form tephra or pyroclasts, the names of debris ejected during an explosive eruption.
Alternating eruptions and lava flow can form composite volcanoes. These are large, tall, cone-shaped mountains formed from repetitive explosive and effusive eruptions.
Also, if they or their hot volcanic materials encounter water, they will cause phreatomagmatic and phreatic eruptions.
However, degassed ones can erupt effusively. Some will form lava domes or plugs. Others will form blocky lava flows. Their surfaces have large angular blocks, flowing slowly and over short distances.
Some, like felsic, i.e., rhyolitic lava flows, the eastern Snake River Plain did flow far. Reasons may include higher eruption temperatures that made them more fluid.
Lastly, rapid cooling of frothy lava will form pumice. Pumice traps lots of gases, creating a glassy but highly vesiculated rock.
What rocks do they form?
Most of these magmas are quite thick. So, most cannot flow to the surface like less fluid mafic counterparts. Instead, they will cool inside the Earth’s crust, forming granite.
However, those that reach the surface will form volcanic rocks like rhyolite, dacite, and obsidian.
Most of these rocks are light-colored save for obsidian, a volcanic glass.
How does felsic magma form?
They form from the fractional crystallization of basaltic or mafic magma. In this process, denser, earlier crystallizing minerals are progressively separated from the melt.
Separation happens when they settle or float lighter feldspar minerals. This change gradually changes the composition of magma.
Fractional crystallization can happen with the assimilation of subcrustal rock, i.e., contamination or not. Contamination occurs when hot magma melts and incorporates subcrustal rocks. This contributes to the composition.
The other ways are partial melting of 1) subcrustal rocks beneath thick crust that bear garnet and 2) hot subducting slabs in relatively young subduction zones.
Also, partial melting of components of pyroxenitic mantle wedge can form these magmas.
Lastly, one study suggests felsic magmas may form by separating hydrous melt from slab-derived supercritical liquids from the sub-arc mantle.
Where do they erupt
These silicic magmas erupt in various tectonic settings, including subduction zones, continental hotspots, and rifts, and sometimes in interoceanic islands.
However, they are most common on continental margins above subduction zones. The thick continental crust allows them to evolve or differentiate.
Some continental hotspots where they occur in the US are Yellowstone in Wyoming, Coso Long Valley Systems and Volcanic Field in California, and Grande Rift in Colorado and New Mexico
That is not all. They also form rifts in East Africa, Taupo in New Zealand, and Aden Volcano in Yemen.
Additionally, bimodal shields, such as in hotspots and rifts in Canada, such as Rainbow Range, Level Mountains in British Colombia, and Ilgachuz Range, erupt felsic magmas. These lavas are peralkaline rhyolites but at much higher temperatures of 1200°C (2192°F).
Lastly, intra-oceanic islands like Galapagos erupt these silicic magmas. However, they are less abundant or common.