Magma refers to hot, molten, or semi-molten rocks under the Earth’s surface, while lava refers to magma that makes it to the surface.
These liquid or semi-solid rocks are made of mainly melt (the liquid part), crystallized minerals, and volatiles. Also, they can have solids entrained by the melt.
Most are rich in silica (SiO2). Such are called silicate magmas, examples are felsic, intermediate, mafic, ultramafic, and alkaline. Silicates are rock-forming minerals with a tetrahedra silica (SiO44-) unit that dominates the Earth’s crust.
On the other hand, a few of the magmas or lavas are non-silicate, i.e., not dominated by silica. Examples are sulfur, carbonatites, and iron oxide.
Silicate magmas
Silicate magmas or lavas vary in silica, volatile content, and composition. These variations influence their properties and how they erupt.
Some of these properties are
Here is more on the various silicate magmas and lavas:
1. Mafic
Mafic magmas and lavas are the most abundant. They have 45-52 wt.% silica and are relatively high in iron and magnesium and low in sodium and potassium.
Also, they have more dark-colored mafic minerals like amphibole, biotite, pyroxene and mica and lesser light-colored or felsic. Felsic minerals include feldspars, quartz, feldspathoids, and muscovite.
Some of the characteristics of mafic lavas and magmas include:
- Erupt at relatively high temperatures of 1000-1200 °C (1832- 2192°F)
- Have relatively low viscosity of 10 to 100 Pa⋅s or 104 to 105 cP. Viscosity is the resistance to flow.
- They are dense due to the relatively high amount of dense elements like iron and magnesium or their minerals. For instance, basalt magma has a density of about 2.65-2.8g/cm3
- They have a low gas content of about 1-3%, as most escapes are less viscous.
Mafic magmas erupt non-explosively via effusive flow out or fountaining. However, interaction with water can cause explosive phreatomagmatic or phreatic eruptions.
Subaerial eruptions will form pahoehoe or aa lava flow. Also, being fluid, they will flow a long distance, forming low-profile shield volcanoes. Large eruptions will form flood basalts.
On the other hand, submarine eruptions will form pillow lava or sheet flows.
Lastly, mafic magmas form from partial melting of peridotites. They are common in mid-ocean ridges and oceanic hotspots like Hawaii. Also, they can occur in back-arc basins and intraplate seamounts, with small amounts forming in subduction zones, continental hotspots, and rifts.
2. Felsic
Felsic magmas or lavas have more than 63 wt.% silica (SiO2) and are relatively high in sodium and potassium and low in iron, magnesium, and calcium. Also, they are high in felsic minerals and lower in mafic minerals.
Some of their characteristics include:
- They erupt at low temperatures of 650°C to 800°C (1202°F to 1472°F).
- Have a high viscosity of about 105 Pa⋅s to 108 Pa⋅s (108 cP to 1011 cP) at temperatures of 1200°C(2192°F) and 800°C (1472F°).
- They are less dense since they are high in lighter elements like silica, potassium, and sodium oxides and less in heavy ones like iron, calcium, or magnesium. Rhyolitic magmas have a density of about 2.18-2.25g/cm3
- Have high gas content, usually 4-7%. Their high viscosity prevents gases from escaping.
Felsic magmas mostly erupt explosively since they are highly viscous and high in gas content. Eruptions can be Vulcanian, Strombolian, Plinian, phreatomagmatic, phreatic, etc.
However, effusive eruptions can occur if degassing happens. Such can form plugs, lava domes, or blocky lava flows. Also, alternating explosive and effusive eruptions will form composite volcanoes.
How do they form? Felsic magma forms partial melting of subcrustal rocks or fractional crystallization of mafic magma. Fractional crystallization can happen with or without subcrustal rock assimilation.
Lastly, they are common above subduction zones. However, they can also occur in continental hotspots, rifts, and rarely interoceanic islands.
3. Intermediate
Intermediate magma and lava have a composition that lies between felsic and mafic. They have 52-63 wt.% silica and are moderate in sodium, potassium, iron, and calcium. Also, they are moderate in both felsic and mafic minerals.
Their characteristics include:
- Erupt at temperatures between mafic and felsic of about 800 and 1100°C (1472°F and 2012°F).
- Their viscosity is moderate, i.e., between mafic and felsic, with values being 3.5 × 106 cP (3,500 Pa⋅s) at 1200°C (2192°F)/
- Moderately dense with andesitic magma having a density of 2.45-2.5g/cm3
- They are moderate in volatiles, i.e., 3-4%.
They erupt explosively or effusively. Effusive eruptions will form aa lava flows, and alternating eruptions will form composite volcanoes.
These magmas form from fractional crystallization, subcrustal contamination, melting of subcrustal rocks, or magma mixing.
Lastly, they occur mostly above subduction zones on continents and islands. Also, they form in oceanic hotspots and mid-ocean ridges but are less common.
4. Ultramafic
Ultramafic magmas and lavas have less than 45 wt.% silica and generally over 18% magnesium oxides.
Also, they are high in iron oxide and have almost entirely mafic minerals, especially pyroxenes and olivine, with minor felsic minerals.
Some of their characteristics include:
- Erupt at very high temperatures. For instance, komatiites likely erupted at 1600°C during the Archean age.
- They are dense since they have primarily heavy elements or minerals, i.e., iron and magnesium.
- Are highly fluid or have very low viscosity of 100 to 1000 cP (0.1 to 1 Pa⋅s), comparable to motor oil. It’s low because the low silica content and high temperature don’t allow them to polymerize.
- Low in volatiles or gases since they escape easily due to their low viscosity.
Most ultramafic lava and magma formed during the Archean age when the Earth was hotter. They likely formed from the high degree of partial melting of mantle rocks at high temperatures exceeding 1600°C (2912°F).
For instance, almost all komatiite lava is of Archean age with no modern examples. There are no younger than Proterozoic. A few Phanerozoic are most likely formed from hot mantle plumes.
Lastly, these ultramafic magmas most likely erupted effusively, being low in gases and viscosity.
5. Alkaline magma or lavas
Alkaline magmas are higher in alkalis (Na2O and K2O) than required by feldspar or are deficient in silica SiO2 with respect to their amount of alkalis. Therefore, they will have feldspathoids and alkali-rich mineral phases like alkali pyroxenes or amphiboles.
These magmas occur commonly in continental rifts like the East African rift. Also, they can occur in intraplate hotspots or above deeply subducted plates.
They likely form partial melting to a small degree of mantles altered metasomatically and their origin is deeper than subalkaline. This partial meeting will form primary alkaline mafic magmas like ankaramite or alkalic picrite basalt.
Their properties, like temperature, viscosity, and gas content, depend on their composition, including silica content. They can be ultramafic, mafic, intermediate, or felsic if they undergo fractionation or evolve.
Lastly, rocks formed from alkaline magmas include nephelinite, tephrite, trachyte, basanite, phonolite, ijolite, tephrite, melilitite, etc.
Non-silicate magma
Non-silicate magmas are dominated by minerals that don’t have silica group (SiO44-). Instead, they are dominated by carbonate, sulfur, or oxides like iron oxide.
Common examples include:
1. Sulfur
Sulfur lava forms from the melting of sulfur deposits at as low as 113°C (235°F) temperature. This forms a low viscosity, nearly like water fast flow melt whose flow resembles pahoehoe.
One volcano known to have sulfur lava flow is Lastarria volcano, Chile. Fumaroles, i.e., openings on volcano surfaces that emit hot gases and vapors, create these sulfur flows.
Some of the most significant sulfur flows on the Lastarria volcano are 350 meters and 250 meters long.
2. Iron oxide lava
Molten iron oxide lava forms an immiscible liquid that separates from calc-alkaline or alkaline parental magma. They erupt at 700-800°C (1292-1472°F) and are a significant iron source.
Examples occurred in Kiruna, Sweden, and the El Laco volcanic complex on the border of Chile and Argentina.
3. Carbonatite and natrocarbonatite magma
Carbonatite magmas or lavas are unusually high in carbonates, most exceeding 75%, with minor amounts of non-carbonate minerals like silicates, oxides, phosphates, sulfides, or halides.
The amount of sodium carbonate may be different from the actual composition of magmas as hydrothermal activities can easily remove highly soluble carbonates like sodium.
Carbonatite lavas are highly fluid with a viscosity of about ≈5 × 10−3 poise since they are low in silica that polymerizes. Their densities are low and have the highest capacity to hold dissolved volatiles like water and halogens. Also, they erupt at relatively lower temperatures.
Studies [1, 2, and 3] point out the origin of these magmas as the mantle. They Form from 1) an extremely low degree of partial melting, immiscible carbonate-silicate separation, or through an unusual and extreme fractional crystallization.
Natrocarbonatite are a type of carbonatite magmas or lavas that occur in Ol Doinyo Lengai in Tanzania.
They are dominated by sodium carbonate and half as much calcium and potassium carbonate each. Also, they have small amounts of other minerals like halides, sulfates, and fluorides.
These lavas are also highly fluid, with viscosity that exceeds water by a small margin. Also, they erupt at relatively low temperatures of 500 to 600°C (932° to 1112°F).
References
- Neukirchen, F., & Ries, G. (2020). The world of mineral deposits: A beginner’s Guide to Economic Geology. Springer.
- Winter, J. D. (2014). Principles of igneous and metamorphic petrology. Pearson Education.
- Philpotts, A. R., & Ague, J. J. (2022). Principles of igneous and Metamorphic Petrology (3rd ed.). Cambridge University Press.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.