Mafic magmas and lavas are the most common on Earth. The other relatively common ones are intermediate and felsic.
Lesser common types are ultramafic and alkaline, while non-silicate examples like sulfur, iron oxide, carbonatite, and natrocarbonatite are rare.
Non-silicate magmas or lavas don’t have silicate groups. A silica group has a basic unit of silicon and oxygen, forming a tetrahedron unit, i.e., SiO44-.
To avoid confusion, magma or lava refers to molten or semi-molted rock. When beneath the surface, it is magma. However, if it makes its way to the Earth’s surface, it is lava.
What are mafic magmas or lavas?
Mafic magmas or lavas are proportionately high in iron and magnesium and low in silica (SiO2), sodium, and potassium. Also, they are relatively highly fluid and occur or erupt at higher temperatures than their intermediate or felsic counterparts.
The term mafic is a blend of two words, i.e., ma from magnesium and fic from ferric. Ferric or ferrum is of Latin origin and means iron.
As you can see, this name emphasizes that these magmas, lavas, or their rocks are relatively high in iron and magnesium minerals.
Sometimes, iron and magnesium minerals are known as ferromagnesium. This name shouldn’t cause any confusion.
Composition
Mafic magma and lavas are low in silica (SiO2). They have 45-52 wt.% silica compared to intermediate with 52-63 wt. % and felsic >63%. Also, they are relatively high in iron and magnesium oxide and low in alkali oxides. Alkali oxides are sodium and potassium.
They often contain dark-colored crystallized minerals such as pyroxene, olivine, hornblende, or biotite and are associated with calcic-plagioclase (calcium-rich).
Like any other magma, they will have gases and may entrain some solids.
Characteristics of mafic magmas and lavas
Some of their characteristics include the following:
1. Form and erupt at high temperatures
Minerals present in mafic magmas melt at high temperatures. Also, their lava erupts at higher temperatures of 1000-1200 °C (1832- 2192°F).
2. Are highly fluid or less viscous
Mafic magmas and lavas are low in viscosity, with typical viscosities of 10 to 100 Pa⋅s or 104 to 105 cP, which is about 10,000 times that of water. Viscosity is the resistance to flow. The higher the material flows, the less vice versa.
These low viscosities make them flow faster and travel far distances. Also, it influences the appearance of the resulting landforms.
Some reasons for low viscosities include their low silica content and high eruption temperatures.
3. Are dense
Mafic magmas and lavas are dense because they are relatively high in iron, magnesium, and calcium oxides and lower in less dense minerals like silica, sodium, or potassium oxides.
Did you know that iron oxides have a density greater than 5g/cm3, magnesium 3.58g/cm3 and calcium 3.34g/cm3? This is higher than sodium, potassium, or silica, with densities less than 2.7g/cm3 abundant in felsic lavas.
4. Low in gas content
Mafic magmas are relatively lower in gases. Their lower viscosity allows most gases to escape.
The lower gas content and low viscosities are the reasons why they often erupt non-explosively or effusively.
How does mafic magma erupt and flow?
Mafic magmas erupt mostly non-explosively. Their lava will either fountain or flow out effusively from vents or fissures. However, some are explosive. This may include when magma or hot volcanic materials interact with water, i.e., phreatomagmatic and phreatic eruptions.
Subaerial eruptions will form relatively thin lava flows, both pahoehoe and aa. Also, they will form shield volcanoes and sometimes lava lakes. However, large eruptions will form flood basalt.
Flood basalts are a type of large igneous provinces (LIPs). LIPs are nothing other than the accumulation of large igneous rocks.
Examples of basaltic flood basalts are Columbia River Basalts in the Northwest of the USA. Others are Deccan Traps, Paraná and Etendeka traps, and Siberian traps.
Most of the landforms from subaerial eruptions have a low profile and cover expansive areas. This happens because they are fluid and make relatively thin flows.
On the other hand, submarine eruptions will form mostly basaltic pillow lavas. However, they can also form sheet flow and lobate lava flows. Also, they can form submarine LIPs like the Ontong Java Plateau. It covers a 1.5 million km2 area.
Lastly, mafic lava flow, especially basaltic, may form features like columnar jointing.
What rocks do mafic magma or lava form?
Mafic magmas that reach the surface form mostly basalt rock. Those cooling inside the Earth’s crust from gabbro or diabase/microgabbro.
However, they can form other rocks, too, including sideromelane, palagonite, norite, palagonite, Pele’s tear, and hair.
How and where does mafic magmas form?
Mafic magmas form from the partial melting of mantle rocks called peridotites.
This partial melting can happen via 1) decompression melting, 2) flux melting, or 3) heat transfer caused by increased temperature.
Upwelling or decompression melting occurs when the solid mantle moves upward. This reduces pressure and lowers the melting point. It is the most common cause of partial melting.
On the other hand, flux melting involves the addition of volatiles like water and sometimes carbon dioxide. They also reduce melting point.
Most of these magmas form at mid-ocean ridges or spreading centers. This is where the accretion of the new ocean crust happens. These magmas are common in oceanic hotspots like Hawaii, back-arc basins, and intraplate seamounts.
Furthermore, they can also form in subduction zones, continental rifts, and hotspots. However, in these areas, they will often transfer heat, causing continental crust to melt.
Continental crust melting will increase their silica, alkali oxide, and other components, making their composition intermediate or felsic. This is essential contamination.
Also, fractional crystallization and different pools of magma mixing can make magma evolve or differentiate.