phenocrysts. Mafic lava Mafic or basaltic lavas are typified by relatively high magnesium oxide and iron oxide content (whose molecular formulas provide the consonants in mafic) and have a silica content limited to a range 52% to 45%. They generally erupt at temperatures of and at relatively low viscosities, around 104 to 105 cP (10 to 100 Pa⋅s). This is similar to the viscosity of ketchup, although it is still many orders of magnitude higher than that of water. Mafic lavas tend to produce low-profile shield volcanoes or flood basalts, because the less viscous lava can flow for long distances from the vent. The thickness of a solidified basaltic lava flow, particularly on a low slope, may be much greater than the thickness of the moving molten lava flow at any one time, because basaltic lavas may "inflate" by a continued supply of lava and its pressure on a solidified crust. Most basaltic lavas are of ʻaʻā or pāhoehoe types, rather than block lavas. Underwater, they can form pillow lavas, which are rather similar to entrail-type pahoehoe lavas on land. Ultramafic lava Ultramafic lavas, such as komatiite and highly magnesian magmas that form boninite, take the composition and temperatures of eruptions to the extreme. All have a silica content under 45%. Komatiites contain over 18% magnesium oxide and are thought to have erupted at temperatures of . At this temperature there is practically no polymerization of the mineral compounds, creating a highly mobile liquid. Viscosities of komatiite magmas are thought to have been as low as 100 to 1000 cP (0.1 to 1 Pa⋅s), similar to that of light motor oil. Most ultramafic lavas are no younger than the Proterozoic, with a few ultramafic magmas known from the Phanerozoic in Central America that are attributed to a hot mantle plume. No modern komatiite lavas are known, as the Earth's mantle has cooled too much to produce highly magnesian magmas. Alkaline lavas Some silicate lavas have an elevated content of alkali metal oxides (sodium and potassium), particularly in regions of continental rifting, areas overlying deeply subducted plates, or at intraplate hotspots. Their silica content can range from ultramafic (nephelinites, basanites and tephrites) to felsic (trachytes). They are more likely to be generated at greater depths in the mantle than subalkaline magmas. Olivine nephelinite lavas are both ultramafic and highly alkaline, and are thought to have come from much deeper in the