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Alkaline magmas are characteristic of continental rifting, areas overlying deeply subducted plates, or at intraplate hotspots. [2]: Ch6 They are more likely to be generated at greater depths in the mantle than subalkaline magmas. [2]: Ch23.6. Alkaline rocks are rare in the Archean, but become common in the Proterozoic.
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in ...
Trachyandesite is a member of the alkaline magma series, in which alkaline basaltic magma experiences fractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma.
Trachyte is the usual silica-rich end member of the alkaline magma series, in which alkaline basaltic magma experiences fractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma to give it a composition close to that of alkali feldspar.
These represent the low-silica end of the calc-alkaline magma series and are characteristic of volcanic arcs above subduction zones. [43] Boninite is a high-magnesium form of basalt that is erupted generally in back-arc basins; it is distinguished by its low titanium content and trace-element composition. [44]
Hawaiite is sodic trachybasalt. Hawaiite is an aphanitic (fine-grained) volcanic rock produced by rapid cooling of lava moderately poor in silica and enriched in alkali metal oxides (potassium oxide plus sodium oxide). It is often impractical to determine the mineral composition of such a fine-grained rock, and so hawaiite is defined chemically.
Volcanic arcs are characterized by explosive eruption of calc-alkaline magma, though young arcs sometimes erupt tholeiitic magma [7] and a few arcs erupt alkaline magma. [8] Calc-alkaline magma can be distinguished from tholeiitic magma, typical of mid-ocean ridges, by its higher aluminium and lower iron content [9]: 143–146 and by its high ...
Dacite (/ ˈdeɪsaɪt /) is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.