Editing Flood basalt (section)

=== Ascent to the surface ===
The composition of flood basalts may reflect the mechanisms by which the magma reaches the surface. The original melt formed in the upper mantle (the ''primitive melt'') cannot have the composition of quartz tholeiite, the most common and typically least evolved volcanic rock of flood basalts, because quartz tholeiites are too rich in iron relative to magnesium to have formed in equilibrium with typical mantle rock. The primitive melt may have had the composition of [[picrite basalt]], but picrite basalt is uncommon in flood basalt provinces. One possibility is that a primitive melt ''stagnates'' when it reaches the mantle-crust boundary, where it is not buoyant enough to penetrate the lower-density crust rock. As a tholeiitic magma differentiates (changes in composition as high-temperature minerals crystallize and settle out of the magma) its density reaches a minimum at a magnesium number of about 60, similar to that of flood basalts. This restores buoyancy and permits the magma to complete its journey to the surface, and also explains why flood basalts are predominantly quartz tholeiites. Over half the original magma remains in the lower crust as [[cumulates]] in a system of dikes and sills.<ref name=Cox1980>{{cite journal |last1=Cox |first1=K. G. |title=A Model for Flood Basalt Vulcanism |journal=Journal of Petrology |date=1 November 1980 |volume=21 |issue=4 |pages=629–650 |doi=10.1093/petrology/21.4.629}}</ref>{{sfn|Philpotts|Ague|2009|p=383}}

As the magma rises, the drop in pressure also lowers the [[liquidus]], the temperature at which the magma is fully liquid. This likely explains the lack of phenocrysts in erupted flood basalt. The ''resorption'' (dissolution back into the melt) of a mixture of solid olivine, augite, and plagioclase—the high-temperature minerals likely to form as phenocrysts—may also tend to drive the composition closer to quartz tholeiite and help maintain buoyancy.{{sfn|Philpotts|Ague|2009|p=382}}{{sfn|Philpotts|Ague|2009|p=383}}