Figure 5: The melting column concept for volcanic arcs, where the amount of melting reflects the ascent height through the asthenosphere. Thin arc crust and lithosphere allow long melting columns and high degrees of melting in the mantle wedge. High degrees of melting, in turn, produce low Na basalts, as at mid-ocean ridges. Conversely, thick arc crust and lithosphere leads to short melting columns and less melting. This model could explain the global correlation between arc crustal thickness and the average Na content of arc basalts (Na6.0 is the Na2O concentration for basalts with 6% MgO). From Plank and Langmuir (1988).