One explanation for flood basalts is that they are caused by the combination of continental rifting and its associated melting , in conjunction with a mantle plume also undergoing decompression melting , producing vast quantities of a basaltic magma . One proposed explanation for flood basalts is that they are caused by the combination of continental rifting and its associated decompression melting , in conjunction with a mantle plume also undergoing decompression melting , producing vast quantities of a tholeiitic basaltic magma .