Mixing of one magma body with another of different composition. Note the relationship between the igneous rock names and Bowen's discontinuous and continous reaction series. Welded tuff - tuff where ash particles have been fused by heat. Volcanic breccia - composed of large volcanic fragments. If near the melting point, then rock will melt as the pressure drops, such as rock moving upward in the mantle.
Volatiles also lower the melting point.
Melting occurs when the geothermal gradient exceeds the melting point at a given depth. Basalts probably originate from the partial melting of peridotite in the upper mantle while granites probably originate from the melting of continental rocks. Andesites may come from basaltic magmas that assimilate crustal rocks.