At Glencoe caldera volcano, friction melts and magmas transformed explosively to froth or spray where they encountered rapid decompression in dilatant sections of superfaults. The friction melts were blasted upwards, plastering free surfaces, and were rapidly followed by fragmented magma and then liquid magma that formed intrusions. Irregular contacts of fault intrusions record explosive disruption of hydrothermal systems cut by the dilational faults, with lithic breccias removed from the rapidly formed cavities before the arrival of melt spray. Layers of lithic breccia are common in ignimbrites around calderas and usually show incorporation of hot and hydrothermally altered rocks. Such layers may specifically reflect initial superfault dilation during caldera collapse.