The Manicouagan resurgent caldera is an octagonal depression 38 miles across, with a mountain of Precambrian rocks about 4 miles north of the centre. The moat between the mountain and the walls of the depression is partly filled with volcanic rocks of the Triassic Manicouagan complex. These rocks are composed of varying fractions of alkali olivine basalt mixed with country rocks. The Precambrian rocks beneath the Manicouagan complex, comprising Archean gabbro, anorthosite, mangerite, and charnockite, overlain by Proterozoic quartz feldspar gneisses containing quartzite- marble- iron-formation pods, have been erratically shock metamorphosed to assemblages containing one or more of the following features: vesicular glass, deformation lamellae, maskelynite, optically anomalous minerals, hornfelsic texture, and metasomatic alteration. Field and experimental evidence suggest that metamorphism occurred under conditions of very high temperature, moderate pressure, and extremely rapid oscillation of both temperature and pressure. These conditions are deduced to be connected with a hydrogen-rich character of the parent magma. Structural analysis of the area reveals that a long history of Precambrian deformation preceded formation of the Manicouagan structure. Development of this structure began with strong upwarping in the areas now occupied by Manicouagan and Mouchalagane lakes. Development continued with radial volcanic blasts followed by subsidence along pre-existing zones of weakness. Doreitic magma was poured out into the depression , and the central mountain rose on the back of intrusions at depth. The structural pattern of the igneous rock sheets agrees with geophysical evidence by indicating that Precambrian rocks beneath the Triassic igneous rocks are essentially unbrecciated. Hypotheses supposing the Manicouagan structure to result from or to be initiated by extraterrestrial impact, are considered and rejected as contrary to field evidence. The geometric form and history of development of the Manicouagan structure are similar to those of resurgent calderas. The structure is concluded to be an unusual variety of resurgent caldera.