A map is presented of the distribution of volcanic features in the lunar maria between longitudes 50°E. and 100°W, and latitudes 50°N. and S. Flood basaltic flows predominate, and where individual flow units can be identified they may be hundreds of kilometres long. Sinuous rilles are considered to be tubes and channels associated with flood volcanism and to indicate directions and lengths of flows. The large areal extent of the flood basalts may result from an exeedingly high eruption rate of low viscosity lava. Mare ridges are thought to be generally shallow in origin and result from magma movement in extensive shallow sills. Other types of volcanism include caldera collapse, shield volcanoes and viscous extrusions together with pyroclastic cones.

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