The 1940 eruption of Mauna Loa was confined to the summit crater, Mokuaweoweo, and the upper southwest rift. The rapid outpouring of the first day produced a lava lake, locally over 50 feet deep, over a large part of the crater. During cooling and solidifying the original surface settled from 0 feet where the flow was a foot or less deep, to 9 feet where the fill was 50 feet deep. Cooling accounts for but a small percentage of this volume decrease. The collapse of the crust that produced slump scarps along the crater walls was rapid enough to pull some of the plastic material on the under side of the crust into the form of stalactites. The intrusion of the lava under the crust of the initial flood raised pressure ridges and pressure plateaus.