A summary of : Melson, W.G. and Saenz, R. 1973 Volume, Energy and Cyclicity of Eruptions of Arenal Volcano, Costa Rica, Bull. Volcanol. 37, 3, 416-437. Description of six phases of eruption at this one of eight volcanoes on the continental divide. Avalanches of hot fallen explosively ejected material accelerated under the influence of gravity more rapidly because of thermal expansion of trapped air and caused many deaths. About 0.03±.02 km³ of tephra emitted in 3 days. New measurements show lower crater to be larger than estimates from Hedervari equations. Estimated energy less by 10 than 1815 eruption of Tambora and less by nearly two orders of magnitude than Santorini 1400 BC. A qualitative model of events leading to emission of new lava is presented.