The 1963–65 eruption of Irazú, like all others of this volcano during the historic period, produced only ash and other fragmental ejecta without lava. The initial outbreak on March 13, 1963 started with a series of great explosions that hurled out much ash, blocks, and bombs, but the activity soon settled down to alternating periods of explosive cruptions and quiet emission of steam.

Ash was deposited mostly along a zone that extended westward from the summit to and beyond the city of San Jose, 24 km away. The prolonged ashfall severely damaged dairy, vegetable, and coffee farms, and for a while made daily life in the affected cities extremely difficult. Accelerated runoff of rainwater from the ash-covered slopes of the volcano caused destructive floods, mudflows, and landslides.

The climax of the cruption probably occurred during December 1963 and January 1964, when ash and incandescent scoria were erupted voluminously and the magma rose to within 100 meters of the lip of the vent. Precise levelling along the highway to the summit in May 1964 by the Geographic Institute revealed the upper part of the volcano upheaved as much as 11 cm above levels determined in 1949. A repetition of the levelling in September 1964 showed a subsidence to approximately the 1949 configuration, indicating a distinct reduction of pressure in the magma chamber.

Substantial amounts of pulverized wallrock were present in the ash along with fragments of scoria and pumice. Progressive caving of the vent walls, which enlarged the diameter of the vent from 200 meters to 525 meters, kept dropping wallrock down onto the exploding magma, and at times stopped the eruption for a day or two by plugging the vent. The scoriaceous and pumiceous bombs were porphyritic two-pyroxene olivine basaltic andesite, and their composition remained remarkably constant throughout the eruption.

The ash section was about 2 meters thick, 800 meters downwind from the vent in June 1964. In the section, deposits of the rainy season could be distinguished by their well developed stratification from those of the dry season. A zone containing three persistent pumice horizons represents the climactic period of December 1963 to January 1964. The cloudburst of December 10, 1963 is recorded by a highly rilled surface, and the strong winds of the dry season of 1964 are indicated by a rippled lag deposit.