The observatory said Tuesday the falling rocks made sharp popping sounds that could be heard by visitors at the Jaggar Museum Overlook at Hawaii Volcanoes National Park.
The first section collapsed into the lava lake at 8:42 a.m. Monday. Moments later the crater ejected gas, ash and a dusty brown plume.
The second section - estimated to be nearly 400 feet long and up to 16 feet wide - collapsed several hours later. The lava lake's level began to drop shortly afterward.
Kilauea is the world's longest continually erupting volcano. The east rift zone began erupting in 1983.