ANYER, Indonesia—The volcano that unleashed a tsunami and killed more than 400 people has collapsed to about a third of its previous height and is ejecting superheated magma into the sea, magnifying the power of the eruptions and sending ash more than a mile into the air.

The volcano, called Anak Krakatau, partially collapsed in a violent spasm on Dec. 22, causing a massive landslide that sent high waves barreling toward both shores of the Sunda Strait, the narrow passage between the islands of Java and Sumatra. The volcano’s...