But despite its undersea status, Zealandia is not made of magnesium- and iron-rich oceanic crust. Instead, it is composed of less-dense continental crust.

…Sutherland and his colleagues now suspect that the changes in Zealandia at this time were part of a larger disturbance that quickly led to the formation of Ring of Fire subduction zones around the western Pacific.

“We don’t know where or why,” Sutherland said in the statement, “but something happened that locally induced movement, and when the fault started to slip, like in an earthquake, the motion rapidly spread sideways onto adjacent parts of the fault system and then around the western Pacific.” 

This process would have taken over a million years, but would have represented a dramatic rearrangement of the geology of the western Pacific.

The lost continent of Zealandia hides clues to the Ring of Fire’s birth | Live Science

hmmm