As the Australian plate sinks, it melts, and some of the magma thus produced erupts violently to the surface.

ECONOMIST: A capricious volcano, but fears ease of another Pompeii

Another piece to the puzzle: Both of these earthquakes occurred along opposite ends of the same tectonic plate, the Australian plate.

CNN: Two quakes in the same region in a day - coincidence?

Where the Pacific plate collides with the Indo-Australian plate, it is forced downwards into the trench, a subduction zone, and the volcanoes are carried with it.

BBC: Undersea mountains march into the abyss

The Australian plate borders the Pacific plate to the East (the Samoan quake occurred along this boundary), while on the Western side, it borders the Eurasian plate (where the Indonesia quake occurred).

CNN: Two quakes in the same region in a day - coincidence?