Paolo wrote: Thu Aug 09, 2018 1:36 pm There are only a few possibilities for that empty space left behind, which Moi mentions.
Something else fills it.
It becomes a stable empty space, a'la a cave.
It collapses in, with or without collapsing the surrounding and surface areas, which would in turn create other stable, empty spaces. A big enough collapse, and it could collapse the surface somewhere. However, as I understand it, these areas are usually far too deep for that.
The caldera at the summit of Kilauea where the active crater Halema'uma'u' was located has been collapsing with each of the near daily earthquakes.
Hawaii County Civil Defense administrator Talmadge Magno discussed it on here:
8/03/video-volcano-alternate-routes-planned-in-case-highway-11-fails/
The caldera floor has been deforming and that has been caught on camera several times.
Apparently, the magma chamber beneath the caldera has been collapsing.
The geologists discuss the possible next stage in this video:
8/07/video-eruption-lull-discussed-by-usgs-hawaiian-volcano-observatory/