The arrival and release of weight impacts the stress of the entire region, potentially triggering earthquakes and volcanoes. Before fracking and injection wells made a mess of the continental interior, the biggest causes of intraplate earthquakes far from plate tectonic boundaries were attributed to the shifting stresses of isostatic rebound. These impacts can be far-reaching in both space and time: despite being ice-free, the infamous 1811 New Madrid earthquake in the American south may have been induced by intraplate stresses induced from the last ice age.
The same thing is happening for volcanoes. A key trigger of eruptions is changing in the subsurface pressure and stress adjustments in the magma chamber. As the lithosphere flexes and recovers, this redistribution can be enough to fuel a surge in volcanic activity. Right now, the released pressure in Iceland could be fuelling a surge in volcanism, magma chambers long kept confined expanding and pushing out into surface eruptions from the flight-disrupting Eyjafjallajökull to the ongoing slow, steady trickle of Bárðarbunga.