I don't think earth elementals are able to
create earth, are they? So rather than earth elementals, wouldn't you want
fire elementals, to agitate the magma from below? Or perhaps both, to arrange an eruption most efficiently?
More on new islands:
Surtsey appeared off the coast of Iceland (in relatively shallow 425-foot-deep water) and took almost four years to reach a maximum size of one square mile. By 2002, wind and wave erosion had caused its area to dwindle to half that, though that harder remainder is expected to last until at least 2100.
Wikipedia has a list of
new islands -- most of them temporary, due to just such erosion.
One of the better options might be
Kavachi: it's already highly eruptive, and located between Papua New Guinea and the Solomon Islands, neither of which has much renown for enforcing territorial claims.
Other possibilities might include
Metis Shoal and
Home Reef, though both are decidedly within the Kingdom of Tonga, so there'd need to be some politicking and horse-trading by any prospective "owners".