Re: Tectonic activity?
'could the appearance and subsequent non-showing of this 'island' be somehow related to the local tectonics? The surrounding area is part of the 'ring of fire' for a reason so maybe the whole thing was formed, and destroyed by natural processes causing sand to rise and subsequently subside?'
Although the Coral Sea is adjacent to the Pacific 'Ring of Fire' it is a distinct geological body.
The sea appears to have been formed by crustal extension and subsidence of Eastern Australia during the fragmentation of an earlier continent (Zealandia). New Caledonia, (which is in the general region of where Sandy Island isn't) is another part of Zealandia and is depressingly geologically inert. Its north and eastern fringes are marked by the San Cristobal and Vanuatu trenches so volcanic and earthquake activity is concentrated in a sharp band along the Solomon Islands and Vanuatu.
There is a spreading centre in the middle of the sea but it is now extinct and my bumper map of all the world's geological wobblings shows no atolls or hot spots in that part of the ocean.
So IMHO, tectonism is out.