Greenland is part of the North American continent, although it is a territory of Denmark.
Iceland is geographically part of Europe so in crossing the relatively narrow Denmark straits between Iceland and Greenland, you have technically crossed the Atlantic.
Iceland is geogically split in half. The continental plates join in the west of the country - there is a trench west of Reykjavik which, if you stand on one side you are on the American continental plate but if you jump to the other side of the ditch you stand on the European/Asian plate. The plates are drifting apart at roughly 1ft per decade, so straddling the ditch is not recommended for long periods !