ABSTRACT We propose a new, sunken continent beneath the North Atlantic Ocean that we name Icelandia. It may comprise blocks of full-thickness continental lithosphere or extended, magma-inflated continental layers that form hybrid continental-oceanic lithosphere. It underlies the Greenland-Iceland-Faroe Ridge and the Jan Mayen microplate complex, covering an area of ~600,000 km 2 . It is contiguous with the Faroe Plateau and known parts of the submarine continental rifted margin offshore Britain. If these are included in a “Greater Icelandia,” the entire area is ~1,000,000 km 2 in size. The existence of Icelandia needs to be tested. Candidate approaches include magnetotelluric surveying in Iceland; ultralong, full-crust-penetrating reflection profiling along the length of the Greenland-Iceland-Faroe Ridge; dating zircons collected in Iceland; deep drilling; and reappraisal of the geology of Iceland. Some of these methods could be applied to other candidate sunken continents that are common in the oceans.

ABSTRACT The true partitioning between continental and oceanic lithosphere in oceans is unclear. According to early models, oceanic-type accretion generates pairs of linear magnetic anomalies, which are indicators of oceanic lithosphere and can be used as isochrons formed by seafloor spreading. However, seaward-dipping reflectors at conjugate volcanic passive margins also generate linear magnetic anomalies. The thick wedges of the inner seaward-dipping reflectors are associated with magnetic anomalies that are clearly distinct in shape and amplitude from those recorded in the distal oceanic realm. However, linear magnetic anomalies indistinguishable from those related to oceanic crust exist in the outer seaward-dipping reflector domain of many volcanic passive margins. Located seaward of the inner seaward-dipping reflectors, the crust of outer seaward-dipping reflectors is thus generally considered to be “oceanic.” However, the outer seaward-dipping reflector crust may be interpreted as tectonically exhumed mid-to-lower magma-intruded continental crust covered with syntectonic basalts. Although both oceanic crust and outer seaward-dipping reflector crust are associated with thick lava sections, the linear magnetic anomalies of outer seaward-dipping reflectors represent pre-oceanization magnetic anomalies that develop along extended continental lithosphere. We illustrate the consequence of these uncertainties on the type of lithosphere between Greenland and Europe. Here, depending on latitude, 20%–100% of the lithosphere previously thought to be oceanic might, on the contrary, be continental. Since more than 50% of passive margins worldwide are volcanic, poor mapping of seaward-dipping reflector–bearing crust types, and misinterpretation of linear magnetic anomaly–bearing distal volcanic passive-margin crust, could have led to widespread overestimation of the age of continental breakup and the extent of oceanic lithosphere in oceans.