The New Hebrides islands overlie an eastward-dipping Benioff zone and are bordered by a submarine trench. They consist of volcanic and volcaniclastic rocks, intrusions, mudstones and limestones. Eastern and Western Belts of islands containing Neogene rocks are bisected by a Central Chain of active and recently extinct volcanoes.
In the Western Belt intense early Miocene basaltic and andesitic volcanism resulted in a thick succession consisting largely of quartz-free marine volcaniclastic rocks; intrusion of hornblende andesites and diorites preceded middle Miocene block faulting and uplift. Late Miocene turbidites derived from this succession and rare tufts were faulted and eroded prior to a Pliocene transgression.
In the Eastern Belt undated basalt and andesite lavas were faulted and intruded by gabbros and norites before the middle Miocene. Emplacement of serpentinites containing amphibolite rafts pre-dated accumulation of pillow basalts and breccias of late Miocene to early Pliocene age.
Most islands of the Central Chain consist of late Pliocene to Recent subaerially-erupted basaltic lavas; some islands in the south contain similar basalts and andesites overlying thick Pliocene successions of pillow lavas and volcaniclastic rocks. Massive reef limestones deposited in all three Belts during the late Pliocene and Quaternary now form tilted plateaux and terraces.
The Western Belt was the site of an early Miocene volcanic arc; a parallel submarine trench may have existed either near the Eastern Belt, or to the west of the Western Belt. In many other island arcs volcanic episodes were contemporaneous with those in the Western Belt and Central Chain of the New Hebrides.