Zealandia is largely made up of two nearly parallel ridges, separated by a failed rift, where the rift breakup of the continent stops and becomes a filled graben. The ridges rise above the sea floor to heights of , with a few rocky islands rising above sea level. The ridges are continental rock, but are lower in elevation than normal continents because their crust is thinner than usual, approximately thick, and consequently, they do not float so high above Earth's mantle as that of most landmasses. About 25 million years ago, the southern part of Zealandia (on the Pacific Plate) began to shift relative to the northern part (on the Indo-Australian Plate). The resulting displacement by approximately along the Alpine Fault is evident in geological maps. Movement along this plate boundary also has offset the New Caledonia Basin from its previous continuation through the Bounty Trough. Compression across the boundary has uplifted the Southern Alps, although due to rapid erosion their height reflects only a small fraction of the uplift. Farther north, subduction of the Pacific Plate has led to extensive volcanism, including the Coromandel and Taupo Volcanic Zones. Associated rifting and subsidence has produced the Hauraki Graben and more recently, the Whakatane Graben and Wanganui Basin. Volcanism Volcanism on Zealandia has taken place repeatedly in various parts of the continental fragment before, during, and after it rifted away from the supercontinent Gondwana. Although Zealandia has shifted approximately to the northwest with respect to the underlying mantle from the time when it rifted from Antarctica, recurring intracontinental volcanism exhibits magma composition similar to that of volcanoes in previously adjacent parts of Antarctica and Australia. Large volume magmatism occurred in two periods, being in the Devonian (370 to 368 million years ago) and the Early Cretaceous (129 to 105 million years ago). This volcanism is widespread across Zealandia, but on present land generally it is of low volume apart from the huge mid to late Miocene shield volcanoes that developed the Banks and Otago Peninsulas. In addition, it took place continually in numerous limited regions all through the Late Cretaceous and the Cenozoic. Some of its causes remain in dispute perhaps because of data gaps. During the Miocene, the northern section of Zealandia (Lord Howe Rise) might have slid over a stationary hotspot, forming the Lord Howe Seamount Chain. It has been suggested that Zealandia may have played an important part