During the Capricorn Expedition of 1952-1953 the research vessels H orizon and S pencer F. B aird spent two weeks in, the Tonga Trench and Archipelago making echo-sounding, coring, seismic-refraction and towed-magnetometer studies. The Tonga Trench is a slightly arcuate furrow convex to the east over most of its length. Between Tafahi and Samoa it bends sharply to the northwest, appearing to follow the andesite line. It is continuous at depths greater than 7000 m for 1100 km, including in this segment three elongate basins deeper than 9000 m and one deeper than 10,000 m. The deepest basin explored lies at about 23° 15′ S., near a sharp bend in the axial trend, where bomb sounding gave a depth of 10,800 ± 200 m. A typical west-to-east profile, 350 km long, across the central Tonga Islands, crosses a chain of active volcanic islands at Kao, crosses the narrow Tofua Trough of 1700 m depth to the coralline island Lifuka, then down the west flank of the trench to a maximum depth of 9000 m in a steep-walled gorge 1–5 km wide. The east flank rises more gently to a broad ridge rising about 350 m above the general level of the sea bottom eastward. At 26° 00′ S. Lat. and at 18° 45′ S. Lat. the trench is constricted by large seamounts on the east flank. The latter seamount caps a smooth rise from 7500 m to 410 m and appears to be a guyot tilted westward. Scattered bottom samples indicate that, except for the calcareous deposits found on the main Tongan ridge, the sea floor in the area is largely volcanic sand and gravel, probably ejected from the near-by active volcanoes. Seismic-refraction profiles, parallel to the axis of the trench, indicate that the sediment foundation throughout the area is a rock, probably volcanic, of compressional wave velocity 5.2 ± 0.2 km/sec. The sediment thickness is about 2 km in the Tofua Trough, very thin, <200 m on the average, in the inner gorge of the trench, and about 400 m on the east flank, 50 km from the trench axis. Beneath the 5.2 km/sec basement the crustal velocity is 7 km/sec in Tofua Trough, 6½ km/sec at the trench axis and also on the east flank. The Mohorovičić discontinuity, characterized by a velocity of 8.1 ± 0.1 km/sec, is estimated to be at a depth of 20 km below sea level at the trench axis, and 12 km below sea level on the east flank. This velocity was not observed beneath Tofua Trough, where the highest velocity was 7.6 km/sec, estimated to occur at a depth of 12 km below sea level. Two profiles of total magnetic field strength, recorded across the trench, have been used to estimate profiles of sediment thickness. When the effects of basement topography are eliminated, a deep-seated asymmetry remains that could indicate either that the Curie point isothermal surface is deeper or that the magnetic susceptibility is greater on the west side than on the east side of the trench.

Reconstruction of the tectonic evolution of the Pacific basin indicates a direct relationship between intraplate volcanism and plate reorganizations, which suggests that volcanism was controlled by fracturing and extension of the lithosphere. Middle Jurassic to Early Cretaceous intraplate volcanism included oceanic plateau formation at triple junctions (Shatsky Rise, the western Mid-Pacific Mountains) and a diffuse pattern of ocean island volcanism (Marcus Wake, Magellan seamounts) reflecting an absence of any well-defined stress field within the Pacific plate. The stress field changed in the Early Cretaceous when accretion of the Insular terrane to the North American Cordillera and the Median Tectonic arc to New Zealand stalled migration of the Pacific-Farallon and Pacific-Phoenix ocean ridges, leading to the generation of the Ontong Java, Manahiki, Hikurangi, and Hess Rise oceanic plateaus. Plate reorganizations in the Late Cretaceous resulted from the breakup of the Phoenix and Izanagi plates through collision of the Pacific-Phoenix ocean ridge with the southwest margin of the basin and development of island arc–marginal basin systems in the northwestern part of the basin. The Pacific plate nonetheless remained largely bounded by spreading centers, and intraplate volcanism followed preexisting lines of weakness in the plate fabric (Line Islands) or resulted from fractures generated by ocean ridge subduction beneath island arc systems (Emperor chain). The Pacific plate began to subduct under Asia in the Early Eocene as inferred from the record of accreted material along the Japanese margin. Further changes to the stress field at this time resulted from abandonment of the Kula-Pacific and the North New Guinea (Phoenix)–Pacific ridges and from development of the Kamchatkan and Izu-Bonin-Mariana arcs, leading to the generation of the Hawaiian chain as a propagating fracture. The final major change in the stress field occurred in the Late Oligocene as a result of breakup of the Farallon into the Cocos and Nazca plates, which caused a hiatus in Hawaiian volcanism; initiated the Sala y Gomez, Foundation, and Samoan chains; and terminated the Louisville chain. The correlations with tectonic events are compatible with shallow-source models for the origin of intraplate volcanism and suggest that the three principal categories of volcanism, intraplate, arc, and ocean ridge, all arise from plate tectonic processes, unlike in plume models, where intraplate volcanism is superimposed on plate tectonics.