The OJP covers an area of ~2.0 x 106 km2 (larger than Alaska and comparable in size with Western Europe), and OJP-related volcanism extends over a considerably larger area into the adjacent Nauru, East Mariana, and, possibly, Pigafetta and Lyra basins (Fig. F1). With a maximum thickness of crust beneath the plateau of 30–35 km (e.g., Gladczenko et al., 1997; Richardson et al., 2000), the volume of igneous rock forming the plateau and filling the adjacent basins could be as high as 6 x 107 km3 (e.g., Coffin and Eldholm, 1994).
Seismic tomography experiments show a rheologically strong but seismically slow upper mantle root extending to ~300 km depth beneath the OJP (e.g., Richardson et al., 2000; Klosko et al., 2001). Gomer and Okal (2003) measured the shear wave attenuation in this root and found it to be low, implying that the slow seismic velocities must be due to a compositional, rather than thermal, anomaly in the mantle. The nature and origin of this compositional anomaly have not yet been established.
The OJP seems to have formed rapidly at ~120 Ma (e.g., Mahoney et al., 1993; Tejada et al., 1996, 2002; Chambers et al., 2002; Parkinson et al., 2002), and the peak magma production rate may have exceeded that of the entire global mid-ocean-ridge system at the time (e.g., Tarduno et al., 1991; Mahoney et al., 1993; Coffin and Eldholm, 1994). Degassing from massive eruptions during the formation of the OJP could have increased the CO2 concentration in the atmosphere and oceans (Larson and Erba, 1999) and led or at least contributed significantly to a worldwide oceanic anoxic event accompanied by a 90% reduction in nannofossil palaeoflux (Erba and Tremolada, 2004).
Collision of the OJP with the old Solomon arc resulted in uplift of the OJP's southern margin to create on-land exposures of basaltic basement in the Solomon Islands (Fig. F1), notably in Malaita, Santa Isabel, and San Cristobal (e.g., Petterson et al., 1999). In addition to these exposures, basaltic basement on the OJP and surrounding Nauru and East Mariana basins has been sampled at 10 DSDP and ODP drill sites (Fig. F2). Leg 192, however, was the first one designed specifically to address the origin and evolution of the OJP (Mahoney, Fitton, Wallace, et al., 2001). Earlier research on the OJP was reviewed by Neal et al. (1997). The principal results of Leg 192 are presented in a Special Publication of the Geological Society, London (Fitton et al., 2004). This publication complements the recent thematic set of papers on the origin and evolution of the Kerguelen Plateau, the world's second largest oceanic LIP, published in the Journal of Petrology (Wallace et al., 2002). The present synthesis summarizes the papers in the Special Publication, as well as results from Leg 192 that are published in the Leg 192 Scientific Results volume and elsewhere.