Ita Mai Tai is a large, uncompensated seamount on the eastern edge of the East Mariana Basin. A large positive gravity anomaly of 254 mgal cbaracterizes the summit and a low of -69 mgal, the surrounding moat. Using polygonal prisms to approximate the bathymetry, the observed gravity was inverted to calculate an average density of 2.59 g/cm³ for the seamount. Other geologic data and seismic reflection profiles from DSDP Legs 20 and 89 provided further modeling constraints such as the density of the volcanic conduit and the density of sediment bodies. The drill sites describe a volcanic edifice formed in the Aptian/Albian on Jurassic/Cretaceous crust. The volcanism is recorded in volcanoclastic and epiclastic deposits in the basins nearby. The guyot was covered initially by a succession of reefal and lagoonal sediments followed by a thick mantling of pelagic sediments after it subsided.

Seismic reflection records indicate two different depths to basement on either side of Ita Mai Tai which may be explained by eruptions along or near an unrecognized fossil plate boundary. Gravity models that adequately match the calculated and observed data sets for Ita Mai Tai show little crustal thickening, suggesting that Ita Mai Tai is almost completely uncompensated isostatically. A study of ninety-six other western Pacific seamounts including Sio Guyot, a completely compensated seamount in the Mid-Pacific Mountains, show a linear inverse relationship between the maximum free-air anomaly and the minimum depths to the tops of seamounts, approximated by g_z= (3800m - z) /16.0. Ita Mai Tai and Sio Guyot are seen as end members with their contrasting degree of compensation. To determine whether the observed anomalies could be explained by crustal "roots", theoretical models at 0 and 100 percent local compensation were derived for all western Pacific seamounts studied. Most of the observed gravity anomalies for seamounts in this study lie in the field between the theoretical lines of 0 and 100 percent compensation. Twenty-two appear to be more than 100 percent compensated.

A map of isostatic compensation shows a distinct break between the eastern and western Marshall Islands, perhaps indicating different ages of seamount formation. There is as much as a 200 mgal difference in the gravity field observed for well-surveyed seamounts and islands of the same size in the western Pacific. The sea floor age at the time of seamount formation seems to vary inversely with the relative amount of compensation. The over-compensated seamounts my have been erupted near Cretaceous ridge crests rather than in a mid-plate event an has been previously thought.