Through thick and thin

摘要

The sea floor around the Hawaiian island chain is unusually shallow. New seismic evidence suggests that this up-raised 'swell' is partly due to heating and thinning of the lithosphere beneath. Earth scientists revere the Hawaiian islands as the inspiration for some of the most fundamental concepts of their discipline. More than 40 years ago, J. T. Wilson suggested that the linear increase in the age of the islands along the chain reflected the steady motion of the Pacific plate over a source of heat, or hotspot, fixed in the mantle beneath. W. Jason Morgan subsequently proposed that hotspots were in fact the tops of mantle plumes - rising currents of hot buoyant material originating deep in the mantle. Meanwhile, shipboard surveys of the Pacific ocean floor had mapped out the Hawaiian 'swell', a large area of anomalously shallow sea floor that surrounds the Hawaiian chain (Fig. 1). What created the Hawaiian swell? The fact that it doesn't simply collapse under its own weight indicates that there must be an upward force that counteracts the pull of gravity. On page 827 of this issue, Xueqing Li et al. weigh in with new data and fresh insight on the origin of the swell-supporting force.