Recent research into understanding the genesis of oceanic crusts shows that crust formation is not equally vigorous everywhere. In Iceland, where the mid-ocean ridge emerges above sea level, new crust is more than 15 km thick. At the other extreme, some ridges lie beneath 5,000 metres of water but have a crust less than four km thick. To understand the reasons for these variations, one must know the depth at which melting begins in the mid-ocean-ridge basalt (MORB) source regions, say researcher B Bourdon and colleagues of the Lamont-Doherty Earth Observatory, New York, US. Bourdon reports that uranium-series radioisotope data from MORB indicates that melting begins at greater depths beneath shallow ridges (which have thick crusts). He says that therefore, tempermure differences between MORB source regions am a primary cause of variations in oceanic crust formation. Seismic velocity data also suggests that earth's mantle temperatures me inversely proportional to ridge depth, and so there am several evidences linking thickness of crusts to temperature variation (Nature, Vol 384, No 6606).