Dr R. W. Girdler writes: In their fascinating paper on the volcanic islands of the southern Red Sea, Gass et al. 1973 give the impression that I consider that only the ‘central median trough’ of the Red Sea is underlain by oceanic crust. This is not so; evidence has been accumulating for a considerable time indicating that much more than the axial trough is oceanic (e.g. Girdler 1966, 1970; Girdler & Darracott 1972).

At the time of my 1958 paper (to which they refer) we had no idea that the thickness of evaporites forming the shelves of the Red Sea would be so enormous. In the 1960s, exploration boreholes showed thicknesses of salt (specific gravity 2.1) of the order of 4 km! It is clearly necessary to make corrections for these in the gravity interpretations and the positive Bouguer anomaly over the centre of the Red Sea then becomes much wider and hence the extent of the ‘intrusive zone’ (oceanic crust) is found to be much wider.

Gass et al. also refer to the existence of large magnetic anomalies over the median trough and their ‘virtual absence elsewhere’. Unfortunately, there are few ships’ tracks over the shelves due to navigational and other difficulties but it is important to realise that absence of data does not imply absence of anomalies. There are aeromagnetic surveys which reveal the existence of anomalies over the shelves (Girdler 1970). For example, an aeromagnetic survey by the Gulf Oil Company showed the presence of sea floor

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