 About five kilometres (3 mi) to the west of the Houtman Abrolhos lies the shelf break, beyond which the seabed falls away much more steeply, averaging around 50 metres per kilometre (260 ft/mi).The islands of each island group arise from a single carbonate platform, so the waters within an island group are mostly shallow.The channels between groups, however, are 40 to 50 metres deep, and therefore no impediment to the exchange of offshore and inshore waters.