Because New Zealand’s marine and terrestrial environments are so variable and dynamic, sediments off the coast are also diverse. The land mass itself produces almost 1% of all the sand and mud entering the world’s oceans. This outflow is caused by rapid uplift of the mountain ranges, frequent earthquakes, high rates of erosion, and an abundance of soft rocks that are easily eroded.
The continental shelf around New Zealand is covered mainly with sediment from the land, except at the northern and southern extremities. Here, a lack of major rivers means that there is no way for material to be washed to the sea, and shelly sediment from once-living sea creatures prevails.
In Cook and Foveaux straits, powerful tides and waves sweep away much of the mud, leaving gravel, coarse sand and shells. In contrast, the sea floor off the eastern North Island is swamped with mud because currents are relatively weak and the supply of sediment from land is the largest in New Zealand.
Off the western North Island, black, iron-rich sand has been formed by wave action on volcanic rock.
Canyons, channels and plateaux
The continental shelf off Otago, Cook Strait, North Canterbury and elsewhere is dissected by submarine canyons. These steep-sided gorges siphon off sediment and guide the turbid flows of sand and mud (called turbidity currents) into deeper water.
Off Cook Strait, submarine canyons merge into the deep Hikurangi Channel. This guides rapidly flowing mud and sand 2,000 kilometres across the 3-kilometre-deep Hikurangi Plateau, then onto the deep Pacific Ocean floor. These currents overflow the channel spreading sandy mud over everything they pass.
In contrast, south of New Zealand, the Campbell Plateau is isolated from sediment coming from land. Sediment there is dominated by calcium carbonate, sand and mud.