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Shaping the Ocean Floor

The continental shelf is the relatively shallow area (down to 200 metres below current sea level) around the coast of New Zealand.

Continental Shelf. Image: GNS Science.

Image: GNS Science.

Deposition and erosion
Debris carried out to sea by rivers, flows onto the continental shelf and beyond to the continental slope. Thick deposits of unstable sediment can build up over tens or hundreds of years.

These get released by earthquake shaking, sending huge, dense debris flows down canyons and channels far across the ocean floor to distances of up to 2000 km from the source area.

This computer generated image of the continental margin near to East Cape shows an enormous rock and mudslide that occurred about 170 000 years ago. It is over 60 km across and involved the collapse of about 3500 cubic kilometres of material! This would have caused a massive tsunami to radiate across the Pacific Ocean.

This computer generated image of the continental margin near to East Cape shows an enormous rock and mudslide that occurred about 170 000 years ago. It is over 60 km across and involved the collapse of about 3500 cubic kilometres of material! This would have caused a massive tsunami to radiate across the Pacific Ocean.

Ocean currents can also transport large volumes of sediments from one place to another.

Ice ages and sea level change
River sediments remain on the continental shelf when the sea level is relatively high (like now) following the melting of ice caps after an ice age.

However, during glacial periods, with sea level roughly 130 metres lower, the shallower parts of the continental shelf were cold, wind-swept coastal plains. The rivers carved valleys across them and deposited their sediments directly onto the continental slopes. Dense flows of sediment rich water descended down submarine canyons on these slopes, or settled out to form unstable deposits on them that would later break loose, often triggered by earthquakes, to move rapidly down to the deep-ocean floor.

Most of the sea floor channels are less active now, when sea levels are high. However, an exception is the Hikurangi Channel, a huge submarine valley that marks the actual collision boundary of the Australian and Pacific plates.