Story: Sea floor geology
Page 6 – How sediment forms
Almost the entire ocean floor is covered with different types of sediment: mud, sand and gravel. This is formed of material from land, the skeletons of plankton and seabed-dwelling animals, chemical reactions, and air-borne dust.
Sediment from land
Material from land makes up 75% of seabed sediment. Most of this is carried to the sea by rivers and dumped on the nearby continental shelf and slope. Some is carried further offshore by wind and ocean currents. Sediment is also deposited through flows of mud and sand (turbidity currents) and submarine landslides, such as those that occur off Kaikōura.
Dead plankton and sea life
In the deep ocean, sediment from land makes up only 20% of the seabed cover. Most is made up of minute organisms or plankton that have died in surface waters and sunk. In the oceans there is a continuous rain of these tiny creatures, which include diatoms, coccoliths, radiolarians and foraminifers.
Debris from once-living sea creatures settles on the continental shelf, especially where the supply of sediment entering the sea from land is low. These deposits commonly contain shells, bryozoans, algae and tube worms. Such deposits are currently forming extensively on the Chatham Rise, Campbell Plateau and Lord Howe Rise, and will one day be limestone.
In the deep ocean basins beyond the New Zealand continent, at depths of 4–5 kilometres, red and brown clays dominate the sea floor. They contain fine particles carried from land by wind and ocean currents, ash from volcanic eruptions, and particles from space. At these depths material from once-living sea creatures is sparse because of the dissolving power of very cold water.
Minerals and chemical processes
Sediment may form from new mineral growth around submarine volcanoes and hot-water vents, or from chemical processes in the sea water. Such sediments comprise less than 1% of the seabed, but can give rise to locally unique and important habitats. The green mineral glauconite, for example, occurs where there is little sediment entering the sea from the land. This occurs on Chatham Rise, where there is enough of it to form large areas of greensands. Chatham Rise also hosts particles so rich in phosphate that they have economic potential. In the deep ocean, fist-sized nodules of manganese and iron form beneath currents flowing along the base of the Campbell Plateau.
Where there are strong currents and waves, such as in Cook Strait, sediment is easily moved. Boulders and pebbles roll and tumble, gradually becoming smooth and round. Moving water also washes out fine material, leaving only grains of similar size. As gravel and sand have large grains, these often accumulate in beach and continental shelf environments.
Ocean currents and tides, especially when intensified by storms, can bring about marked changes in the sea floor. In contrast, the protected waters of harbours, estuaries and fiords encourage silt and clay to settle.
As water depth increases further from the New Zealand coast, the effects of waves and currents decrease. Gravel and sand beaches give way to finer-grained deposits, and at depths below about 30 metres the seabed is often covered with mud.