Most sand on the world's continental shelves was dumped there during the last ice age
The shallow seabed fringing the continents is really drowned land. Around 60 to 70 percent of its sediment was laid down during the last ice age, when sea level stood 100 to 120 metres lower. Even the depth where the shelf suddenly drops away appears to be an ice-age fingerprint.
A continental shelf is part of a continent submerged beneath a shallow shelf sea; around islands it is called an insular shelf. Much of it lay exposed during glacial periods. Seaward, the shelf ends at the shelf break, remarkably often at about 140 metres deep, likely marking where the shoreline sat when ice sheets held more water. Beyond it the continental slope plunges at an average of 3 degrees, sometimes 10, often cut by submarine canyons whose origin was not well understood until the 1960s. Below lies the continental rise, a gentler apron of sediment that can reach 500 km out before merging into the abyssal plain.
Shelves cover about 27 million square kilometres, roughly 7 percent of the ocean surface. They average about 80 km wide and are generally shallower than 100 metres, sloping only about half a degree. Width varies enormously. The Siberian Shelf in the Arctic stretches 1,500 km, and the Sunda Shelf beneath the South China Sea links Borneo, Sumatra and Java to mainland Asia. Off Chile or western Sumatra, where an ocean plate dives beneath the continent, there is almost no shelf at all.
That contrast reflects plate tectonics. Passive margins, like most Atlantic coasts, build wide shallow shelves from thick wedges of sediment eroded off the neighbouring land. Active margins shake with frequent earthquakes that send sediment tumbling into deep water, leaving shelves narrow and steep. Sediment tends to grade from sand near shore, where waves stir the bottom, to silt and clay in calmer water offshore, piling up 15 to 40 centimetres every thousand years, far faster than in the deep ocean.
The waters above are unusually lively. Tides, wind and fresh water from rivers mix the shallow shelf seas, and although they cover only about 8 percent of the ocean surface, they account for 15 to 20 percent of global primary productivity. The North Sea and the Persian Gulf both sit on continental shelves.
Source: Continental shelf
