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Oceans are big wet blue things that cover quite a lot of the earth. They are very salty, extremely polluted and contain many delicious edibles.
Oceans formed over the course of millions of years after the shiny new Earth cooled enough for water to settle on its surface and fill up the holes. As the crust drifted about, continents and supercontinents formed and moved around; the spaces in between these land masses would be individual seas. Pangea was the most recently formed of these supercontinents and split 250-150 million years ago, dividing the large Tethys ocean into several smaller oceans. The Atlantic Ocean is the youngest of these; as measured by the record of changes in the magnetic field in the rocks below it — it is thought to date back less than a hundred million years.
In historic times, the oceans posed an insurmountable barrier to most cultures. During the Middle Ages, better shipbuilding techniques finally enabled regular, safe crossings. The result of this was that five or six small countries on the northeastern Atlantic coast were able to carve up most of the world into a few massive empires, the consequences of which are still readily apparent today.
According to some Young-Earthers, the amount of salt in the ocean suggests they formed 62 million years ago. The young Earthers make their argument based on a starting point of zero salt, which isn't true — the oceans actually used to be saltier than they are today. This is due to the fact that a large amount of salt is deposited on the shore when seawater evaporates outside of the ocean — the first great lowering of salinity occurred in the late Precambrian, when huge amounts of salt were deposited in Neoproterozoic evaporite basins. These deposits result in a mineral known as "evaporite".
In other evidence for the extreme age of the oceans, not long after they formed, iron-manganese nodules started forming, incredibly slowly — see here for more information.
The greatest variety of life occurs in coral reefs, but the vast open ocean has the most productivity.
There are plenty of fish in the sea - the oceans contain more than (insert really big number here) species. They also teem with plant life and invertebrate animals, and many species of aquatic mammal, including dolphins and whales. Most of the really weird stuff, however, is in the deep sea trenches. The deepest recorded sea trench is the Mariana Trench which is thought to be roughly just under 11,000 m, or about 36,000 ft.
The oceans have also provided food for seagoing communities (a resource currently being depleted faster than it can be replaced), and are also used as a huge rubbish dump. Both of these may well have a permanent effect on sea life — it is possible that jellyfish may become the sea's dominant life form after all the fish have been eaten or choke on plastic bags.
Whales are probably the most famous example of overfishing (overmammaling?). The result is this:
* The Atlantic population of gray whales went extinct in the late-17th century. It is not listed as a part of IUCN's red list.
Despite the mess whaling made of the marine mammal population, people still do it (even though commercial whaling is banned by international law). As of 1987, Japan has been whaling for "scientific purposes", which apparently include distributing the meat of what may be the second most intelligent species on Earth to school children for lunch, and (one can safely assume) resurrecting commercial whaling.
- This is the duration of various phases of breaking up, not a ridiculous uncertainty. After all, drift is slow, you don't wake up to find the Earth completely resurfaced overnight!
- A young Earth indeed.
- Weird stuff. From Greenpeace. It's just a video, so just open it.
- Invasion of the jellyfish
- Eschrichtius robustus (Grey whale) Geographic Range IUCN Red list of threatened species