Oceans cover 70% of the Earth and they are the largest single animal habitat.
Scientists divide the ocean into two main environments – the pelagic (which is the water itself), and the benthic (which is the seabed).
Most benthic animals live in shallow waters around the continents. They include worms, clams, crabs and lobsters, as well as bottom-feeding fish.
Scientists call the sunny surface waters the euphotic zone. This extends down 150 m and it is where billions of plankton (microscopic animals and plants) live.
Green plant plankton (algae) in the oceans produce 30% of the world’s vegetable matter each year.
Animal plankton includes shrimps and jellyfish.
The surface waters are also home to squid, fish and mammals such as whales.
Below the surface zone, down to about 2,000 m, is the twilight bathyal zone. Here there is too little light for plants to grow, but many hunting fish and squid live.
Below 2,000 m is the dark abyssal zone, where only weird fish like gulper eels and anglerfish live (see strange sea creatures).
The Sargasso is a vast area in the western Atlantic where seaweed grows thick. It is a rich home for barnacles and other sea creatures.
Ocean surface currents are like giant rivers, often tens of kilometers wide, 100 m deep and flowing at 15 km/h.
The major currents are split on either side of the Equator into giant rings called gyres.
In the Northern Hemisphere the gyres flow round clockwise; in the south they flow anticlockwise.
Ocean currents are driven by a combination of winds and the Earth’s rotation.
Near the Equator water is driven by easterly winds (see wind) to make westward-flowing equatorial currents.
When equatorial currents reach continents, the Earth’s rotation deflects them polewards as warm currents.
As warm currents flow polewards, westerly winds drive them east back across the oceans. When the currents reach the far side, they begin to flow towards the Equator along the west coasts of continents as cool currents.
The North Atlantic Drift brings so much warm water from the Caribbean to SW England that it is warm enough to grow palm trees, yet it is as far north as Newfoundland.
By drying out the air cool currents can create deserts, such as California’s Baja and Chile’s Atacama deserts.
The West Wind Drift around Antarctica moves 2000 times as much water as the Amazon.
The oceans are over 2000 m deep on average.
Along the edge of the ocean is a ledge of land – the continental shelf. The average sea depth here is 130 m.
At the edge of the continental shelf the sea-bed plunges thousands of meters steeply down the continental slope.
Underwater avalanches roar down the continental slope at over 60 km/h. They carve out deep gashes called submarine canyons.
The gently sloping foot of the continental slope is called the continental rise.
Beyond the continental rise the ocean floor stretches out in a vast plain called the abyssal plain. It lies as deep as 5000 m below the water’s surface.
The abyssal plain is covered in a thick slime called ooze. It is made partly from volcanic ash and meteor dust and partly from the remains of sea creatures. The abyssal plain is dotted with huge mountains, thousands of meters high, called seamounts. Flat-topped seamounts are called guyots. They may be volcanoes that once projected above the surface. The deepest places in the ocean floor are ocean trenches – made when tectonic plates are driven down into the mantle. The Mariana Trench is 10,863 m deep.
Huge numbers of sea creatures live in the pelagic zone – the surface waters of the open ocean beyond the continental shelf. Underwater volcano Deep-sea trench Ocean ridge Oceanic.