Oceans
Oceans occupy about 70 per cent of the Earth’s surface. There
are five oceans in the world. They are the Atlantic, Pacific,
Indian, Arctic and Antarctic oceans. The surface under the
oceans is called the ocean floor. Like land, the ocean floor also
has natural features like plains, valleys and mountains.
Quick Q’s:
1. Which is the largest
of all oceans?
The Pacific Ocean is
the world’s largest and
deepest ocean. It has
an average depth of
over 4,000 metres
(13,100 feet). It has the
world’s deepest trench
– the Mariana Trench
near Japan. The
Challenger Deep in the
Mariana Trench is the
deepest point on Earth
– about 11,033 metres
(36,200 feet) deep.
2. What causes an
ocean current?
An ocean current is
a mass of water that
keeps moving in one
direction. Surface
currents are caused by
wind and the Earth’s
rotation. Underwater
currents are the result
of differences in
temperature and salt
content of the water.
3. What is a
black smoker?
When water seeps into
the crust of the ocean
floor through cracks, it
may be heated by the
magma below. As
pressure builds up
within the crust, the
hot water shoots up
through these cracks.
These jets of warm
water are often black
due to their mineral
content, so they are
called black smokers.
Q Which ocean is also known as the
Southern Ocean?
A The Antarctic Ocean is also called the
Southern Ocean. Until recently, the Antarctic
Ocean was considered to be a part of the
other main oceans, as it was actually formed
from parts of the Pacific, Atlantic and Indian
oceans. In the year 2000, however, it was
officially named the ‘Southern Ocean’.
Q What is the ocean floor like?
A The ocean floor is far from flat. The
edges of islands and continents gently slope
into the surrounding water to form an area
called a continental shelf that is higher than
rest of the ocean floor. A continental shelf
usually extends about 75 kilometres (47 miles)
out to sea but some, like the Siberian shelf
in the Arctic Ocean, can extend up to 1,500
kilometres (932 miles). The continental shelf
contains large deposits of petroleum, natural
gas and minerals. It also receives the most
sunlight, so marine life thrives here. The
point where the continental shelf starts to
plunge steeply towards the deep ocean floor
is called the continental slope. It is here that
the deep canyons of the ocean are found.
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Q What causes the formation of mountains
and valleys on the ocean floor?
A Like the rest of the Earth, the ocean
floor is divided into tectonic plates. The
movement of these plates is responsible for
features like ridges, trenches, and valleys.
Ridges are formed when two plates drift
apart. Boiling rock from inside the Earth,
called magma, oozes out through the cracks
between the plates and cools to form a ridge.
Trenches are formed when a heavier plate
sinks down under a lighter one.
Not a flat floor
The ocean floor has mountains and valleys. Some of the
mountains are tall enough to rise above the surface and
form islands.
Ocean currents
The water in the oceans and seas is always in motion,
due to the rotation of the Earth, the gravitational pull
of the Sun and Moon, and the difference in temperature
and salt content of the water. These movements form
strong currents in the oceans, both at the surface
and deep down.
Oceans of the world
The five oceans of the world cover over 70 per cent of
the surface of the Earth.
Pacific
Ocean
Atlantic
Ocean
Indian
Ocean
Antarctic Ocean
Arctic Ocean
Island Arc Active Volcano
Oceanic Trench
N. Equatorial
N. Equatorial
N. Equatorial
S. Equatorial S. Equatorial
S. Equatorial
West Wind Drift
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