Page 63 - Geography for Secondary Schools Student’s Book Form One

Geography for Secondary Schools
Student’s Book Form One
Continental shelf Ocean deep

A continental shelf is a gently sloping margin An ocean deep (trench) is a long narrow
of a continent that is occupied by shallow steep sided depression plunging from the
waters that extend from the coastline to deep sea-plain to the depths of 10000 metres
a depth of about 200 metres towards the or more. The deepest known ocean trench
continental slope. Some continental shelves in the world is Mariana Trench, which is
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provide excellent fishing grounds in the approximately 11000 metres deep. It is
world because they are shallow, and they located 11035 metres off the Island of Guam
allow the sun’s rays to reach the shelf floor. in the Pacific Ocean.
Hence, they provide good conditions for
the growth of plankton, which fish feed on. Island
An island is a piece of land surrounded
Continental slope by water. Islands may occur in oceans,
A continental slope is found at the point seas, lakes or rivers. A group of islands
where the continental shelf forms a steep is called an archipelago. Examples of an
slope towards the ocean basin. archipelago include Islands of Zanzibar
and the Comoros. There are three types of
islands: continental islands, oceanic islands,
Mid-ocean ridge and coral islands.

A mid-ocean ridge is a raised part of the
ocean floor. Some of these ridges appear (a) Continental islands
above the surface of the ocean as oceanic A continental island is a large unbroken
islands such as Azores in Portugal and landmass similar to a continental landmass
Bouvet in Norway. Examples of the mid- rising from a continental shelf. Examples
oceanic ridges are the Mid-Atlantic Ridge of continental islands include Mafia,
in the Atlantic Ocean and the Mid-Indian Pemba, Unguja, Philippines, Indonesian,
Ridge in the Indian Ocean.
Greenland, and Sri Lanka.

Deep sea plain (b) Oceanic islands
Deep sea-plains are extensive undulating Oceanic Islands rise directly from the
areas of the ocean floor. It is also known as ocean floor. Some of the oceanic islands
abyssal plains. Large parts of these plains are the tops of oceanic ridges, plateaus,
are covered by thin layers of sediments and ocean floor volcanoes. Examples of
of both organic and inorganic materials oceanic islands are Hawaii, Cape Verde,
brought in by rivers and some from broken Iceland, Canary, and Seychelles.
rocks of the ocean floor and cliffs.
(c) Coral islands
Abyssal hill A coral island is an island, which is built
Abyssal hill are small sharp hills rising up from limestone rocks made of skeletons
from abyssal plain surface. of very small marine organisms called

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