Page 128 - SST Class 07

also cause horizontal and ARCTIC OCEAN ARCTIC OCEAN
vertical movements. Arctic Circle

g These movements give rise
to a variety of relief features. Rainier EUROPE
NORTH ASIA
NORTH
The sudden movements are PACIFIC AMERICA NORTH Stromboli Etna Vesuvius Fujiyama NORTH
ATLANTIC
OCEAN
OCEAN PACIFIC
Popocatepetl OCEAN
experienced through erup- Tropic of Cancer
Mauna Loa
tion of volcanoes and earth- Pelee AFRICA
Equator Cotopaxi
quakes. Kilimanjaro INDIAN OCEAN
SOUTH Krakatoa
VOLCANOES Tropic of Capricorn AMERICA
AUSTRALIA
A volcano is simply an area where SOUTH Aconcagua SOUTH
PACIFIC ATLANTIC
OCEAN OCEAN
magma or molten rock, from the
VOLCANOES
earth’s mantle reaches the earth’s
SOUTHERN OCEAN
Antarctic Circle
surface, becoming lava. Most ANTARCTICA
volcanoes occur at plate bound- World—Distribution of volcanoes
aries, where two plates are mov- The higher the level of gas, the more pressure
ing away (diverging) or together (converging). A that builds and the more violent an explosion.
few volcanoes like the Hawaiian Islands form a The combination of silica and dissolved gas
hot spot, or a weak spot in the earth’s crust, levels determines the type of eruption and shape
where magma forces its way to the surface. of the volcano.
Volcanic eruptions may be explosive or effusive, Types of Volcanoes
depending on the lava chemistry (amounts of
On the basis of the frequency of eruption, volca-
silica and dissolved gases). Silica is a mineral
noes are classified into two types :
found in nature as sand or quartz. High levels of
Active Volcanoes : Active volcanoes erupt
silica mean very viscous (thick) lava and low
frequently and throw out lava, gases, ashes and
levels mean more fluid lava. Dissolved gases
rocks. These volcanoes are also called living
build up inside the volcano, much like a can of
volcanoes. Stromboli and Etna in Italy are active
soda or other carbonated beverage.
volcanoes.

Volcanic bombs Dormant Volcanoes : Extinct
volcanoes have not erupted for
Ash cloud
hundreds of years. There is no
Crater
Ash possibility of any eruption in future.

Secondary Cone These volcanoes are also called dead
Secondary vent volcanoes. Kilimanjaro in Africa and
Lava flow
Rainier in USA are extinct volcanoes.
Distribution of volcanoes

Main vent Most of the world’s active volcanoes
Layers of ash and lava
form a circular belt around the Pacific
Ocean. That is why this belt is called
the ‘Pacific Ring of Fire’. This ring
Magma chamber
coincides with the edge of the
Components of a volcano lithospheric plates.

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