C A S T R I E S
C O M P R E H E N S I V E S E C O N D A R Y
G E O G R A P H Y
S C H O O L
VOLCANOES
S H I E L D A N D C O M P O S I T E VO L C A N O E S
WHAT IS A VOLCANO?
• Volcanism is the process by which liquid rock, or magma is forced through vents into the
earth’s crust or onto its surface.
• A volcano is a natural vent or fissure in the earth’s crust through which hot, gaseous, molten or
solid materials may escape to the surface. Once it reaches the surface, magma is called
lava.
• Volcanoes and earthquakes are two geologically and physically related phenomena.
• Volcanoes erupt around lines of weaknesses, where folding and fracturing have occurred.
STRUCTURE OF A VOLCANO
STRUCTURE OF A VOLCANO
• A volcano is a conical structure with a funnel- shaped depression or crater in the top.
• In the bottom of the crater there is a conduit or hole, called the vent or pipe.
• the conical shape is produced by the accumulation of materials that have been forced up
through the vent and have erupted over the top of the volcano. These build up over successive
eruptions, during which lava solidifies down the sides.
• Volcanoes are classified according to their shape, their size and the nature of the materials
ejected. They type of lava maybe basic or acidic determines the shape of the volcanic cones.
DISTRIBUTION OF VOLCANOES
• Volcanic zones occur along the edges of tectonic plates that form the earth’s crust.
• About 75% of volcanoes are situated in a circle around the Pacific Ocean known as the Pacific
Ring of Fire.
FORMATION OF VOLCANOES
C O M P O S I T E VO L C A N O E S
• Composite/strata volcanoes have alternating
layers of lava and ash. They have steep sides
• They occur at convergent plate boundaries and
are associated with EXPLOSIVE and VIOLENT
eruptions.
• When plates collide as in the collision of oceanic
and continental crust, composite/strata volcanoes
are formed.
• When the oceanic crust is subducted, where it
melted by high temperatures, some magma may
block the vent or pipe of the volcano, building up
pressure inside.
• These volcanoes are also associated with acidic
lava which tends to have a high silica content
making the lava very viscous (sticky) all
contributing to the highly explosive volcano.
• E.g Mt. Pelee (Martinique), volcanoes in the Pacific
Ring of Fire, Soufriere in St.Vincent and
Monsterrat.
S H I E L D VO L C A N O E S
• They occur at divergent plate boundaries
and Hotspots and are associated with
frequent but gentle eruptions.
• Shield volcanoes are formed from the
accumulation of basic lava, that has a low
viscosity due to less silica. (It is a runny,
flowy lava)
• They can flow for long distances and form
lava sheets.
• The sides of shield volcanoes are broad
and gentle, usually inclined at 7 degrees or
less.
• Their craters are wide as for example in
Hawaii, where Mauna Loa is the world’s
largest volcano.
COMPOSITE VS SHIELD VOLCANOES
C O M P O S I T E VO L C A N O
S H I E L D VO L C A N O
END