Volcanoes
There are 3 types of volcanoes:
1. Shield – VERY large
2. Composite – medium
3. Cinder - small
Differences between volcanoes
Origin
• Where does magma
come from?
• Shield – Basaltic magma
forms deep down
• Composite – andesitic
and granitic magma
forms from subduction
zone
• Cinder – granitic and
andesitic magma formed
at subduction zone.
Content
• What magma is made of?
• Silica content –
– Shield – low silica contentbasaltic – “runny”
– Composite – high silica content –
granitic/rhyolitic – viscous (thick)
– Cinder – high silica content –
granitic/rhyolitic – viscous (thick)
Differences continued
• Gas Content
– Gas is held in rock because of high pressure
underground (like CO2 in soda). When pressure is
reduced the gas tries to escape.
• Shield – low gas content because it cools so quickly –
volcano therefore doesn’t have the explosive “power”.
• Composite – high gas content kept underground
therefore has much explosive power since gas escapes
all at once.
• Cinder – high gas content kept underground therefore
has much explosive power since gas escapes all at once.
Explosive Power
• Shield – called “quiet” volcano due to its lack of
explosiveness ( low silica and gas content)
sputters out not viscous
• Composite – explosive – very viscous so pressure
builds up inside when gas is released the volcano
explodes releasing pressure immediately.
• Cinder - explosive – very viscous so pressure
builds up inside when gas is released the volcano
explodes releasing pressure immediately.
Pyroclastic materials
• Definition – particles produced in volcanic
eruptions
– Basaltic lava – viscous rhyolitic magma is charged
with gases as the gas expands it blows fragments
from the vent.
Factors
• Composition –
• Temperature of magma –
• Dissolved gases -
Review Questions
• 1. What factors determine the type of
volcanic eruption?
• 2. List the materials that can be ejected from
volcanoes.
• 3. Describe the three types of volcanoes.