Let’s Review…
• Copy in Notebook and Answer:
– Compare and contrast magma and lava.
– What plate boundaries will result in formation
of a volcano?
Magma
What caused Mt. St. Helen’s to erupt?
How does Magma form?
• Magma is molten rock beneath the Earth’s
surface
• High Temperature:
– Rocks melt at
800-1200°C
Viscosity
• Viscosity – internal resistance to flow
• Low Viscosity
– Moves fast (Less thick)
• High Viscosity
– Moves slow (More thick)
Types of
Magma
Type of
Magma
Viscosity
Silica
Content
Gas
Content
Explosiveness
Type of
Volcano
Example
Type of
Magma
Basaltic
Viscosity
Silica
Content
Low
1-2%
Gas
Content
Explosiveness
50%
Least
Type of
Volcano
Shield
Example
Hawaii
Type of
Magma
Basaltic
Viscosity
Silica
Content
Low
1-2%
Andesitic Medium 3-4%
Gas
Content
Explosiveness
50%
Least
60%
Type of
Volcano
Shield
Medium Composite
Example
Hawaii
Mt. St.
Helens
Type of
Magma
Basaltic
Viscosity
Silica
Content
Low
1-2%
Andesitic Medium 3-4%
Rhyolitic High
4-6%
Gas
Content
Explosiveness
50%
Least
Type of
Volcano
Shield
60%
Medium Composite
70%
Greatest
Cinder
Cone
Example
Hawaii
Mt. St.
Helens
Yellowstone
*The higher the gas content, the more explosive the magma is
going to be
Volcanic Material
• Tephra
– Rock fragments thrown into the air during a
volcanic eruption
• Pyroclastic Flow
– Rapidly moving volcanic material.
– Speeds up to 200km/h
– May contain hot, poisonous gases
Tephra
Pyroclastic Flow
Shield Volcano
• Broad Gentle Sloping Sides
• Non-Explosive
Composite Volcano
• Alternates Lava and Tephra
• Potentially most dangerous
Cinder Cone Volcano
• Steep sides caused by tephra landing around vent
• Explosive