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Let’s Review…
• Copy in Notebook and Answer:– Compare and contrast magma and lava.– What plate boundaries will result in formation
of a volcano?
MagmaWhat 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
Explosive-ness
Type of Volcano
Example
Type of Magma
Viscosity Silica Content
Gas Content
Explosive-ness
Type of Volcano
Example
Basaltic Low 1-2% 50% Least Shield Hawaii
Type of Magma
Viscosity Silica Content
Gas Content
Explosive-ness
Type of Volcano
Example
Basaltic Low 1-2% 50% Least Shield Hawaii
Andesitic Medium 3-4% 60% Medium CompositeMt. St. Helens
Type of Magma
Viscosity Silica Content
Gas Content
Explosive-ness
Type of Volcano
Example
Basaltic Low 1-2% 50% Least Shield Hawaii
Andesitic Medium 3-4% 60% Medium CompositeMt. St. Helens
Rhyolitic High 4-6% 70% Greatest Cinder Cone
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