1. Eruptions

2. Making Magma Activity depends on composition of magma
Lava can be thin & runny or thick & lumpy

3. Magma Temperature Most rocks melt between 800-1200 °C
In Crust & Upper Mantle
Pressure
Increase with depth because of weight of rocks
High pressure increases melting temperature

4. Composition of Magma Amount of Gas & Silica
Increase gas dissolved = increased explosiveness
Dissolved gases = carbon dioxide, water vapor, sulfur oxide, sulfuric acid
Water vapor determines where magma forms
Water vapor decreases melting temperature

5. Composition Viscosity = Resistance to flow
Temperature & silica content affect viscosity
High viscosity = cool magma
High silica = thick & sticky
Traps gases, produces explosive eruptions
Low silica = thin & runny
Flow easily, produce quiet, non-explosive eruptions

6. Types of Magma - Basaltic
Less then 50% silica content
Low-viscosity magma
Gas easily escapes
Quiet eruptions
Ex: Kilauea & Mauna Loa

7. Types of Magma - Andesitic
50-60% silica content
Oceanic-continental subduction zones
Intermediate viscosity
Intermediate explosiveness
Send high volumes of ash & debris
Ex: Colima Volcano in Mexico

8. Types of Magma - Rhyolitic
More than 60% silica content
Magma mixes with water and silica
High viscosity
Large volumes of gas trapped
Very explosive

9. Explosive Eruptions
If lava is too viscous to flow, pressure builds up until an explosion
Tephra = pieces of solidified lava or pieces of crust
Classified by size
Ash = smallest tephra, diameter is less then 2 mm
Blocks = largest tephra

10. Ash Can rise into atmosphere
Tiny sulfuric acid droplets remain in stratosphere and block sun’s rays and decrease global temperature

11. Pyroclastic Flows Tephra + gases VERY HOT

12. In-Class Assignment
18.2 Study Guide WKT