How many volcanoes are considered active today?
~57
What two processes occur as magma rises?
Crystallisation and exsolution of volatiles
What are the three most common volcanic gases?
H2O (56%), CO2 (28%), SO2 (14%)
Name three less common volcanic gases.
CO, H2S, HCl (also H2, CH4, HF)
What happens when magma reaches volatile saturation?
Bubbles nucleate and grow
What three factors control eruptive style?
Silica content, volatile content, viscosity
How does silica content affect explosivity?
High SiO2 increases explosivity
How does volatile content affect explosivity?
High H2O and CO2 increase explosivity
How does viscosity affect explosivity?
High viscosity increases explosivity
What is a crater?
Steep-walled depression at the summit, usually <1 km wide
What is a vent?
Opening connected to the magma reservoir
What is unique about crater lakes?
Hot (20–50°C), pH <2, acidic, often ephemeral
What is a lava dome?
Bulbous, steep-sided lava mass formed by slow eruption
What hazards can lava domes create?
They can block openings causing explosive eruptions and pyroclastic flows
What are shield volcanoes composed of?
Primarily basaltic lava
What eruption style is typical of shield volcanoes?
Mild, effusive eruptions producing large lava volumes
What are cinder/scoria cones made of?
Ejected lava fragments (cinder-sized)
What are the slopes and size of cinder cones?
Steep (~30°), <400 m tall, bowl-shaped crater
Where do cinder cones often form?
On flanks of larger volcanoes as parasitic cones
What are composite/stratovolcanoes made of?
Alternating lava flows and pyroclastic deposits
What is the typical magma composition of stratovolcanoes?
Andesitic to dacitic
How do stratovolcanoes usually erupt?
Explosively, with eruption columns tens of km high
What is a caldera?
Large depression formed after magma withdrawal or collapse
What are fissure eruptions?
Eruptions from long cracks or fractures, often radial from a volcano
