GEOG 4

Basaltic

Roughly 40-50% silica content. 

Low viscosity

Low gas content

Eruption temp = 1100 degrees

Eruption type = flowing

Thin lava flows, forming shield volcanoes.

Examples: Hawaiian Islands chain

Andesitic

60-65% silica content. 

Moderate viscosity.

Moderate gas content.

Eruption temp = 800-1000 degrees.

Eruption type = flowing and moderatly explosive.

Thick, rubbly lava flows and ash falls. Creating stratovolcanoes.

Examples: Mount Etna, St Helens, Mount Fuji and Nevada Del Ruiz.

Rhyolitic

Roughly 70% silica content. 

Very high viscosity.

High gas content.

Eruption temp = 750-850 degrees

Eruption type = Very explosive

Pumice and ask fall, pyroclastic flows also occur. Lava flows when the magma is low in gas.

Examples: Caldera volcanoes. Such as Yellowstone and Chaiten.

Found at convergent or destructive plate boundaries.

Magmas with high gas content, high silica content and low temperature are viscous and produce explosive, hazardous volcanoes.