- Locations: form in the ocean (oceanic crust).
- Process: hot spots under ocean crust melts oceanic crust
- Structure: countless layers of lava erupted from main vent, often with a crater on top (some side vents); broad, huge volcanoes with very gentle slopes.
- Eruptive violence: low.
- Magma: low SiO2 and H2O magma of oceanic crust.
- Lava type(s): basalt
- Materials ejected: free flowing lava
- Examples: Hawaii, Galapagos Islands.
- Locations: forms on continents (rift valleys).
- Process: continental crust opens up to form a new spreading center (eventually a new ocean)
- Structure: layers of individual volcanic rocks (cinders, tephra) build up as blobs of lava are expelled from the one central vent; steep sides; usually very symmetrical; small volcanoes.
- Eruptive violence: low to moderate.
- Magma: high SiO2 and H2O magma from mantle.
- Lava type(s): usually basalt, sometimes andesite.
- Materials ejected: blobs of lava
- Examples: Sunset Crater, AZ. Schreiber's Meadow , S P crater, AZ
Stratovolcano / Composite Volcano
- Locations: form on the edges of continents or as a chain of islands in ocean.
- Process: at a subduction zone, oceanic crust melts and combines with melting continental crust.
- Structure: alternating layers of lava, ash and volcanic debris; normally one central vent; moderately steep sides; mid-sized volcanoes.
- Eruptive violence: moderate to high.
- Magma: medium to high SiO2 and H2O magma.
- Lava type(s): mostly andesite with some rhyolite.
- Materials ejected: lava, ash, cinders and other volcanic debris.
- Examples: Mt. Baker, Mt. St. Helens, Mt. Fuji
- Locations: form on the continents.
- Process: hot spots under continents melt continental crust.
- Structure: huge volcano that does not look like typical volcano, maybe very flat, gigantic crater forms when the top of the volcano collapses into the huge magma chamber.
- Eruptive violence: explosive eruptions.
- Magma: high SiO2 and H2O magma.
- Lava type(s): rhyolite
- Materials ejected: ash and cinders
- Examples: Long Valley Caldera, CA; Yellowstone
The bar in the diagrams represents 1 mile. Images courtesy of USGS