Volcanoes

Earthquake Hazard Map

It contains 53 to 65% silica. This lava forms from magma that melts through oceanic crust that is rich in silica. This type of lava often forms at convergent plate boundaries.

Intermediate lava (andesitic)

It contains more than 65% silica. The high percentage of silica increases the lava’s viscosity, or resistance to flow. This type of lava traps gases, producing explosive volcanic eruptions.

Felsic lava (rhyolitic)

It contains less than 45% silica. This lava is extremely rare on the planet today. It formed early in Earth’s history when the planet’s surface was hotter and the crustal material was still primarily mafic.

Ultramafic lava

It contains 45 to 52% silica. This lava contains a high percentage of magnesium and iron. The lower silica content produces lava with a lower viscosity. This type of lava produces relatively nonexplosive eruptions.

Mafic lava (basaltic)

• Occur at convergent plate boundaries where one plate

Subduction Boundary Eruptions

• Occur at divergent plate boundaries and it tends to be effusive

Rift Eruptions

• These are characterized by eruptions that produce lava flows that are very fluid and spread out over a large area, often forming lava plains. 

Icelandic

• Characterized by fluid, basaltic lava flows that erupt from vents or fissures, often creating "fire fountains". 

Hawaiian

Icelandic

Hawaiian

• Molten, incandescent lava spurts from a fissure on the volcano's rift zone, feeding lava streams that flow downslope. 

Fissure

Explosive Eruptions

These eruptions are characterized by a large amount of gas, dust, ash, and lava fragments blown out laterally by the collapse of a cinder cone, forming pyroclastic flows. 

Pelean

• These are the most violent and dangerous eruptions, associated with volcano collapse, and can produce eruption columns that reach up to 40 km into the sky. 

Plinian

Explosive Eruptions

Pelean

Plinian

Mt. Pelee

• These are explosive eruptions of intermediate composition lava, producing pyroclastic flows and eruptive columns from 5 to 10 km high. 

Vulcanian

• These are characterized by moderate explosions that eject incandescent lava fragments, cinders, and lava bombs into the air. 

Strombolian

Vulcanian

Strombolian

Anak Krakatau

These eruptions occur when magma interacts with water, resulting in a violent explosion that can produce a variety of volcanic debris. 

Phreatomagmatic

Surtseyan

a high-energy volcanic eruption that occurs when sea or lake water floods into the top of an active, open vent, leading to violent explosions

Phreatomagmatic

Surtseyan

• These are steep-sided volcanoes built up from layers of lava, ash, and other volcanic debris, often associated with explosive eruptions. 

Composite Volcanoes (Stratovolcanoes)

• These are broad, gently sloping volcanoes formed by effusive eruptions of low-viscosity lava, like those found in Hawaii. 

Shield Volcanoes

Composite Volcanoes

Shield Volcanoes

Mauna Loa

Mt. St. Helens

• These are small, rounded mounds of hardened lava formed by viscous lava that doesn't flow far from the vent. 

Lava Domes

• These are small, steep-sided volcanoes formed by the accumulation of pyroclastic material (cinders, ash, and pumice) ejected during explosive eruptions. 

Cinder Cones

Lava Domes

Cinder Cones

Paricutin

Lassen Peak

• Volcanoes that have not erupted in a long time but could erupt again in the future. 

Dormant Volcanoes

Volcanoes that are currently erupting or have erupted recently. 

Active Volcanoes

Volcanoes that are unlikely to erupt again. 

Extinct Volcanoes