Lecture 7: Volcanism

Lava

Molten rock effusively erupted by a volcano.

Pahoehoe Lava

Occurs in basaltic lava; shows smooth, undulating and ropy surface; thin flows, slow velocity.

A’a Lava

Rough, rubbly surface; faster movement, higher velocity; may occur in other types of magma.

Pillow Lava

Basaltic to andesitic magma associated with water; pillow-shaped lobes with quickly chilled margins.

Tephra

Umbrella term for pyroclastic material (rock & glass fragments) explosively erupted by a volcano.

Pyroclasts

Individual pyroclastic (rock/glass) fragments classified based on size.

Pyroclastic Flow

High velocity avalanche of hot gases infused with incandescent ash and pyroclastic fragments.

Nuee Ardente

Glowing avalanches composed of a low-density cloud of hot expanding gases containing hot fine ash.

Lahar Flow

Slurry or volcanic mudflow from saturated volcanic and pyroclastic debris.

Pyroclastic Density Currents (PDCs)

The umbrella term for fast, concentrated density currents of pyroclastic material.

Fall

Ash eventually settles after being carried high above the volcano in a vertical plume.

Flow

Fast, concentrated density current of steaming pyroclastic material.

Surge

Extremely fast, dilute density current of steaming pyroclastic material; travels fastest at the top.

Volcanic Gases

Gases exsolved and separated from magma as they reach the surface.

Fumaroles

Fracture or opening from which volcanic gases and steam escape.

Solfatara

Fumarole emitting SO2 or H2S, precipitating sulfur deposits.

Volcano

Opening or rupture in the crust where lava, volcanic gases and pyroclasts are released.

Shield Volcano

Dome-shaped, low gradient and gently sloping volcano with basaltic flows.

Stratovolcano

Tall, conical volcano with alternating layers of lava flows and pyroclastic materials.

Composite Volcano

Another name for stratovolcano; characterized by a mix of lava flows and pyroclastic materials.

Scoria Cones

Mounds of scoria (< 200-300 m high) with a crater in the middle.

Maars

Explosive volcanic formations that result from magma interacting with groundwater.

Tuff Rings

Surface accumulations of highly fragmented basaltic scoria from near-surface explosions.

Caldera

Large depressions (> 2km in diameter) formed from summit collapse or colossal eruptions.

Effusive Eruption

An eruption characterized by the oozing of lava flows.

Explosive Eruption

Eruption driven by gas, resulting in tephra column formation.

Volcanic Explosivity Index (VEI)

Index used to describe the explosivity of an eruption.

Dense Rock Equivalent (DRE)

Characterization of the size of an eruption based on erupted magma volume.

Hawaiian Eruption

Quiescent, effusive eruption that begins as fissures evolving to central vent flows.

Strombolian Eruption

Mild blasts of incandescent cinders and lava bombs; least violent explosive eruption.

Vulcanian Eruption

Discrete ejection of solid materials and gases from more viscous magmas.

Peléan Eruption

Large amounts of gas, dust, and ash driven by lava dome collapse.

Plinian Eruption

Largest, most violent eruption from fragmentation of gassy magma.

Phreatic Eruption

Eruption caused by heated water interacting with magma or hot rocks.

Phreatomagmatic Eruption

Generated by interaction of magma with abundant surface water.

Surtseyan Eruption

Occurs in an undersea volcano breaking the water’s surface.

Submarine Eruption

Volcanic eruptions occurring underwater along mid-oceanic ridges.