3.8 Types of Volcanoes

GEOL1340

Shield Volcanoes

Broad, gently sloping dome-shaped volcanoes (slopes $2^\circ\text{--}10^\circ$, up to $200\text{ km}$ across) built from low-viscosity, effusive mafic/basaltic lavas (e.g., Mauna Loa, Kilauea).

Composite Volcanoes (Stratovolcanoes)

Large, steep-sided symmetric cones (slopes $10^\circ\text{--}30^\circ$) formed at subduction zones from alternating layers of lava flows and explosive pyroclastic tephra (e.g., Mount St. Helens, Mount Rainier).

Cinder Cones (Pyroclastic Cones)

Small ($<200\text{ m}$ high), steep ($>20^\circ$) volcanic cones built rapidly from accumulated vesicular mafic fragments (scoria/lapilli) ejected during short-lived, monogenetic fire-fountaining eruptions (e.g., Eve Cone, Parícutin).

Monogenetic Volcanoes

Volcanoes (like most cinder cones) that form during a single eruptive phase lasting weeks to months and then become permanently extinct.

Crater

A bowl-shaped, steep-sided summit depression formed by the outward explosive ejection of rock during an eruption.

Caldera

A large, basin-shaped volcanic depression ($>2\text{ km}$ in diameter) formed by the inward structural collapse of a volcano summit into an emptied underlying magma chamber (e.g., Kilauea Caldera).

Fissure Eruption

An eruption that issues lava along a linear fracture or rift zone rather than through a central circular pipe vent.

Volcanic Neck (Plug)

A prominent landform consisting of hardened, solidified magma left behind in a central vent after the softer surrounding ash cone erodes away.

Lava Field / Plain

A large, flat expanse of land covered by extensive surficial or subaqueous flows of highly fluid basaltic lava.

Fumarole

A vent in or near a volcanic region that emits steam and volcanic gases ($SO_2, CO_2, H_2S$).

Hot Spring

A thermal discharge where geothermally heated groundwater freely flows to the surface.

Geyser

A specialized hot spring characterized by intermittent, turbulent discharges of boiling water and steam caused by underground plumbing constrictions.

Pāhoehoe Lava

Hot, non-viscous mafic lava that cools into smooth, billowy, or ropey surface textures as it flows gently.

ʻAʻā Lava

Cooler, stiffer, blocky/rubbly mafic lava formed when lava is forced to flow faster down slopes than its viscosity allows.

Pillow Lava

Bulbous, toothpaste-like lava structures formed when fluid mafic lava erupts subaqueously and instantly quenches against cold ocean water.

Lava Tube

An underground tunnel formed when the surface of a mafic lava flow cools and hardens into a solid roof while molten lava continues flowing inside; allows lava to travel tens of kilometers without cooling.

Volcanic Ash

Fine pyroclastic material measuring $<2\text{ mm}$ in particle diameter.

Lapilli / Cinders

Intermediate pyroclastic fragments measuring $2\text{--}64\text{ mm}$ in diameter.

Volcanic Blocks and Bombs

Large pyroclastic fragments ($>64\text{ mm}$); volcanic bombs are ejected as molten lava that streamlines aerodynamically while flying through the air.

Tuff

A lithified pyroclastic rock formed from compacted and cemented volcanic ash ($\ge 75\%$).

Volcanic Breccia

A lithified pyroclastic rock composed predominantly of consolidated, large, angular volcanic blocks and fragments.

Lahar

A fast-moving, highly destructive volcanic mudflow triggered when explosive eruptions melt snow and ice or destabilize water-saturated volcanic ash layers (e.g., Nevado del Ruiz).

Pyroclastic Flow

A fast-moving, high-density current of superheated gas, ash, and rock fragments that travels down volcanic slopes during explosive eruptions (e.g., Mount Pelée).

Active Volcano

A volcano currently erupting or showing signs of internal seismic/magmatic unrest.

Dormant Volcano

An inactive volcano that retains an intact magma supply capable of erupting in the future.

Extinct Volcano

A volcano that has cut off completely from its magma supply and will not erupt again.

Large Igneous Provinces (LIPs)

Enormous regional accumulations of flood basalts (up to millions of $\text{km}^2$) generated by rapid, high-volume bursts of mantle plume activity (e.g., Columbia River Basalts, Siberian Traps).

Kimberlites

Ultramafic pipe-like volcanic structures originating deep in the mantle ($150\text{--}450\text{ km}$); brought rapidly to the surface, transporting diamond-bearing rocks (e.g., Ekati Mine).

Columnar Jointing

Hexagonal cooling fractures that form perpendicular to cooling surfaces in thick, homogeneous basaltic lava flows or sills.