7: Volcanoes

Volcano

A vent, hill, or mountain from which molten rock, hot rock fragments, and gases are ejected onto the Earth's surface.

Volcanism

The process of magma rising, erupting, and forming igneous rock; can be a geological hazard, a rock-forming process, or an energy source.

Magma

Molten rock located beneath the Earth's surface, containing dissolved gases.

Lava

Magma that has erupted onto the Earth's surface.

Magma Chamber

A large underground reservoir of molten rock that feeds a volcano.

Conduit

A pipe or channel that carries magma from the magma chamber to the Earth's surface.

Vent

Any opening at the Earth's surface through which magma or volcanic gases are emitted; can be central or on the flank (side vent).

Crater

A bowl-shaped depression at the summit of a volcano, often surrounding the main vent.

Caldera

A large, basin-shaped volcanic depression formed by the collapse of a volcano's summit after a massive eruption or magma chamber evacuation.

Divergent Boundary

A tectonic plate boundary where plates move apart, typically producing mafic magma and effusive (calm) eruptions (e.g., mid-ocean ridges, continental rifts).

Convergent Boundary

A tectonic plate boundary where plates move toward each other, involving subduction and typically producing intermediate to felsic magma and explosive eruptions.

Transform Boundary

A tectonic plate boundary where plates slide horizontally past each other; typically lacks volcanism due to absence of magma generation.

Mantle Plume (Hot Spot)

A localized upwelling of hot mantle material that can produce volcanism, often forming shield volcanoes, independent of plate boundaries (e.g., Hawaii).

Effusive Eruption

A relatively gentle volcanic eruption characterized by the passive outpouring of low-viscosity lava, allowing gases to escape easily.

Explosive Eruption

A violent volcanic eruption characterized by the fragmentation and violent ejection of magma, ash, and gases due to high-viscosity magma trapping gases.

Viscosity

The resistance of a fluid to flow; in magma, it is primarily controlled by silica content and temperature. High silica = high viscosity.

Mafic Magma

Magma low in silica (45-52%) and high in iron and magnesium; has low viscosity, high temperature, and leads to effusive eruptions.

Felsic Magma

Magma high in silica (>63%) and aluminum; has high viscosity, lower temperature, and leads to explosive eruptions.

Partial Melting

The process where only a portion of a rock melts due to heat and/or pressure change, producing magma that is more silica-rich than the source rock.

Decompression Melting

Partial melting triggered by a decrease in pressure as mantle rock rises, commonly at divergent boundaries and hot spots.

Flux Melting

Partial melting triggered by the addition of water (a flux) from a subducting plate, which lowers the melting point of the overlying mantle rock at convergent boundaries.

Magmatic Differentiation

The process by which magma evolves in composition, becoming more felsic, through mechanisms like assimilation and fractional crystallization.

Assimilation

The process where hot magma melts and incorporates surrounding country rock as it rises, changing its composition.

Fractional Crystallization

The process where early-forming minerals (mafic) crystallize and settle out of the magma, leaving the remaining melt more silica-rich.

Shield Volcano

A broad, gently sloping volcano built by many layers of low-viscosity basaltic lava flows; typical of divergent boundaries and hot spots (e.g., Mauna Loa).

Stratovolcano (Composite Volcano)

A steep-sided, conical volcano built by alternating layers of viscous lava flows, tephra, and pyroclastic deposits; typical of convergent boundaries (e.g., Mayon, Mt. St. Helens).

Cinder Cone

A small, steep-sided volcano built from ejected lava fragments (scoria) that accumulate around the vent; often forms on the flanks of larger volcanoes.

Large Igneous Province (LIP)

A massive accumulation of igneous rock, often from a brief, intense outpouring of mafic lava covering vast areas (e.g., Columbia River Basalts).

Pyroclastic Material

Solid volcanic fragments ejected during an explosive eruption, including ash, lapilli, blocks, and bombs.

Tephra

A general term for all pyroclastic material (solid fragments) ejected into the air during a volcanic eruption.

Pyroclastic Density Current (PDC)

A fast-moving, ground-hugging flow of hot volcanic ash, rock fragments, and gases; includes pyroclastic flows and surges. It is the deadliest volcanic hazard.

Lahar

A destructive volcanic mudflow or debris flow composed of a slurry of water and volcanic sediment; can occur during or long after an eruption.

Debris Avalanche

A rapid, massive slope failure of a volcanic edifice, involving rock, soil, and volcanic debris; can trigger tsunamis if it enters water.

Lateral Blast

A sideways-directed, low-angle explosive eruption from the flank of a volcano, often associated with dome collapse or flank failure.

Volcanic Tsunami

A tsunami generated by volcanic activity, such as a submarine eruption, flank collapse into water, or pyroclastic flow entering the sea.

Volcanic Gas

Gases released from magma, primarily water vapor, carbon dioxide, and sulfur dioxide, which can cause acid rain, climate effects, and health hazards.

Vog (Volcanic Smog)

A hazy air pollutant mixture of sulfur dioxide gas and sulfate aerosols, formed by volcanic gas reacting with oxygen and moisture.

Laze Plume

A corrosive steam plume produced when lava flows into seawater, generating hydrochloric acid gas and fine glass particles.

Lava Dome

A steep-sided, often bulbous mound of viscous lava extruded from a volcanic vent; commonly associated with explosive eruptions.

Fumarole

A vent that emits volcanic gases, such as steam and hydrogen sulfide, often from a persistent heat source.

Holocene Volcano

A volcano that has been active at any time during the current Holocene Epoch (last ~11,700 years), considered potentially active.

PHIVOLCS

The Philippine Institute of Volcanology and Seismology, the national agency responsible for monitoring volcanic and seismic activity in the Philippines.

Permanent Danger Zone (PDZ)

A designated high-risk area around an active volcano where permanent habitation is prohibited due to constant volcanic hazards.

Volcano Alert Level

A standardized scale used by monitoring agencies (like PHIVOLCS) to communicate the status of a volcano and recommend actions, ranging from normal to hazardous eruption.

Pacific Ring of Fire

A major area in the basin of the Pacific Ocean where many earthquakes and volcanic eruptions occur due to the presence of numerous subduction zones.

Island Arc

A curved chain of volcanic islands formed by subduction of one oceanic plate beneath another (ocean-ocean convergence).

Continental Arc

A chain of volcanoes on a continental margin, formed by subduction of an oceanic plate beneath a continental plate (ocean-continent convergence).

Basanite

A type of mafic volcanic rock; the specific composition is less critical than understanding it as a mafic, low-silica rock typical of divergent settings.

Phreatomagmatic Eruption

An explosive eruption caused by the interaction of magma with external water (groundwater or surface water), producing fine ash and steam.

Magmatic Eruption

An eruption driven primarily by the expansion of gases originally dissolved in the magma.

Phreatic Eruption

A steam-driven explosion that occurs when groundwater is superheated by magma, ejecting old rock but no new magma.

Pyroclastic Fall

The settling of tephra (ash, lapilli) from an eruption column under gravity, affecting downwind areas.

Ballistic Projectile

Large, solid rock fragments (blocks and bombs) ejected on parabolic trajectories near the volcanic vent during an explosion.

Ground Deformation

The swelling, subsidence, or cracking of the ground around a volcano due to magma movement, often a precursor to eruption.

Secondary Explosion

An explosion caused by the interaction of hot volcanic deposits (like a recent lava flow) with water, generating new ash clouds or small PDCs.