Chapter 5 – Volcanoes and Volcanic Hazards

Mount St. Helens (1980)

Explosive eruption blew out the entire north flank, lowering the summit by ~400 m (1,350 ft). “The blast blew out the entire north flank of the volcano, leaving a gaping hole.”

Kilauea (1983–present)

Gentle, effusive lava flows; nonexplosive but long-lived

Mount St. Helens vs. Kilauea Key Difference

Silica content and gas pressure—St. Helens = viscous & gas-rich; Kilauea = fluid & gas-poor.

Viscosity + Gas Content

= eruption style

• “The more silica present in magma, the greater its viscosity.”

Mafic (Basaltic)

low silica, low gas → fluid, quiet eruptions

Felsic (Rhyolitic)

high silica, high gas → viscous, explosive eruptions

Temperature effect

hotter magmas = more fluid

Effusive Eruptions

low-viscosity magma allows gases to escape easily

Explosive Eruptions

trapped gases expand rapidly, shattering magma into ash and pumice.

Materials Extruded During an Eruption

• Lava Flows

• Pyroclastic Material (Tephra)

• Volcanic Gases (Volatiles)

Lava Flows

• >90% of Earth’s lava = basaltic

• Pahoehoe

• Aa

• Lava tubes

• Block Lava

• Pillow Lavas

Lava Flows: >90% of Earth’s lava

= basaltic

Lava Flows: Pahoehoe

smooth, “ropy” surface; hotter, fluid lava

Lava Flows: Aa

rough, jagged blocks; cooler, more viscous

Lava Flows: Lava tubes

“Cave-like tunnels that once served as conduits carrying lava from an active vent to the flow’s leading edge.”

Lava Flows: Block Lava

thick, short flows of andesitic/rhyolitic lava

Lava Flows: Pillow Lavas

bulbous, underwater lava forms; indicator of submarine eruptions

Pyroclastic Material (Tephra)

• Ash & Dust

• Lapilli (Cinders)

• Bombs & Blocks

• Scoria

• Pumice

Pyroclastic Material: Ash & Dust

<2 mm; can fuse into welded tuff

Pyroclastic Material: Lapilli (Cinders)

2–64 mm fragments

Pyroclastic Material: Bombs & Blocks

>64 mm, ejected during eruptions; bombs form streamlined shapes

Pyroclastic Material: Scoria

basaltic, vesicular fragments (dark/red)

Pyroclastic Material: Pumice

felsic, vesicular, light enough to float.

Volcanic Gases (Volatiles)

• 1–8% of magma by weight.

• Major gases: H₂O (most abundant), CO₂, SO₂.

• Gases control eruption violence—“Low-viscosity basalt allows easy escape; high-viscosity rhyolite traps gases, causing violent eruptions.”

Major volcanic gases

H₂O (most abundant), CO₂, SO₂

Anatomy of a Volcano

• Conduit

• Vent

• Crater

• Caldera

• Parasitic Cones

• Fumaroles

Anatomy of a Volcano: Conduit

magma pathway

Anatomy of a Volcano: Vent

surface opening for lava/gas

Anatomy of a Volcano: Crater

depression at summit (<1 km wide).

Anatomy of a Volcano: Caldera

collapsed depression >1 km, after major eruption

Anatomy of a Volcano: Parasitic Cones

secondary vents on flanks

Anatomy of a Volcano: Fumaroles

vents emitting gases (steam, SO₂)

Shield Volcanoes

Built by low-viscosity basaltic lava with lateral flow; covers large areas

Shield Volcanoes Shape

broad, gently sloping (like a warrior’s shield)

Shield Volcanoes Lava

fluid, basaltic

Shield Volcanoes Examples

Mauna Loa – world’s largest, over 9 km tall from seafloor

Kilauea

Hawaii’s most active shield; continuous eruptions since 1983

Cinder Cones (Scoria Cones)

Built from ejected lava fragments (mostly cinders and bombs)…relatively small—usually less than 300 meters (1000 feet) in height

Cinder Cones Formed From

pyroclastic fragments (mainly scoria)

Cinder Cones Shape

steep-sided, small (30–300 m tall)

Cinder Cones Example

Parícutin (Mexico) – erupted 1943–1952, buried nearby town

Composite Volcanoes (Stratovolcanoes)

Most are located in the Ring of Fire

Composite Volcanoes Structure 

alternating layers of lava + pyroclastics

Composite Volcanoes Magma Type

andesitic to rhyolitic (viscous)

Composite Volcanoes are Highly Explosive

due to trapped gas

Composite Volcanoes Examples

Mt. Fuji, Mt. St. Helens, Mt. Rainier, Mt. Vesuvius

Composite Volcanoes Hazards 

pyroclastic flows, lahars

Volcanic Hazards

• Lava Flows

• Pyroclastic Flows

• Lahars

• Ash Fall

• Landslides

• Gas Emissions

Volcanic Hazards: Lava Flows

destroy property, rarely kill people

Volcanic Hazards: Pyroclastic Flows

 “avalanches of hot ash and gas up to 300 km/h” – most deadly hazard.

Volcanic Hazards: Lahars

volcanic mudflows; water + ash → cement-like flows

Lahars Example 

Nevado del Ruiz (1985) killed 23,000+

Volcanic Hazards: Ash Fall

roofs collapse, aviation hazards (abrasive, engine-clogging)

Volcanic Hazards: Landslides

flank collapses can trigger eruptions (Mt. St. Helens)

Volcanic Hazards: Gas Emissions

SO₂ + H₂O → sulfuric acid; toxic clouds

Volcanic Explosivity Index (VEI)

logarithmic scale measuring eruption magnitude

Calderas

collapse after massive eruptions (Crater Lake, Yellowstone)

Lava Domes

viscous lava piles over vent

Fissure Eruptions

long cracks produce flood basalts (e.g., Columbia River Plateau)

Volcanic Necks & Pipes

hardened magma in conduit, later exposed (e.g., Ship Rock, NM)

Divergent Boundaries

mid-ocean ridges → basaltic magma (pillow lavas)

Convergent Boundaries

subduction zones → andesitic/rhyolitic, explosive arcs (Ring of Fire)

Hot Spots

intraplate volcanism – oceanic (Hawaii) & continental (Yellowstone)

Continental Rifts

mixed magmas from mantle & crust (East African Rift, Basin & Range)

Volcanic Activity Precursors

• Seismic activity

• Ground deformation

• Gas emissions

• Thermal changes

Volcanic Activity Precursors: Seismic activity

earthquake swarms

Volcanic Activity Precursors: Ground deformation

bulging slopes

Volcanic Activity Precursors: Gas emissions

SO₂ increase

Volcanic Activity Precursors: Thermal changes

infrared satellite data

Monitoring Volcanic Activity Tools

tiltmeters, GPS, thermal imaging, gas sensors, remote sensing

Monitoring Volcanic Activity Purpose

predict eruptions and create Hazard Maps for evacuation planning

Active Volcanoes 

erupting or likely soon

Dormant Volcanoes

long rest but potential

Extinct Volcanoes

magma source cutoff

Supervolcano

eruption >1,000 km³ ejecta, VEI 8 (e.g., Yellowstone)

Pillow Basalts

form where basalt erupts underwater and cools instantly

Thermophiles

heat-loving bacteria in hot springs (Yellowstone)

Aerosols

“extremely small solid particles or droplets” from gas emissions

Tephra

general term for all pyroclastic material

Volcanic Winter

global cooling from large SO₂ and ash injection