Volcanoes & Volcanic Hazards - Lecture 5 Notes
5.1 Mount St. Helens Versus Kilauea
Mount St. Helens, Washington, 1980
Explosive eruption that blew out the upper part of the mountain, removing the north flank.
Uppermost portion removed: about $400~\mathrm{m}$ (approximately 1350 feet) of summit broken away on the north side.
Immediate effects included a blast of gas and ash with a downrange impact of about $400~\mathrm{km^2}$ of forest destroyed.
Resulted in mudflows, ash falls, and emission of hot gases.
Fatality toll: 59 people.
Kilauea, Hawaii, 1983 (ongoing eruption began in 1983)
Relatively gentle by comparison to St. Helens.
Emitted fluid lava flows (effusive activity).
Lava flows destroyed houses and other structures in their path, but the eruption was not highly explosive.
Comparative takeaway
Different eruption styles (explosive vs. effusive) reflect fundamental controls on eruption dynamics, notably magma properties and gas content.
5.2 The Nature of Volcanic Eruptions
Factors determining the violence of an eruption:
Composition of the magma
Temperature of the magma
Dissolved gases in the magma
These three factors control magma viscosity, which largely governs the eruption style.
Viscosity definition
Viscosity is a measure of a material’s resistance to flow. Higher viscosity means more resistance to flow.
Temperature and composition effects on viscosity
Temperature: hotter magmas are less viscous.
Silica content (SiO$_2$): higher silica content increases viscosity.
Examples by composition:
Mafic (basaltic) lava: low silica, low viscosity (more fluid).
Intermediate: andesitic lava: intermediate silica and viscosity.
Felsic (rhyolitic/granitic) lava: high silica, high viscosity (more explosive potential).
Representative temperatures (and associated viscosities):
Basaltic magma: approx. $1000$–$1250^{\circ}\mathrm{C}$, low viscosity.
Andesitic: approx. $800$–$1050^{\circ}\mathrm{C}$, intermediate viscosity.
Rhyolitic: approx. $650$–$900^{\circ}\mathrm{C}$, high viscosity, more explosive behavior.
Dissolved gases in magma
Gas content typically ~ by weight, mainly water vapor and carbon dioxide.
Gases come out of solution as magma rises and experiences decreasing pressure near the surface, increasing magma mobility and eruption violence as gases try to escape.
Summary of eruption styles by viscosity
Fluid basaltic lavas: generally quiet eruptions.
Highly viscous lavas (rhyolite/andesite): tend to produce more explosive eruptions.
5.3 Materials Extruded During an Eruption
Lava flows (lavas)
Basaltic lavas are relatively fluid.
Pahoehoe lava: ropey, twisted texture.
Aa lava: rough, jagged, blocky texture.
Pillow lavas: form at divergent plate boundaries on the ocean floor.
Andesitic/Rhyolitic lavas are more viscous.
Block lavas: travel short distances due to high viscosity.
Dissolved gases (in magma)
Gas content: typically $1 ext{-}6\%$ by weight (mainly H$2$O and CO$2$).
Pyroclastic materials (tephra) – fire fragments
Volcanic ash: particles $< 2~\mathrm{mm}$ in diameter.
Pumice: porous rock formed from frothy lava.
Lapilli (cinders): $2$–$64~\mathrm{mm}$.
Cinders: often used interchangeably with lapilli for small vesicular fragments.
Volcanic bombs: ejected as hot lava, larger than $64~\mathrm{mm}$.
Volcanic blocks: ejected hardened or cooled rock fragments, typically larger than $64~\mathrm{mm}$.
Particle sizes (tephra) at a glance
Volcanic ash: $<2~\mathrm{mm}$
Lapilli (cinders): $2$–$64~\mathrm{mm}$
Volcanic bombs: $>64~\mathrm{mm}$
Volcanic blocks: $>64~\mathrm{mm}$
Note: The fine fraction below 0.063 mm (dust) is often referred to as volcanic dust.
5.4 Anatomy of a Volcano
General features
Crater: a steep-walled depression at the summit, typically < $1~\mathrm{km}$ in diameter.
Caldera: a summit depression usually > $1~\mathrm{km}$ in diameter, formed by collapse after a massive eruption.
Vent: opening connected to the magma chamber via a conduit.
Additional structural components depicted in diagrams
Magma chamber, conduit, crater, vent, and parasitic cones (secondary cones on the flank) as part of a typical volcanic system.
5.5 Shield Volcanoes
Characteristics
Broad, gently sloping, dome-shaped profile.
Primarily built from basaltic lava.
Typically emit mild eruptions that erupt large volumes of lava quiescently or effusively.
5.6 Cinder Cones
Characteristics
Built from ejected basaltic lava fragments that cool into cinders.
Form steep slopes but are relatively small in size.
Frequently occur in groups or clusters.
5.7 Composite Volcanoes (Stratovolcanoes)
Characteristics
Large, classic cone shape, thousands of feet high and several miles wide at the base.
Composed of interbedded lava flows and layers of pyroclastic debris.
Eruptions tend to be violent and explosive.
Most are located adjacent to the Pacific Ocean (the Pacific “Ring of Fire”).
Examples mentioned
Fujiyama (Mount Fuji), Mount St. Helens, Mt. Rainier (illustrative examples in the regional context).
5.8 Volcanic Hazards
Pyroclastic flows
Fiery flows composed of hot gases and ash/debris moving down volcano slopes.
Speeds can reach up to ~.
Lahars
Mudflows generated by interaction of volcanic material with water, following valleys and streams.
Other hazards
Tsunamis: triggered by collapse of volcanic landforms into the ocean.
Volcanic ash and aviation hazards: ash clouds can damage aircraft.
Volcanic gas: poisonous gases posing respiratory hazards.
Climate effects: dust and gases can alter global temperatures and climate patterns.
5.9 Other Volcanic Landforms
Calderas
Steep-walled depressions typically larger than $1~\mathrm{km}$ and formed by collapse of an emptied magma chamber.
Fissure eruptions and basalt plateaus
Fluid basaltic lava erupts from crustal fractures called fissures, forming plateau regions (e.g., Columbia River Plateau).
Lava domes
Bulbous masses of congealed lava; often associated with explosive activity of silica-rich magma.
Can collapse to produce blocky pyroclastic flows.
Volcanic necks
Sturdy, resistant vents left standing after erosion removes the surrounding cone (e.g., Shiprock, New Mexico).
5.10 Plate Tectonics & Volcanism
Global distribution
Volcanic activity is not randomly distributed; most volcanoes align with plate margins.
Divergent boundaries: greatest volume of lava produced.
Convergent boundaries (subduction zones): result in more explosive volcanism.
Ring of Fire
A major, circum-Pacific volcanic belt with many active volcanoes (examples listed in the lecture visuals).
Intraplate volcanism (hot spots)
Activity within a tectonic plate, associated with mantle plumes (hot spots).
Produces basaltic magma sources in oceanic crust (e.g., Hawaii, Iceland) and granitic magma sources in continental crust (e.g., Yellowstone).
Concept: volcanism on a plate moving over a hot spot creates a chain of volcanoes and volcanic features as the plate migrates over the plume head and tail.
Diagrammatic concepts described
A rising mantle plume with a large bulbous head generates large flood basalts when the head rapidly decompresses.
Plate motion over the plume tail creates a linear chain of smaller volcanic structures downstream from the hotspot.
The head forms large continental-scale basaltic plateau volcanism; the tail forms a volcanic trail along the plate.
Summary takeaway on plate tectonics and volcanism
Plate tectonics strongly controls where volcanism occurs and the style of eruptions, with hot spots providing examples of intraplate volcanism and plate boundaries driving most of the global volcanic activity.