Volcanoes & Volcanism – Comprehensive Study Notes

Lesson Flow and Context

The session is structured around the teacher’s standard routine:
- Review of the Past Lesson ➜ checks prior knowledge.
- Preview of the New Lesson ➜ primes students for fresh content.
- Discussion / Group Activity / PETA (Performance Task) / Exit Ticket ➜ multiple modes of engagement and assessment.
Knowing this flow allows you to anticipate formative questions at the start ("What have we tackled last time?") and diagnostic questions ("What do you know about today’s lesson?").

Volcanism – the geologic process in which magma escapes from Earth’s interior, eventually cooling and hardening into rock at or below the surface.
- Extrusive (surface) component ➜ lava, volcanoes, igneous surface rocks.
- Intrusive (sub-surface) component ➜ plutons, sills, dikes, batholiths.

A volcano is a landform featuring an opening (vent) through which magma, gases, and pyroclastic material reach the surface.
Although “mountain” is colloquially used, some volcanoes are fissures or depressions; the essential trait is the opening.

Main Vent – primary channel that conveys magma to the surface.
Crater – bowl-shaped (or elongated) surface opening where magma, gases, and tephra exit.
Magma Chamber – sub-surface reservoir storing molten rock; may feed multiple vents.
- (Analogy) Think of the magma chamber as an underground “pressure cooker”; when the lid/vent opens, contents escape.

Extrusive Volcanism – magma breaches the surface; lava flows, volcanic cones, and ash deposits form.
- Leads directly to igneous (extrusive) rocks such as basalt, andesite, rhyolite.
Intrusive Volcanism – magma cools and solidifies below the surface; produces plutonic bodies.
- Results in coarse-grained igneous rocks (granite, gabbro).

All igneous rocks arise from cooling molten material:
- Fast cooling (surface) ➜ fine-grained, glassy textures.
- Slow cooling (sub-surface) ➜ coarse crystals.

Viscosity = resistance to flow.
- High viscosity ➜ sluggish, dome-forming eruptions (rhyolitic).
- Low viscosity ➜ fluid, fountain-like eruptions (basaltic).
Temperature effect: higher temperature → \downarrow viscosity → faster flow.
Silica effect: higher \text{SiO}_2 content → \uparrow viscosity.
- Basalt (~50–52\% silica) is runny; rhyolite (~>70\%) is sticky.
Volatiles ((\text{H}2\text{O}, \text{CO}2)) lower viscosity but can drive explosivity when trapped.

Hotspot – surface expression of a stationary (relative to plates) mantle plume.
- Deep, buoyant column of hot rock rises, partially melts, delivers magma.
Formation Sequence
1. Plume head melts crust, producing volcano on ocean floor.
2. Plate motion carries the volcano away; activity ceases → extinct volcano.
3. New volcano forms directly above plume ➜ chain develops.
Hawaiian Example
- Island progression (old ➜ young): Kauai → Oahu → Molokai → Maui → Hawaii.
- Direction of Pacific Plate motion is toward the northwest; plume remains fixed.
- Youngest, most active center = Hawaii (Big Island) situated above plume.

Rising magma fractures rock, producing earthquakes (precursors).
Continuous tremor + shallow quakes near the volcano often herald an eruption.
Material that builds cones or shields accumulates layer by layer, shaping new landforms.

During eruption, a volcano can simultaneously emit:
- Lava flows (basaltic dominant).
- Pyroclastic materials (ash, lapilli, bombs, blocks).
- Volcanic gases (major driver of atmospheric impact).
- Earthquakes (generally mild to moderate, but can trigger landslides or tsunamis if undersea).

Chemical composition frequency
- Basaltic ≈ > majority.
- Andesitic = moderate fraction.
- Rhyolitic = small fraction.
Flow Morphology
- Pahoehoe – smooth, ropey; forms when surface skin cools while interior stays fluid.
- Aa – rough, jagged, clinker-covered; forms when viscosity increases and the flow fractures.
- Rule of thumb: Pahoehoe can transition to Aa as it cools and degasses.

Typical composition by volume:
- Water vapor ≈ 70\%.
- Carbon dioxide ≈ 15\%.
- Nitrogen ≈ 5\%.
- Sulfur dioxide ≈ 5\%.
- Minor: hydrogen sulfide, hydrogen halides ((\text{HCl}, \text{HF}, \text{HBr})).
Implications:
- Health & environmental – acid rain, respiratory issues.
- Climate – (\text{SO}_2) aerosols can induce short-term cooling.

Volcanic ash – tiny (<2 mm) fragments of glassy volcanic rock.
- Can disrupt aviation, damage machinery, bury crops.
Larger ejecta: lapilli (2–64 mm), bombs/blocks (>64 mm).

Soil Fertility – ash and weathered lava supply nutrients ((\text{K}, \text{Fe}, \text{Mg})).
Ore Genesis – hydrothermal systems mobilize metallic minerals: Cu, Au, Ag.
Atmospheric Cooling – stratospheric aerosols reflect sunlight.
Landform Creation – islands (e.g., Surtsey), plateaus, fertile plains.
Water Production – volcanic outgassing historically contributed to Earth’s hydrosphere.

Plate Tectonics – hotspot tracks record plate velocities.
Hazard Mitigation – monitoring gases & seismicity saves lives (e.g., Pinatubo 1991).
Climate Science – large eruptions (Tambora 1815) can trigger “Year Without a Summer.”
Agriculture & Economy – volcanic soils sustain high-population regions (Java, Andes).
Ethics & Policy – balancing geothermal exploitation vs. cultural significance (e.g., Māori sacred sites near Taupō).

Viscosity ∝ \dfrac{\text{SiO}_2}{T} (qualitative: more silica, lower temperature → higher viscosity).
Gas release proportion: \text{H}2\text{O}:\text{CO}2}:\text{N}2}:\text{SO}2}=70:15:5:5 (remainder trace gases).
Relative age along the Hawaiian chain:
\text{Age}\;\uparrow\;\Leftrightarrow\;\text{Distance from Hotspot}\;\uparrow