Week 3: Volcanic Eruption Styles & Magma Properties
Grade 7 Science | Rosche
The Phenomenon: The Explosive vs Oozing Mystery
Anchoring Context & Focus Question
Why do some volcanoes explode violently while others ooze slowly?
Before We Begin: Activate Your Prior Knowledge
Think back to Weeks 1 & 2: You learned that volcanoes form at plate boundaries. This week, we investigate WHY the same process — magma reaching the surface — produces such wildly different eruptions. The answer lies in the chemistry of the magma itself.
Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!
| Term | Spanish | Definition |
|---|---|---|
| viscosity | viscosidad | A liquid's resistance to flow (high viscosity = thick, low = runny) |
| pyroclastic flow | flujo piroclástico | Fast-moving cloud of hot gas, ash, and rock (700°C, 100+ km/hr) |
| magma | magma | Molten rock beneath Earth's surface |
| silica | sílice | Silicon dioxide (SiO₂) — determines magma viscosity |
| lahar | lahar | Volcanic mudflow created when water mixes with volcanic ash |
| eruption | erupción | Violent expulsion of magma, ash, and gases from a volcano |
| volcano | volcán | Opening in Earth's crust where magma reaches the surface |
Hook – The Explosive vs Oozing Mystery
Compare Mt. St. Helens and Hawaiian volcano eruption styles.
The Explosive vs Oozing Mystery
What You'll Do
- Compare Mt. St. Helens (1980) and Kilauea eruption footage or images
- Record observations about how each eruption looks and behaves
- Generate a hypothesis: what causes the difference?
- Connect to plate boundary types from Week 1
Eruption Comparison Data
| Feature | Mt. St. Helens (WA) | Kilauea (Hawaii) |
|---|---|---|
| Eruption Style | Explosive — massive ash cloud | Effusive — slow lava flows |
| How Dangerous? | 57 killed in 1980 | Tourists watch safely nearby |
| Plate Boundary | Convergent (Juan de Fuca) | Hot spot (mid-plate) |
| Lava Color | Lighter gray-pink | Darker reddish-black |
Click to expand
Worked Example and Simulation – Predicting Eruption Style from Magma Composition
Worked Example and Simulation: Predicting Eruption Style
The Causal Chain: Silica → Viscosity → Eruption Style
(thick, sticky)
(can't escape)
eruption
| Magma Type | Silica % | Viscosity | Eruption Type | Example |
|---|---|---|---|---|
| Basaltic | 45–52% | Low | Effusive (Hawaiian) | Kilauea, Iceland |
| Andesitic | 52–63% | Medium | Moderate | Pinatubo, Merapi |
| Rhyolitic | 63–77% | High | Explosive (Plinian) | Mt. St. Helens, Yellowstone |
Simulation: Magma Viscosity
PREDICT (before running the sim)
Look at the simulation controls. Before changing any variables, predict what will happen when you adjust them. Write your prediction down.
OBSERVE (while using the sim)
Change one variable at a time. Record what happens after each change. Use the data journal to capture at least 3 trials.
EXPLAIN (after collecting data)
Compare your observations with your prediction. Use scientific vocabulary to explain the patterns you found. What surprised you? What confirmed your thinking?
Station 1 – Magma Viscosity Investigation
Connect silica content to magma flow and eruption style.
Viscosity Investigation
Viscosity is a liquid's resistance to flow. High-viscosity magma is thick and sticky (like honey); low-viscosity magma flows easily (like water). Silica molecules form chains that "tangle" and resist movement.
| Magma Sample | Silica Content | Flow Rate | Texture | Eruption Type |
|---|---|---|---|---|
| Sample A (Basalt) | 48% | Fast | Runny, like water | Effusive |
| Sample B (Andesite) | 57% | Medium | Like syrup | Moderate |
| Sample C (Rhyolite) | 70% | Slow | Like cold honey | Explosive |
Pattern to Find: As silica content increases, what happens to viscosity? What happens to eruption style?
Station 2 – Eruption Type Analysis
Compare explosive vs effusive eruptions and predict hazards.
Eruption Type Comparison
| Feature | Hawaiian (Effusive) | Plinian (Explosive) |
|---|---|---|
| Magma Type | Basaltic (low silica) | Rhyolitic (high silica) |
| Eruption Speed | Slow lava flows | Instant explosion |
| Main Hazards | Lava flows, laze | Pyroclastic flows, ash, lahars |
| Plate Boundary | Divergent or hot spot | Convergent (subduction) |
Volcanic Hazard Ranking
- Pyroclastic flows — 700°C gas + rock cloud, 700 km/h. Most deadly.
- Lahars — volcanic mudflows. Buried entire towns (Nevado del Ruiz, 1985).
- Ash fall — collapses roofs, destroys crops over wide areas.
- Lava flows — slow, usually survivable but destroys property.
- Volcanic gases — CO₂/SO₂ can asphyxiate in valleys.
Station 3 – Design a Volcano Monitoring System
Apply eruption knowledge to hazard mitigation.
Design a Volcano Monitoring System
Volcanologists use multiple data streams to predict eruptions. No single signal is reliable alone — you need a system. Your job is to design a monitoring network for a high-risk volcano near a city.
| Warning Sign | What It Detects | Sensor Type | Warning Time |
|---|---|---|---|
| Seismicity | Magma movement causes small earthquakes | Seismometer | Days to weeks |
| Ground Deformation | Magma inflates volcano like a balloon | GPS / Tiltmeter | Days to weeks |
| Gas Emissions | Rising SO₂ means fresh magma | Gas spectrometer | Hours to days |
| Temperature | Groundwater heats as magma rises | Thermal camera | Hours to days |
Design Challenge: If you could place only 3 sensors on a volcano near a city of 50,000 people, which would you choose and where? Justify your choices in your form response.
Exit Ticket – Volcanic Systems Integration
Synthesize understanding of eruption styles and magma properties.
Show What You Know
You've investigated magma viscosity, eruption types, volcanic hazards, and monitoring systems. Now connect it all: trace the path from silica content → eruption style → specific hazards → monitoring need.
Sentence Starters:
- "Rhyolitic magma produces explosive eruptions because..."
- "The most dangerous volcanic hazard is pyroclastic flows because..."
- "Volcanoes near convergent plate boundaries are more dangerous than those at hot spots because..."
Enrichment & Extension
Optional content if you finish early or want to go deeper.
Week 3 Complete!
Next Week: Earth's Interior Structure & Evidence — how do we know what's inside?