Volcanic Eruption Styles & Magma Properties

REF STUDY GUIDE: Big Ideas Use to answer form questions

1.Silica & Viscosity: Magma's silica content determines its viscosity, which controls eruption style. High-silica magma (rhyolitic, 63–77% SiO₂) is thick and traps gases — when pressure builds, it explodes violently (Plinian eruption). Low-silica magma (basaltic, 45–52%) flows easily and releases gases gently, producing slow lava flows (Hawaiian eruption). The causal chain is: silica → viscosity → gas trapping → eruption violence.

2.Eruption Types: Hawaiian and Plinian eruptions differ in nearly every measurable way. Hawaiian eruptions (like Kilauea) produce rivers of runny basaltic lava that tourists can safely watch from nearby. Plinian eruptions (like Mt. St. Helens, 1980) launch columns of ash 30+ km high and generate pyroclastic flows at 700°C traveling 100+ km/hr. The eruption type depends on the tectonic setting: convergent boundaries recycle silica-rich crust, producing explosive volcanoes, while hot spots and divergent boundaries tap low-silica mantle material.

3.Volcanic Hazards & Monitoring: Contrary to what most people assume, lava is not the deadliest volcanic hazard. Pyroclastic flows (superheated gas-and-rock clouds) and lahars (volcanic mudflows that can bury entire towns, as at Nevado del Ruiz in 1985) cause far more deaths. Scientists monitor seismicity, ground deformation, and gas emissions to predict eruptions days to weeks in advance — no single warning sign is reliable alone.

DRAW KEY TERMS Google each → Images tab → sketch what you see

Viscosity

A liquid's resistance to flow (high viscosity = thick, low = runny)

Pyroclastic Flow

Fast-moving cloud of hot gas, ash, and rock (700°C, 100+ km/hr)

Silica

Silicon dioxide (SiO₂) — determines magma viscosity

Lahar

Volcanic mudflow created when water mixes with volcanic ash

OTHER VOCAB: magma · eruption · volcano · effusive · explosive · basaltic · rhyolitic

WRITE FOCUS QUESTION Your initial thinking

In 1980, Mt. St. Helens in Washington exploded with the force of 500 atomic bombs, killing 57 people and flattening 230 square miles of forest. Meanwhile, tourists in Hawaii safely photograph Kilauea's lava flows from just meters away. Both are volcanoes — both involve magma reaching the surface — yet their eruptions could not be more different.

Why do some volcanoes explode violently while others ooze slowly?

I think...

WRITE AFTER HOOK FORM Reflect before continuing

I learned that...

Station 1 — Magma Viscosity Investigation

I observed that...

This shows that...

The key concept is...

CHECK DAY 1 CHECK-IN Before you leave

Q1: Based on today's investigation of volcanic eruption styles & magma properties, explain one key finding using evidence from your data. Connect your answer to this week's focus question: Why do some volcanoes explode violently while others ooze slowly?

Confidence:

Still building Getting clearer Ready to explain

Station 2 — Eruption Type Analysis

The data shows...

This connects to what we learned because...

I can conclude that...

Station 3 — Eng. Design

The main idea is...

This connects to what we learned because...

I can apply this by...

DRAW CONCEPT MODEL Show your understanding visually

Draw a model showing the silica → viscosity → eruption style chain. Include labels and arrows to explain relationships.

Labels included

Relationships shown

Key terms used

CHECK TEST YOUR THINKING Rate each: FALSE ← 1–2–3–4–5 → TRUE

1. All volcanic eruptions are violent explosions like Mt. St. Helens.

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2. Lava is the most dangerous volcanic hazard and kills the most people.

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3. Volcanic eruptions happen randomly without any warning signs.

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4. All magma is the same — the differences between eruptions come from the shape of the volcano.

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5. High-silica magma traps gases and produces explosive eruptions, while low-silica magma releases gases gently and produces slow lava flows.

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Pick any TWO statements — especially any you rated 2, 3, or 4. Justify your rating with evidence:

Statement #___ — I rated it ___ because:

THINK SCIENTIST CIRCLE DEBRIEF Reflect after class discussion

AHA! Moment

The one I changed my thinking most about is...

Remaining Question

I still wonder...

WRITE FOCUS QUESTION REVISIT Final written response

Why do some volcanoes explode violently while others ooze slowly?