Eruption Slides
Earth Science Grades 6–8 Unit: Plate Tectonics
Planetary Geology In Action
ERUPTION!
Investigating magma chambers, tectonic fault lines, and the explosive forces shaping Earth's crust.
Interactive Science Lecture Slide 1 / 6
Core Distinction
Magma vs. Lava
Slide 2 / 6
MAGMA
Subsurface
Molten rock stored beneath Earth's crust
- Trapped under extreme lithospheric pressure
- Contains dissolved volatile gases (H₂O, CO₂, SO₂)
- Cools slowly to form intrusive igneous rock (granite)
LAVA
Surface
Molten rock that has breached the surface
- Directly contacts atmospheric air or ocean water
- Gases rapidly escape into the air during venting
- Cools quickly to form extrusive igneous rock (basalt)
Memory Hook: Magma is inside the Mantle; Lava has Left the crust!
Morphology & Anatomy
Three Volcano Architectures
Slide 3 / 6
Shield
Broad & Gentle
Formed by low-viscosity, runny basaltic lava spreading across vast distances.
Eruption Style: Effusive, steady
Example: Mauna Loa, Hawaii
Composite
Steep & Towering
Alternating strata of explosive pyroclastic ash and thick, sticky lava flows.
Eruption Style: Highly explosive
Example: Mt. St. Helens, Fuji
Cinder Cone
Small & Cone-Shaped
Constructed from ejected tephra and gas-charged cinders that rain around the vent.
Eruption Style: Short-lived bursts
Example: Parícutin, Mexico
Magma silica content determines viscosity: higher silica = stickier, more violent eruptions.
Global Tectonics
The Pacific Ring of Fire
Slide 4 / 6
75%
Of all active and dormant volcanoes on Earth reside along this horseshoe rim.
90%
Of global earthquakes occur along these convergent tectonic margins.
The Subduction Engine
Dense oceanic tectonic plates plunge beneath lighter continental plates at convergent boundaries.
1 Water-rich ocean crust descends into the hot asthenosphere.
2 Subducting slab releases fluids, melting overlying mantle rock.
3 Buoyant magma plumes rise upward to punch volcanic arcs.
Key Geographic Zones: The Cascades, Andes, Aleutian Islands, Japan, New Zealand
Thermodynamics
Why Do Volcanoes Explode?
Slide 5 / 6
Gas Expansion
Underground pressure keeps gases dissolved in liquid magma. As magma rises toward the surface, pressure drops precipitously.
The Soda Can Analogy: Opening a shaken can releases trapped CO₂ bubbles violently. Magma chamber degassing behaves identically!
Viscosity Trap
Lava viscosity (resistance to flow) is governed by silica chemistry. Thick magma prevents gases from escaping peacefully.
The Pressure Cork: High-viscosity magma plugs the volcanic conduit until gas pressure shatters the mountain peak.
High Gas Concentration + High Viscosity Magma = Catastrophic Eruption
Review & Reflection
Check Your Knowledge
Slide 6 / 6
Q1
Location & Cooling
Why does magma cool substantially slower than lava, and what texture does this produce in the resulting rock?
Q2
Viscosity Comparison
Why are Hawaiian shield volcanoes generally safer to observe than stratovolcanoes like Mount St. Helens?
Q3
Tectonic Mystery
If 75% of volcanoes are on plate boundaries (Ring of Fire), what geologic anomaly created Hawaii in the middle of the Pacific Plate?
Turn & Talk: Share your hypothesis for Q3 with your table partner. Classroom Discussion
Eruption Analysis Worksheet
Earth Science Investigation
ERUPTION ANALYSIS WORKSHEET
Magma Chambers, Landform Architecture & Tectonic Forces
Name:
Date: Period:
1 Comparative Matrix: Magma vs. Lava
Fill in each feature for molten rock below and above the crust
| Geologic Characteristic | Magma (Subsurface) | Lava (Surface) |
|---|
| Primary Location | | |
| | |
|
| Gas Behavior & Pressure |
|
|
| Cooling Rate (Speed) |
|
|
| Igneous Rock Type Formed |
|
|
| Example Rock Formed |
|
|
2 Volcano Architectures & Eruption Profiles
Match key dynamics to each volcano type
Shield Volcano e.g. Mauna Loa
Broad, gentle slopes with low-viscosity basaltic lava flows.
Eruption Style:
Viscosity & Silica Level:
Sketch Profile Outline:
Composite / Strato e.g. Mt. Rainier
Steep peaks made of alternating tephra, ash, and sticky lava.
Eruption Style:
Viscosity & Silica Level:
Sketch Profile Outline:
Cinder Cone e.g. Parícutin
Steep, smaller conical hills built from piled explosive cinders.
Eruption Style:
Viscosity & Silica Level:
Sketch Profile Outline:
Unit 3: Earth’s Dynamic Crust Turn page for Plate Tectonics & Viscosity Analysis → Page 1 of 2
Eruption Analysis • Page 2
Tectonic Drivers & Pressure Physics
Student Name:
3 The Subduction Engine & The Pacific Ring of Fire
Explain how convergent plate boundaries generate the magma feeding over 75% of active global volcanoes. Complete the 3-step sequence below:
Step 1: Plate Collision & Subduction
Step 2: Flux Melting in the Mantle Wedge
Step 3: Magma Plume Ascent & Volcanic Arc Formation
Ring of Fire Synthesis Question:
Why does the Pacific plate boundary create volatile, explosive volcanoes rather than quiet lava flows?
4 Pressure Cooker Physics: Gas & Viscosity Scenarios
Scenario A: The Uncapped Soda Bottle
In your own words, explain how opening a vigorously shaken carbonated beverage models dissolved volcanic gas decompression when magma ascends.
