Volcano Viscosity Reading Worksheet
Unit 1: Magma, Viscosity & States of Matter
Volcano Viscosity & Eruption Styles
Independent Science Reading
Sub-Ready Credit Recovery
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The Guiding Phenomenon
Why do Hawaiian shield volcanoes gently pour runny red lava into the ocean, while stratovolcanoes like Mount St. Helens explode violently into ash and dust?
1 Part 1: The Two Types of Eruptions
Volcanoes around the world erupt in remarkably different ways. In Hawaii, shield volcanoes erupt basalt lava that is red-hot and runny. This lava bubbles gently out of ground vents or flows as glowing liquid rivers that move slowly down hillsides. Tourists and volcanologists can often stand safely a short distance away to observe.
In contrast, stratovolcanoes like Mount St. Helens in Washington State erupt with sudden, catastrophic violence. Instead of smooth lava streams, stratovolcanoes blast giant columns of gray ash miles into the atmosphere, pulverize solid rock into dust, and unleash superheated avalanches down their slopes.
2 Part 2: What is Viscosity?
The primary scientific property controlling these eruptions is viscosity. Viscosity is a liquid's resistance to flow—essentially how thick or sticky a liquid behaves.
Think of water versus cold honey. Water has low viscosity: it is thin and runny, so it pours immediately. Cold honey has high viscosity: it is thick and sticky, so it flows very slowly. Hawaiian basalt magma has low viscosity, letting it flow smoothly like warm syrup. Stratovolcano magma has high viscosity, making it thick and pasty like cold peanut butter or wet cement.
3 Part 3: Particles & Trapped Gas Pressure
Deep underground, all magma contains dissolved gases (such as water vapor and carbon dioxide). As thermal energy (heat) increases, gas particles absorb kinetic energy and bounce around much faster.
In runny, low-viscosity magma, expanding gas bubbles can rise rapidly and escape harmlessly into the air. But in thick, high-viscosity magma, the sticky molten rock traps the gas bubbles. As the trapped gas particles heat up and expand, they hammer against the sticky magma, building immense pressure until the rock blows apart in an explosive eruption.
Hawaiian Shield Volcanoes
Low Viscosity (Runny) → Gas escapes easily → Gentle, oozing lava
Stratovolcanoes (Mt. St. Helens)
High Viscosity (Thick) → Gas trapped inside → Violent explosion
Turn page over to answer Questions 1–5 on the back
Student Assessment Page
Check for Understanding
Score: _____ / 15 pts
Directions: Use the reading passage on the front page to complete Questions 1–5 independently.
Part A: Multiple Choice (Circle the letter of the best answer)
1. According to Part 2, what does the scientific property viscosity describe?
A How hot rock gets before melting
B A liquid's resistance to flow (thickness)
C The height of a volcano's ash cloud
D The color of cooled volcanic rock
2. Why do Hawaiian shield volcanoes erupt gently with runny lava instead of exploding?
A Low-viscosity magma lets gas bubbles escape easily
B Their magma contains zero gas particles
C Ocean water cools the volcano until it freezes
D Their magma is thick like cold peanut butter
3. According to Part 3, what causes violent pressure to build inside stratovolcanoes like Mt. St. Helens?
A Rainwater fills the crater from above
B Gas particles slow down and stop moving
C Fast-moving gas particles are trapped in thick magma
D Wind pushes the mountaintop downward
Part B: Compare & Contrast (Question 4)
4. Circle the correct word choice inside each pair of brackets to contrast the two magma types:
• Hawaiian Magma: Has [ low / high ] viscosity, which means it flows [ easily / slowly ] and erupts [ gently / violently ].
• Mt. St. Helens: Has [ low / high ] viscosity, which means it flows [ easily / slowly ] and erupts [ gently / violently ].
Part C: Scientific Explanation CER (Question 5)
5. Fill in the blanks below to complete the Claim-Evidence-Reasoning scientific explanation:
Claim & Cause:
"I claim that stratovolcanoes explode violently because their magma is (high / low) viscosity, which causes trapped gas to
Particle Motion & Pressure Reasoning:
"When heat increases, gas particles move (faster / slower) . Because the thick magma will not let gas escape, pressure builds until
Sub Work Checklist: Read Front Q1–3 Circled Q4 Completed Q5 CER Filled
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