Volcanoes

Volcanoes

Middle School

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Learning Objectives

• Define what a volcano is and identify its main parts.

• Explain how tectonic plate movement leads to volcano formation.

• Differentiate between the main types of volcanoes and their eruption styles.

• Describe volcanic hazards and impacts, and classify volcanoes by their activity.

Key Vocabulary

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Magma

Molten rock with dissolved gases and crystals that is found beneath the Earth's surface.

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Lava

Molten rock that flows from a volcano or vent during an eruption onto the surface.

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Viscosity

A measure of a fluid's resistance to flow, which affects a volcano's eruption style.

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Tectonic Plates

Massive, rigid slabs of the Earth's lithosphere that are constantly moving and interacting with each other.

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Hotspot

A fixed area in the mantle where hot plumes of magma rise to create volcanoes.

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Caldera

A large depression formed when a volcano collapses inward on itself after a massive eruption.

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What is a Volcano?

• A volcano is a vent in the Earth's crust for escaping magma.

• Molten rock is magma below the surface, and lava after it erupts.

• Main parts are the magma chamber, vent, crater, and cone-shaped mountain.

• Eruptions can be constructive, building land, or destructive, causing massive explosions.

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Multiple Choice

What is the primary difference between magma and lava?

• Magma creates destructive eruptions, while lava creates constructive ones.

• Magma is molten rock beneath the Earth's surface, while lava is molten rock on the surface.

• Magma is found in the crater, while lava is in the vent.

• Magma is solid, while lava is liquid.

How and Where Volcanoes Form

• Volcanoes form when one tectonic plate subducts, or sinks, under another.

• Volcanoes also form where two tectonic plates are pulling apart from each other.

• Hotspots create volcanoes as a tectonic plate moves over a plume of magma.

Multiple Choice

How is a hotspot volcano chain, like the Hawaiian Islands, formed?

• Two oceanic plates collide, and one subducts beneath the other.

• A tectonic plate moves over a stationary plume of hot mantle material.

• Two tectonic plates pull apart, allowing magma to fill the gap.

• A continuous series of earthquakes creates a long volcanic belt.

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Magma Chemistry and Eruption Style

Low-Silica Magma

• ​This magma has low viscosity, meaning it is thin and fluid, allowing it to flow very easily.

• ​​Dissolved gases can escape easily from the fluid magma, preventing a large buildup of pressure.

• ​This results in gentle, effusive eruptions where lava flows steadily from the volcano's vent.

High-Silica Magma

• ​This magma has high viscosity, which means it is thick, sticky, and does not flow easily.

• ​​Gas bubbles get trapped within the thick magma, causing a massive buildup of pressure as it rises.

• ​The trapped pressure is released suddenly, leading to powerful and dangerous explosive eruptions.

Multiple Choice

Which combination of properties is most likely to cause an explosive volcanic eruption?

• Low silica content and low levels of dissolved gases.

• High silica content and high levels of trapped gases.

• High viscosity and easily escaping gases.

• Low viscosity and high levels of trapped gases.

Types of Volcanoes

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Composite Volcanoes

• ​These have large, steep-sided cones with alternating layers of material.

• ​​They are formed from thick, high-viscosity magma causing explosive eruptions.

• ​These volcanoes are typically found at convergent plate boundaries.

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Shield Volcanoes

• ​These are broad volcanoes that have very gentle slopes.

• ​​They are built from fluid, low-viscosity lava flows.

• ​Their eruptions are non-explosive and lava flows out effusively.

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Cinder Cones

• ​These are the smallest and simplest type of volcano.

• ​​They are steep, cone-shaped hills made of ejected lava fragments.

• ​They often form on the sides of other larger volcanoes.

Multiple Choice

A large, broad volcano with gentle slopes is discovered. What type of volcano is it, and what kind of eruption does it likely have?

• A shield volcano with effusive eruptions.

• A cinder cone with mildly explosive eruptions.

• A composite volcano with explosive eruptions.

• A stratovolcano with effusive eruptions.

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Volcanic Hazards

Lava Flows

• Lava flows are streams of molten rock that pour from a volcanic vent.

• They are very destructive to property but are typically slow-moving, posing little threat to people.

• Another hazard, ash fall, can cover huge areas and cause roofs to collapse.

Pyroclastic Flows

• These are fast-moving avalanches of superheated gas, ash, and rock that destroy everything.

• As the most dangerous volcanic hazard, they are a direct threat to life.

• Mudflows, or lahars, are also highly destructive and can travel far from the volcano.

Multiple Choice

What is a pyroclastic flow?

• A fast-moving, superheated avalanche of gas, ash, and rock.

• A slow-moving stream of molten rock.

• A cloud of volcanic ash that falls from the sky.

• A mixture of volcanic debris and water that flows like concrete.

Impacts and Monitoring

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Positive Impacts

• ​Volcanic activity helps to create very fertile soils for farming.

• ​​It provides geothermal energy, a source of renewable power.

• ​New landforms and landscapes are created, which attracts tourism.

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Volcanic Features

• ​Calderas are large craters formed when a volcano collapses.

• ​​Geysers and hot springs are created by underground heat.

• ​Hardened magma forms intrusive features like dikes and sills.

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Classification & Monitoring

• ​Volcanoes are classified as active, dormant, or extinct.

• ​​Active ones erupt, while dormant ones are temporarily quiet.

• ​Scientists monitor ground changes and gas to predict eruptions.

Multiple Choice

Which of the following is a positive impact of volcanic activity?

• The creation of fertile soils for agriculture.

• The formation of destructive mudflows or lahars.

• The release of ash, which disrupts air travel.

• The collapse of a volcano's summit to form a vent.

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Common Misconceptions About Volcanoes

Misconception	Correction
All volcanic eruptions are violent and explosive.	Many eruptions are gentle, with steady lava flows.
Lava flows are the greatest danger to people.	Pyroclastic flows and lahars are much more dangerous.
Volcanoes are only found in hot climates.	They are found in cold regions, based on plate tectonics.
An extinct volcano can never erupt again.	A dormant volcano is quiet but could reawaken.

Multiple Choice

Why do volcanoes at divergent boundaries, like mid-ocean ridges, typically have non-explosive, effusive eruptions?

• The magma has high silica content, which makes it less fluid.

• These areas experience fewer earthquakes to trigger an eruption.

• The pressure from the overlying water prevents explosive eruptions.

• Magma there has low silica content and low viscosity, allowing gases to escape easily.

Multiple Choice

How does the formation of a volcanic island arc at a convergent boundary differ from the formation of a volcanic chain at a hotspot?

• Island arcs are made of shield volcanoes, while hotspot chains are made of composite volcanoes.

• Island arcs form in the middle of a plate, while hotspot chains form at plate edges.

• Island arcs form from a subducting plate, while hotspot chains form as a plate moves over a stationary magma plume.

• Island arcs are formed by effusive eruptions, while hotspot chains are formed by explosive eruptions.

Multiple Choice

A newly discovered volcano is large and broad with very gentle slopes. What can you predict about its magma composition and typical eruption style?

• It is a cinder cone with mildly explosive eruptions of gas-rich lava.

• It likely has low-viscosity, low-silica magma and effusive eruptions.

• It likely has high-viscosity, high-silica magma and explosive eruptions.

• It is a lava dome with thick, rhyolitic lava that piles up.

Multiple Choice

If scientists monitoring a dormant composite volcano detect increased ground swelling, frequent small earthquakes, and more sulfur dioxide gas, what might this indicate?

• That low-viscosity magma is rising, which will lead to a gentle, effusive eruption.

• That the volcano is cooling down and becoming extinct.

• That high-viscosity magma and gas are accumulating, potentially leading to an explosive eruption.

• That a lava dome is collapsing within the crater.

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Summary

• Volcanoes form at plate boundaries and hotspots due to tectonic plate movement.

• Magma's silica and gas content control if eruptions are explosive or effusive.

• The main volcano types are composite, shield, and cinder cone.

• They have hazards like ash fall and benefits like creating fertile soil.

• Scientists monitor volcanoes to predict eruptions by tracking ground changes and gas emissions.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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