Volcanoes
Hawaii's Kilauea has been erupting almost continuously since 1983, building new land as lava flows into the sea. Vesuvius buried Pompeii in seconds. Same planet, completely different volcanoes. In this lesson you'll discover what drives volcanic eruptions, why some are gentle and others explosive, and why Australia has extinct volcanoes despite having no active ones today.
Printable Worksheets
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Q1 · Australia has no active volcanoes today. But we have extinct ones. Does that mean Australia never had volcanic activity? When might it have been active?
Q2 · Two volcanoes: one erupts gently with slow-flowing lava; another explodes violently. What determines how violently a volcano erupts?
● Know
- The three settings where volcanoes form (divergent, subduction, hotspot)
- The three main types of volcanoes and their characteristics
- Australia's volcanic history and the Newer Volcanics Province
● Understand
- Why silica content determines eruption style
- How the Hawaiian hotspot chain formed
- The benefits and hazards of volcanic activity
● Can do
- Compare shield and composite volcanoes in terms of shape, lava, and eruption
- Explain why Australia has no active volcanoes today
- Identify volcanic landforms in Australia
- Volcano
- Magma
- Lava
- Shield volcano
- Composite volcano
- Molten rock above Earth's surface
- An opening in Earth's crust through which magma reaches the surface
- Tall, steep-sided; built from alternating ash and lava; explosive eruptions
- Molten rock below Earth's surface
- Broad, low profile; gentle eruptions of runny basaltic lava
Hawaii's Kilauea volcano has been erupting almost continuously since 1983. Lava flows slowly into the ocean, building new land. Now picture Vesuvius burying Pompeii in seconds. Same planet, completely different volcanoes. What's different? The silica content of their magma.
Magma = molten rock underground. Lava = molten rock above ground. Volcanoes form where magma finds a path to the surface. Three settings:
- Divergent boundaries: Plates pull apart; basaltic magma (low silica, runny) rises to fill the gap. Gentle eruptions. Forms mid-ocean ridges and rift valleys. Most volcanic activity on Earth happens here, mostly underwater.
- Subduction zones (convergent): Dense oceanic plate sinks into the mantle. Water trapped in the oceanic rock lowers the melting point of the mantle above. Silica-rich magma rises through continental crust. High viscosity → explosive eruptions. Forms volcanic arcs (e.g. the Pacific Ring of Fire).
- Hotspots: Plumes of exceptionally hot mantle punching through a plate anywhere, including in the middle of plates. Examples: Hawaii, Yellowstone (USA). The plate moves over the fixed hotspot, creating a chain of volcanoes.
- Divergent boundary → low-silica basaltic lava, gentle eruptions, mid-ocean ridges.
- Subduction zone → high-silica viscous magma, explosive eruptions, volcanic arcs.
- Hotspot → fixed mantle plume, plate moves over it → chain of volcanoes (e.g. Hawaii).
| Type | Shape | Lava type | Eruption style | Example |
|---|---|---|---|---|
| Shield | Broad, low, dome-shaped (like an upside-down shield) | Low-silica basaltic; runny (low viscosity) | Gentle, effusive, lava flows freely | Mauna Loa, Hawaii |
| Composite / Stratovolcano | Tall, steep-sided, symmetrical cone | High-silica; thick, viscous | Violent, explosive, gas trapped, builds pressure | Mt Fuji (Japan), Krakatoa, Ruapehu (NZ) |
| Cinder cone | Small, steep, bowl-shaped crater | Basaltic pyroclasts (loose rock fragments) | Moderate; short-lived; single eruption event | Tower Hill, Victoria; many in Newer Volcanics Province |
Caldera: a large crater left when a magma chamber empties and the summit collapses. Example: Crater Lake, Oregon (USA).
The key difference between shield and composite: silica content. High silica → viscous magma → gas can't escape → pressure builds → explosion. Low silica → runny magma → gas escapes easily → gentle flow.
Australia has no currently active volcanoes, but has extensive evidence of past activity:
- Newer Volcanics Province (Victoria and South Australia): the most recently active volcanic field in Australia. Last erupted approximately 5,000 years ago (Mt Gambier, SA; Mt Eccles, Vic). More than 400 volcanic centres including Tower Hill, Mt Napier, Mt Schank.
- Atherton Tableland (Queensland): volcanic plateau with fertile basaltic soils.
- Hotspot chain: as the Indo-Australian Plate moved north over a hotspot, it left a trail of progressively older, more eroded extinct volcanoes to the north.
Benefits of volcanoes:
- Volcanic soil is extremely fertile, rich in minerals. Australia's best wine regions (Hunter Valley, Barossa) and coffee-growing areas in QLD sit on volcanic soils.
- Volcanoes create new land (Hawaii) and contribute to Earth's long-term carbon cycle.
Hazards: lava flows, ash fall (disrupts aviation and agriculture), pyroclastic flows (fastest and most deadly, superheated gas and rock), lahars (volcanic mudflows).
Two volcanoes: Volcano A has magma with very high silica content. Volcano B has magma with low silica content. Predict: which volcano is more likely to erupt explosively, and why? What role does silica content play in eruption style?
How close was your prediction?
Excellent, silica content → viscosity → gas trapping → explosion is the key chain of reasoning.
Remember: high silica = thicker magma = trapped gas = explosion. Low silica = runny magma = gas escapes = gentle flow.
In your workbook, complete this comparison table for the three main volcano types:
Columns: Volcano type | Shape | Lava viscosity | Eruption style | Example volcano | Where it forms
Fill in all rows for shield, composite, and cinder cone volcanoes.
Answer these questions about Australian volcanic history:
- Where is the Newer Volcanics Province, and when did it last erupt?
- Why does Australia have a chain of extinct volcanoes getting older from south to north?
- Give one benefit of past volcanic activity for Australian farming.
- Should Australians be concerned about future volcanic eruptions? Explain your reasoning.
At the start of the lesson, you read that the last volcano in Victoria erupted around 5,000 years ago, and that Indigenous Australians likely stood witness to it.
Now that you've studied how volcanoes work, imagine you were there. Would it have been a gentle lava flow or a violent explosion? Use what you know about silica and viscosity to explain your answer.
Q1. Describe two different settings where volcanoes form and explain why magma reaches the surface in each. (4 marks)
Q2. Compare shield volcanoes and composite (strato) volcanoes in terms of shape, lava type, and eruption style. (4 marks)
Q3. Explain why Australia has no currently active volcanoes, yet has evidence of past volcanic activity. (3 marks)
Answers
▾MCQ 1
B Low-viscosity (runny) lava flows freely and allows dissolved gases to escape easily, producing gentle effusive eruptions rather than explosive ones.
MCQ 2
B High silica content makes magma viscous (thick and sticky). Viscous magma traps dissolved gases. As the magma rises and pressure drops, the gas can't escape gradually, pressure builds until it ruptures explosively.
MCQ 3
B The Hawaiian Islands formed as the Pacific Plate moved over a fixed hotspot in the mantle. The chain of islands gets progressively older from south-east (Big Island, youngest, Kilauea still active) to north-west (oldest, most eroded).
MCQ 4
D The Newer Volcanics Province in Victoria and South Australia contains more than 400 volcanic centres. It last erupted approximately 5,000 years ago, well within human history and likely witnessed by Indigenous Australians.
MCQ 5
B Volcanic rock (basalt, etc.) is rich in minerals like potassium, calcium, iron and magnesium. When volcanic rock weathers into soil over thousands of years, those minerals become available to plants, making volcanic soils extremely productive for agriculture.
Short Answer 1
Model answer: Setting 1, Subduction zones: where a dense oceanic plate sinks beneath a continental plate, water in the oceanic rock lowers the melting point of surrounding mantle rock. This melts to form silica-rich magma that is less dense than surrounding rock and rises through the continental crust to the surface. Setting 2, Hotspots: a fixed plume of unusually hot mantle material melts through the plate above it, regardless of plate boundary location. Magma rises to the surface through the plate, forming a volcano. As the plate moves, a chain of extinct volcanoes is left behind.
Short Answer 2
Model answer: Shield volcanoes are broad and flat (like an upside-down shield), built from many layers of low-viscosity (runny) basaltic lava. Their eruptions are gentle and effusive, lava flows freely. Example: Mauna Loa, Hawaii. Composite (stratovolcanoes) are tall, steep-sided cones built from alternating layers of ash and lava. Their magma is high in silica, making it viscous, this traps gas under pressure, causing violent explosive eruptions. Example: Mt Fuji, Japan.
Short Answer 3
Model answer: Australia has no currently active volcanoes because it sits in the stable interior of the Indo-Australian Plate, far from active plate boundaries where most volcanic activity occurs. The plate has moved northward over millions of years, moving away from the hotspot that previously fuelled volcanic activity. However, Australia has extensive evidence of past volcanic activity: the Newer Volcanics Province in Victoria and South Australia erupted as recently as ~5,000 years ago, and older extinct volcanic fields are found across Queensland and elsewhere, formed as the plate passed over various hotspots.