Mountains of fire
A volcano is an opening in Earth’s crust where melted rock, gases and ash escape. Over time, the erupted material builds up into a mountain or hill. There are about 1350 potentially active volcanoes on land around the world, and many more under the oceans.
Inside a volcano
| Part | Description |
|---|---|
| Magma chamber | Underground pool of magma |
| Conduit (pipe) | Channel that carries magma upward |
| Vent | Opening at the surface |
| Crater | Bowl-shaped dip at the top |
| Side vents | Smaller openings on the volcano’s slopes |
Magma is called lava once it reaches the surface. Lava can be hotter than 1100 °C.
Why do volcanoes erupt?
Magma is less dense than the solid rock around it, so it rises. It also contains dissolved gases (mostly water vapour and carbon dioxide). As magma rises, pressure drops and the gases form bubbles, like opening a shaken soda bottle. If the pressure builds up enough, the volcano erupts.
Where volcanoes form
| Location | Why | Examples |
|---|---|---|
| Subduction zones | One plate sinks under another and melts | Andes, Japan, Cascades (Mount St. Helens) |
| Divergent boundaries | Plates pull apart; magma rises | Iceland, mid-ocean ridges |
| Hot spots | A plume of hot mantle rises under a plate | Hawaii, Yellowstone |
About three-quarters of the world’s active volcanoes are in the Ring of Fire around the Pacific Ocean.
Types of volcanoes
| Type | Shape | Lava | Eruptions | Example |
|---|---|---|---|---|
| Shield volcano | Wide, gentle slopes | Runny basalt | Gentle lava flows | Mauna Loa, Hawaii |
| Stratovolcano (composite) | Tall, steep cone | Thick, sticky | Explosive | Mount Fuji, Mount St. Helens |
| Cinder cone | Small, steep hill | Bits of lava blown into the air | Short-lived | Parícutin, Mexico |
| Caldera | Huge collapsed crater | Varies | Can be enormous | Yellowstone, Crater Lake |
Thick, sticky lava traps gas, leading to violent explosions. Runny lava lets gas escape easily, causing gentler eruptions.
Volcanic hazards
| Hazard | Danger |
|---|---|
| Lava flows | Destroy property, but usually move slowly enough to escape |
| Pyroclastic flows | Avalanches of hot gas and ash moving over 100 km/h at hundreds of degrees; the deadliest hazard |
| Ash clouds | Collapse roofs, ruin crops, damage airplane engines |
| Lahars | Mudflows of ash and water that bury towns |
| Volcanic gases | Can suffocate or poison |
Famous eruptions
| Volcano | Year | What happened |
|---|---|---|
| Vesuvius, Italy | 79 CE | Buried Pompeii and Herculaneum under ash, preserving them |
| Tambora, Indonesia | 1815 | Largest eruption in recorded history; caused the "Year Without a Summer" in 1816 |
| Krakatoa, Indonesia | 1883 | Explosion heard about 4800 km away; tsunamis killed tens of thousands |
| Mount St. Helens, USA | 1980 | The side of the mountain blew out; 57 people died |
| Pinatubo, Philippines | 1991 | Ash cooled global temperatures by about 0.5 °C for a year or two |
| Hunga Tonga, Tonga | 2022 | Underwater eruption sent a pressure wave around the world |
Volcanoes in Canada
Canada has volcanoes too! Most are in British Columbia and Yukon. Mount Meager in BC had a large eruption about 2400 years ago. The Tseax Cone in northern BC erupted around 1700, and its lava flow killed many Nisga’a people. Mount Garibaldi, near Whistler, is also a volcano.
Benefits of volcanoes
- Fertile soil: volcanic ash makes rich farmland.
- Geothermal energy: Iceland heats most of its homes with volcanic heat.
- New land: Hawaii and Iceland were built by volcanoes.
- Minerals: volcanoes bring up valuable minerals and gemstones.
Predicting eruptions
Scientists called volcanologists watch for warning signs:
- small earthquakes beneath the volcano
- the ground swelling as magma rises
- changes in gas released from vents
- rising temperatures
These signs helped save thousands of lives at Pinatubo in 1991.
Common mistakes
- Thinking all volcanoes erupt explosively.
- Thinking lava is the deadliest hazard. Pyroclastic flows and lahars are far deadlier.
- Thinking volcanoes only exist in hot places. Iceland and Antarctica have active volcanoes.
Try it yourself
- Build a model volcano and use baking soda and vinegar to simulate an eruption. What does the fizzing represent?
- Compare honey and water poured down a ramp. Which is like runny lava? Which would trap more gas?
- Find the Ring of Fire on a world map and mark five volcanoes.
Quick recap
- Volcanoes are openings where magma, gas and ash erupt.
- Most form at plate boundaries; some form over hot spots.
- Shield volcanoes erupt gently; stratovolcanoes erupt explosively.
- Pyroclastic flows and lahars are the deadliest hazards.
- Volcanoes also create fertile soil, new land and geothermal energy.