What Is a Volcano?
A volcano is a vent or opening in Earth's crust through which magma (molten rock underground), gases, and ash erupt onto the surface. Once magma reaches the surface, it is called lava.
Over many eruptions, the lava and ash build up to form a mountain — a volcanic mountain. The cone shape typical of many volcanoes is built up layer by layer from solidified lava flows and ash deposits.
Structure of a Volcano
A typical volcano has these key features:
- Magma chamber: A large pool of molten rock deep underground where magma collects.
- Main vent: The central channel through which magma rises to the surface.
- Secondary/side vents: Smaller vents that branch off the main vent.
- Crater: The bowl-shaped depression at the top, around the vent opening.
- Cone: The mountain built up from layers of solidified lava and ash.
Some volcanoes have a caldera — a huge, wide crater much larger than a normal volcanic crater. Calderas form when a volcano empties its magma chamber during a massive eruption or when the top collapses inward. Yellowstone in the USA sits on a supervolcano with a caldera 72 km × 55 km wide.
Where Do Volcanoes Form?
Most volcanoes form at tectonic plate boundaries:
1. Destructive (convergent) boundaries: When an oceanic plate is subducted (pushed under) a continental plate, it melts. The magma rises and erupts as a volcano. This creates chains of volcanoes parallel to the coast — such as Mount St Helens in the USA.
2. Constructive (divergent) boundaries: Where plates pull apart, magma rises to fill the gap. Underwater volcanoes form along mid-ocean ridges. Iceland sits on the Mid-Atlantic Ridge and has very active volcanoes.
3. Hot spots: Some volcanoes occur far from plate boundaries above fixed "hot spots" in the mantle where a plume of hot rock rises. The Hawaiian islands are a chain of volcanic islands formed as the Pacific Plate slowly moves over a hot spot.
Types of Volcanoes
| Type | Shape | Eruption style | Example |
|---|---|---|---|
| Shield volcano | Wide, gently sloping | Gentle, fluid lava flows | Mauna Loa (Hawaii) |
| Stratovolcano (composite) | Tall, steep cone | Explosive, mixed lava and ash | Mount Fuji, Mount St Helens |
| Cinder cone | Small, steep | Short eruptions of cinders | Parícutin (Mexico) |
The Pacific Ring of Fire is a band around the Pacific Ocean where about 75% of the world's volcanoes and 90% of earthquakes occur. It follows the boundaries of the Pacific Plate where it meets other plates. Countries on the Ring of Fire include Japan, New Zealand, Chile, and the USA (Alaska).
Volcanic Hazards and Benefits
Hazards: Lava flows, pyroclastic flows (fast-moving hot gas and ash), ash clouds, lahars (volcanic mudflows), and tsunamis triggered by eruptions.
Benefits: Volcanic soil is extremely fertile (rich in minerals). Geothermal energy from volcanic heat is used for electricity in Iceland and New Zealand. Volcanic islands create new land (Hawaii).
Real world: Iceland — living on a volcanic island
Iceland sits directly on the Mid-Atlantic Ridge, where the North American and Eurasian plates pull apart. This makes it one of the most volcanically active places on Earth. Icelanders use geothermal energy from volcanic heat to heat 90% of their homes for free, and Iceland generates nearly all its electricity from renewable volcanic and hydro sources. In 2010, the Eyjafjallajökull eruption shut down European air travel for weeks.
Real world: Mount Etna and Sicilian agriculture
Mount Etna in Sicily, Italy, is Europe's most active volcano and has been erupting regularly for hundreds of thousands of years. Despite (and because of) this, the slopes of Etna are densely farmed — volcanic soil is exceptionally fertile, rich in minerals. The region produces grapes, olives, and citrus fruits that are sold across Europe.
Real world: The 2018 Kilauea eruption in Hawaii
In 2018, the Kilauea shield volcano on Hawaii's Big Island erupted continuously for months, sending rivers of lava through residential neighbourhoods and destroying over 700 homes. Because it is a shield volcano, the lava flowed slowly enough for most residents to evacuate, unlike the explosive eruptions of stratovolcanoes like Mount Pinatubo (Philippines, 1991) which killed hundreds instantly.
Where do you see this in real life? When flights are cancelled due to volcanic ash, or when news reports show lava flowing into the ocean in Hawaii — you are seeing the same volcanic processes described here in action.
Practice Problems
What is the difference between magma and lava?
Show answer
Explain why the Hawaiian islands are not located at a plate boundary but still have active volcanoes.