Not to scale: the volcano is drawn much steeper and the eruption smaller than in real life. The eruption styles come from a simplified model: real eruptions also depend on how much magma there is, how fast it rises and whether it meets water.
Gas is dissolved in magma like the bubbles in a fizzy drink. As magma rises, the pressure drops and the gas forms bubbles. In runny magma the bubbles escape easily; in sticky, silica-rich magma they are trapped, the pressure builds, and the magma can shatter into ash. Tap or drag on the chart to choose a magma.
Tap the chart to choose a magma. Dots: real volcanoes.
Inside the vent: bubbles grow as the magma rises.
The VEI measures how big an explosive eruption is, mainly from the volume of ash and rock thrown out. From VEI 2 up, each step is about 10 times more material. The highlighted boxes are the sizes this kind of eruption usually reaches; how big a real eruption gets depends mostly on how much magma comes out.
The shape of a volcano depends mostly on its magma: runny lava spreads out into a broad shield, sticky lava and ash pile up into a steep cone, and frothy lumps of cinder build a small, steep hill.
Other volcanic features: calderas (huge hollows left when the ground collapses after an eruption, such as Yellowstone or Krakatau), lava domes (mounds of very sticky lava), fissure vents (long cracks that pour out lava, common in Iceland) and thousands of underwater volcanoes along the mid-ocean ridges. In the Smithsonian list of 1,215 volcanoes active in the last 11,700 years, 727 are stratovolcanoes or complex volcanoes, 113 are shields, 190 are cinder cones and volcanic fields, 85 are calderas and 100 are other kinds.
Every triangle is one of the 1,215 volcanoes that have been active in the last 11,700 years (the Holocene), from the Smithsonian Global Volcanism Program. Turn on the plate boundaries and look for the pattern: most volcanoes sit where plates sink under each other, especially around the Pacific Ocean.
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Drag to move, scroll or pinch to zoom, tap a triangle for its facts. The Ring of Fire here means the volcanoes at subduction zones around the Pacific, from New Zealand through Indonesia, Japan and Alaska to the Andes. Data: Smithsonian Institution Global Volcanism Program, Volcanoes of the World (2026 snapshot); plate boundaries: Bird (2003); Earth image: NASA.
The volcano cross-section you draw and label in a test: the magma chamber, main vent, crater, side vent, dike and sill, the layers of a stratovolcano, and an eruption. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A printable worksheet: label the volcano diagram, sort gentle and explosive eruptions, match the three types, multiple choice and explain questions, with an answer key.
Earth’s mantle is very hot, but almost all of it is solid rock. In a few places a small part of it melts: where plates move apart and the pressure on the rock drops, where a sinking plate releases water that lowers the melting point of the rock above it, and over hotspots. The molten rock is called magma.
Magma is less dense than the solid rock around it, so it slowly rises, like a blob in a lava lamp. It often collects a few kilometres down in a magma chamber. When enough magma and gas build up, it forces its way up through cracks and the main vent and erupts at the crater. Magma that reaches the surface is called lava.
Magma holds dissolved gas, mostly water vapour, carbon dioxide and sulfur gases, like a fizzy drink holds carbon dioxide. Near the surface the pressure falls and the gas forms bubbles. Basalt magma (about 50% silica) is runny, so the bubbles escape and the lava flows out gently. Andesite, dacite and rhyolite magma (about 57–75% silica) is thousands to billions of times stickier: the bubbles cannot escape, the pressure builds, and the magma shatters into ash in an explosive eruption.
Many eruptions of runny lava build a broad shield volcano. Sticky lava and ash pile up in layers into a steep stratovolcano. Lumps of frothy lava thrown from one vent build a small cinder cone.
Choose Shield and press Erupt!: lava flows quietly. Then choose Pinatubo 1991 and compare. In the chart, keep the gas the same and slide the silica from left to right: watch the bubbles get trapped and the pressure gauge rise.
| Setting | What happens | Share of volcanoes* | Examples |
|---|---|---|---|
| Subduction zone (plates moving together) | An ocean plate sinks under another plate. Water from the sinking plate helps the mantle above it to melt; the magma is often sticky and gas-rich, so eruptions are often explosive. | 69% | The Ring of Fire: Japan, Indonesia, the Andes, the Cascades (Mount St. Helens) |
| Rift zone (plates moving apart) | The mantle rises and melts as the pressure on it falls. Runny basalt lava; mostly gentle eruptions, many of them underwater along mid-ocean ridges. | 18% | Iceland, the East African Rift (Nyiragongo, Erta Ale) |
| Intraplate (hotspot) | A plume of hot rock rises from deep in the mantle, far from plate boundaries. As the plate moves over it, a chain of volcanoes forms. | 12% | Hawaii, Réunion, the Galápagos, the Canary Islands |
*Of the 1,215 volcanoes active in the last 11,700 years in the Smithsonian list (2026). About two thirds of them, and about 85% of the eruptions recorded since 1900, are on the Ring of Fire around the Pacific. These numbers count volcanoes on land and known underwater ones; many more eruptions on the deep sea floor are never seen.
| VEI | Volume thrown out | Column height | Description | Examples |
|---|---|---|---|---|
| 0 | less than 10,000 m³ | less than 100 m | Non-explosive | Mauna Loa 2022 |
| 1 | more than 10,000 m³ | 100 m – 1 km | Gentle | Stromboli’s everyday bursts |
| 2 | more than 1 million m³ | 1–5 km | Explosive | Kīlauea 1959 |
| 3 | more than 0.01 km³ | 3–15 km | Severe | Nevado del Ruiz 1985 |
| 4 | more than 0.1 km³ | 10–25 km | Cataclysmic | Eyjafjallajökull 2010, Merapi 2010 |
| 5 | more than 1 km³ | more than 25 km | Paroxysmal | Vesuvius AD 79, Mount St. Helens 1980 |
| 6 | more than 10 km³ | more than 25 km | Colossal | Krakatau 1883, Novarupta 1912, Pinatubo 1991 |
| 7 | more than 100 km³ | more than 25 km | Super-colossal | Tambora 1815 |
| 8 | more than 1,000 km³ | more than 25 km | Mega-colossal (“supervolcano”) | Toba about 74,000 years ago; Yellowstone about 630,000 years ago |
VEI values of the dated eruptions are from the Smithsonian Global Volcanism Program; Toba and Yellowstone are from published studies of their deposits.
| Hazards | Benefits |
|---|---|
| Pyroclastic flows: avalanches of hot ash and gas, the deadliest hazard near a volcano Lahars: mudflows of ash and water that can travel tens of kilometres down valleys Ash fall: collapses roofs, damages crops and lungs, and stops aeroplanes Lava flows: destroy buildings and roads, but usually move slowly Volcanic gases: carbon dioxide and sulfur dioxide; large eruptions can cool the climate for a year or two Tsunamis: from collapsing or exploding island volcanoes (Krakatau 1883) |
Fertile soil from weathered ash, good for farming Geothermal energy: Iceland gets most of its heating from hot water underground Tourism: hot springs, national parks, lava viewing Minerals: sulfur, and many metal ores formed by hot volcanic fluids New land: islands such as Hawaii and Iceland were built by volcanoes |
Before most eruptions, rising magma sends warning signs. Scientists watch for swarms of small earthquakes (rock cracking as magma moves), the ground swelling (measured with GPS and satellites), changes in volcanic gases such as sulfur dioxide, and rising temperatures. In 1991 these signs let scientists warn people around Pinatubo; tens of thousands were evacuated before the eruption, saving many lives. Exactly when, and how big, an eruption will be is still hard to forecast.
Mantle rock melts only partly, by decompression melting (at rifts, mid-ocean ridges and hotspots), flux melting (water from the subducting slab lowers the melting temperature) or by heating of the crust. Silicon–oxygen tetrahedra in the melt link into chains and networks; the more silica, the more linked (polymerised) and viscous the melt. Viscosity rises from about 10–1,000 Pa·s for basalt at about 1,100–1,200 °C to 108–1011 Pa·s for rhyolite at about 700–850 °C (water itself is 0.001 Pa·s). Water is much more soluble in magma at high pressure, so it comes out of solution as the magma rises. When bubbles take up roughly 75% of the volume, or the magma is stretched too fast to flow, it breaks apart (fragments) into ash and gas: the start of an explosive eruption. The style chart in this sim is a teaching simplification of these ideas.
Deep underground, a small part of the hot mantle melts into magma. Magma is lighter than the solid rock around it, so it rises and often collects in a magma chamber a few kilometres down. When the pressure of the magma and its gas becomes high enough, the magma forces its way up the vent and comes out at the surface as lava, ash and gas.
Key takeaway: light, gas-rich magma rises and forces its way out.They are the same molten rock. Underground it is called magma; once it comes out onto the surface it is called lava. Lava has lost much of its gas.
Key takeaway: magma underground, lava at the surface.It depends mainly on how sticky (viscous) the magma is and how much gas it contains. Runny basalt magma lets gas bubbles escape, so lava pours out gently, as in Hawaii. Sticky, silica-rich magma traps the gas; the pressure builds until the magma shatters into ash in an explosion, as at Mount St. Helens in 1980.
Key takeaway: sticky + gassy = explosive; runny = gentle.Shield volcanoes are broad with gentle slopes and are built by runny lava (Mauna Loa). Stratovolcanoes, or composite volcanoes, are steep cones of alternating ash and lava layers with often explosive eruptions (Mount Fuji). Cinder cones are small, steep hills of loose cinders around a single vent (Parícutin).
Key takeaway: shield, stratovolcano (composite) and cinder cone.The Pacific is surrounded by subduction zones, where ocean plates sink beneath other plates. Water released from the sinking plate helps the mantle above it to melt, and the magma rises to form long chains of volcanoes. This horseshoe of volcanoes and earthquakes is called the Ring of Fire; it has about two thirds of the volcanoes active in the last 11,700 years.
Key takeaway: subduction zones surround the Pacific.Usually between about 700 °C and 1,200 °C. Runny basalt lava, like Hawaii’s, is the hottest, at about 1,100–1,200 °C. Sticky rhyolite lava is cooler, at about 700–850 °C.
Key takeaway: about 700–1,200 °C; basalt is the hottest.Often they can tell that an eruption is becoming more likely. Rising magma causes small earthquakes, makes the ground bulge and releases more gas, and scientists measure all of these. In 1991 warnings at Pinatubo allowed tens of thousands of people to be evacuated. Predicting the exact day and size is still difficult.
Key takeaway: yes, often weeks to days ahead, but not exactly.Near the volcano, pyroclastic flows are the deadliest: fast avalanches of hot ash, rock and gas. Lahars (volcanic mudflows) can travel far down river valleys; in 1985 one buried the town of Armero in Colombia. Lava flows, which people often fear most, usually move slowly enough to escape.
Key takeaway: pyroclastic flows and lahars, not usually lava.An informal name for a volcano that has had an eruption of VEI 8, throwing out more than 1,000 km³ of material. Examples are Toba (Indonesia), about 74,000 years ago, and Yellowstone (USA), about 630,000 years ago. No eruption this large has happened in recorded history.
Key takeaway: a volcano that has had a VEI 8 eruption.Yes. Most of Earth’s volcanic activity happens unseen on the sea floor along the mid-ocean ridges. Olympus Mons on Mars is the largest known volcano in the Solar System, and Jupiter’s moon Io is the most volcanically active world we know.
Key takeaway: most eruptions on Earth are underwater; Mars and Io have volcanoes too.